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福島第一原子力発電所事故
Satellite image on 16 March of the four damaged reactor buildings |
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| Date | 11 March 2011 time = 14:46 JST (05:46 UTC)
– 16 December 2011 |
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| Location | Ōkuma, Fukushima, Japan |
| Outcome | INES Level 7 (ratings by Japanese authorities as of 11 April)[1][2] |
| Reported injuries | 37 with physical injuries,[3] 2 workers taken to hospital with radiation burns[4] |
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| External videos | |
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| 24 hours live camera for Fukushima Daiichi nuclear disaster on YouTube, certified by Tokyo Electric Power Co. Inc. | |
The Fukushima Daiichi nuclear disaster (福島第一原子力発電所事故 Fukushima Dai-ichi () genshiryoku hatsudensho jiko) is a series of equipment failures, nuclear meltdowns, and releases of radioactive materials at the Fukushima I Nuclear Power Plant, following the Tōhoku earthquake and tsunami on 11 March 2011.[5][6] It is the largest nuclear disaster since the Chernobyl disaster of 1986.[7]
The plant comprises six separate boiling water reactors originally designed by General Electric (GE), and maintained by the Tokyo Electric Power Company (TEPCO). At the time of the quake, Reactor 4 had been de-fuelled while 5 and 6 were in cold shutdown for planned maintenance.[8] The remaining reactors shut down automatically after the earthquake, and emergency generators came online to control electronics and coolant systems. The tsunami broke the reactors' connection to the power grid, leading the reactors to begin to overheat. The flooding and earthquake damage hindered external assistance.
In the hours and days that followed, reactors 1, 2 and 3 experienced full meltdown.[9][10] As workers struggled to cool and shut down the reactors, several hydrogen explosions occurred.[11] The government ordered that seawater be used to attempt to cool the reactors—this had the effect of ruining the reactors entirely.[12] As the water levels in the fuel rods pools dropped, they began to overheat. Fears of radioactivity releases led to a 20 km (12 mi)-radius evacuation around the plant, while workers suffered radiation exposure and were temporarily evacuated at various times. Electrical power was slowly restored for some of the reactors, allowing for automated cooling.[13]
Japanese officials initially assessed the accident as Level 4 on the International Nuclear Event Scale (INES) despite the views of other international agencies that it should be higher. The level was successively raised to 5 and eventually to 7, the maximum scale value.[14][15] The Japanese government and TEPCO have been criticized in the foreign press for poor communication with the public[16][17] and improvised cleanup efforts.[18] On 20 March, the Chief Cabinet Secretary Yukio Edano announced that the plant would be decommissioned once the crisis was over. Japan's foreign minister criticized foreign media reports over the accident as overdone and excessive.[19]
The Japanese government estimates the total amount of radioactivity released into the atmosphere was approximately one-tenth as much as was released during the Chernobyl disaster.[20] Significant amounts of radioactive material have also been released into ground and ocean waters. Measurements taken by the Japanese government 30–50 km from the plant showed radioactive caesium levels high enough to cause concern,[21] leading the government to ban the sale of food grown in the area. Tokyo officials temporarily recommended that tap water should not be used to prepare food for infants.[22][23] There were no deaths or serious injuries due to direct radiation exposures, but a few of the plant's workers were severely injured or killed by the disaster conditions resulting from the earthquake. At least six workers have exceeded lifetime legal limits for radiation and more than 300 have received significant radiation doses. Future cancer deaths due to accumulated radiation exposures in the population living near Fukushima have been estimated to be between 100[24] and 1,000.[20] Fear of ionizing radiation could have long-term psychological effects on a large portion of the population in the contaminated areas. Decommissioning of the plant will likely take 40 years to complete.[25]
The Fukushima I Nuclear Power Plant consists of six light water, boiling water reactors (BWR) designed by General Electric driving electrical generators with a combined power of 4.7 gigawatts, making Fukushima I one of the 25 largest nuclear power stations in the world. Fukushima I was the first GE designed nuclear plant to be constructed and run entirely by the Tokyo Electric Power Company (TEPCO).
Unit 1 is a 439 MWe type (BWR3) reactor constructed in July 1967. It commenced commercial electrical production on 26 March 1971.[26] It was designed for a peak ground acceleration of 0.18 g (1.74 m/s2) and a response spectrum based on the 1952 Kern County earthquake.[27] Units 2 and 3 are both 784 MWe type BWR-4 reactors, Unit 2 commenced operating in July 1974 and Unit 3 in March 1976. The earthquake design basis for all units ranged from 0.42 g (4.12 m/s2) to 0.46 g (4.52 m/s2).[28][29] All units were inspected after the 1978 Miyagi earthquake when the ground acceleration was 0.125 g (1.22 m/s2) for 30 seconds, but no damage to the critical parts of the reactor was discovered.[27]
Units 1–5 have a Mark 1 type (light bulb torus) containment structure, Unit 6 has Mark 2 type (over/under) containment structure.[27] From September 2010, Unit 3 has been partially fuelled by mixed-oxide (MOX) fuel.[30]
At the time of the accident, the units and central storage facility contained the following numbers of fuel assemblies:[31]
| Location | Unit 1 | Unit 2 | Unit 3 | Unit 4 | Unit 5 | Unit 6 | Central Storage |
|---|---|---|---|---|---|---|---|
| Reactor Fuel Assemblies | 400 | 548 | 548 | 0 | 548 | 764 | 0 |
| Spent Fuel Assemblies | 292 | 587 | 514 | 1331 | 946 | 876 | 6375 |
| Fuel | UOx | UOx | UO2/MOX | UOx | UOx | UOx | UO2/MOX |
| New Fuel Assemblies[32] | 100 | 28 | 52 | 204 | 48 | 64 | N/A |
Power reactors work by splitting atoms, typically uranium, in a chain reaction. The reactor continues to generate heat after the chain reaction is stopped because of the radioactive decay of unstable isotopes, fission products, created by this process. This decay of unstable isotopes, and the decay heat, cannot be stopped.[33][34] Immediately after shutdown, this decay heat amounts to approximately 6% of full thermal heat production of the reactor.[33] The decay heat in the reactor core decreases over several days before reaching cold shutdown levels.[35] Nuclear fuel rods that have reached cold shutdown temperatures typically require another several years of water cooling in a spent fuel pool before decay heat production reduces to the point that they can be safely transferred to dry storage casks.[36]
In order to safely remove this decay heat, reactor operators must continue to circulate cooling water over fuel rods in the reactor core and spent fuel pond.[33][37] In the reactor core, circulation is accomplished by use of high pressure systems that pump water through the reactor pressure vessel and into heat exchangers. These systems transfer heat to a secondary heat exchanger via the essential service water system, taking away the heat which is pumped out to the sea or site cooling towers.[38]
To circulate cooling water when the reactor is shut down and not producing electricity, cooling pumps can be powered by other units on-site, by other units off-site through the grid, or by diesel generators.[37][39] In addition, boiling water reactors have steam-turbine driven emergency core cooling systems that can be directly operated by steam still being produced after a reactor shutdown, which can inject water directly into the reactor.[40] Steam turbines results in less dependence on emergency generators, but steam turbines only operate so long as the reactor is producing steam. Some electrical power, provided by batteries, is needed to operate the valves and monitoring systems.
If the water in the Unit 4 spent fuel pool had been heated to boiling temperature, the decay heat has the capacity to boil off about 70 tonnes of water per day (12 gallons per minute), which puts the requirement for cooling water in context.[41] On 16 April 2011, TEPCO declared that Reactors 1–4's cooling systems were beyond repair and would have to be replaced.[42]
The reason that cooling is so essential for a nuclear reactor, is that many of the internal components and fuel assembly cladding is made from zircaloy. At normal operating temperatures (of approximately 300 degrees Celsius), zircaloy is inert. However when heated to above 500 degrees Celsius in the presence of steam,[43] zircaloy undergoes an exothermic reaction where the zircaloy oxidises and produces hydrogen.
The reactor's emergency diesel generators and DC batteries, crucial components in powering the reactors' cooling systems in the event of a power loss, were located in the basements of the reactor turbine buildings. The reactor design plans provided by General Electric specified placing the generators and batteries in that location, but mid-level engineers working on the construction of the plant were concerned that this made the back-up power systems vulnerable to flooding. TEPCO elected to strictly follow General Electric's design in the construction of the reactors.[44]
The Fukushima Daiichi nuclear power complex was central to a falsified-records scandal that led to the departure of a number of senior executives of TEPCO. It also led to disclosures of previously unreported problems at the plant,[45] although testimony by Dale Bridenbaugh, a lead GE designer, purports that General Electric was warned of major design flaws in 1976, resulting in the resignations of several designers who protested GE's negligence.
In 2002, TEPCO admitted it had falsified safety records at the No. 1 reactor at Fukushima Daiichi. As a result of the scandal and a fuel leak at Fukushima, the company had to shut down all of its 17 nuclear reactors to take responsibility.[46] A power board distributing electricity to a reactor's temperature control valves was not examined for 11 years. Inspections did not cover devices related to cooling systems, such as water pump motors and diesel generators.[47]
On 30 October 1991 one of two backup generators of reactor nr. 1 did fail, after it was flooded in the basement of the reactor buildings. Seawater used for the cooling of the reactor was leaking into the turbine-building from a corroded pipe at a rate of 20 cubic meters per hour. This was told by former TEPCO employees to the Japan Broadcasting Corporation news-service in December 2011. An engineer told, that he informed his superiors about this accident, and that he mentioned the possibility that a tsunami could inflict damage to the generators in the turbine-buildings near the sea. After this TEPCO did not move the generators to higher grounds, but in stead TEPCO installed doors to prevent water leaking into the generator rooms. The Japanese Nuclear Safety Commission commented that it would revise the safety guidelines for designing nuclear plants and would enforce the installation of additional power sources. On 29 December 2011 TEPCO admitted all these facts: its report mentioned, that the emergency power system room was flooded through a door and some holes for cables, but the power supply to the reactor was not cut off by the flooding, and the reactor was stopped for one day. One of the two power sources was completely submerged, but its drive mechanism had remained unaffected. [48][49] [50]
In March 2006 the Japanese government opposed a court order to close a nuclear plant in the west part of the country over doubts about its ability to withstand an earthquake. Japan's Nuclear and Industrial Safety Agency believed it was "safe" and that "all safety analyses were appropriately conducted".[51]
In 2007 TEPCO did set up a department to supervise all its nuclear facilities, and until June 2011 its chairman was Masao Yoshida, the chief of the Fukushima Daiichi power plant. An in-house study in 2008 pointed out that there was an immediate need to improve the protection of the power station from flooding by seawater. This study mentioned the possibility of tsunami-waves up to 10.2 meters. Officials of the department at the company's headquarters insisted however that such a risk was unrealistic and did not take the prediction seriously. [52]
In addition to concerns from within Japan, the International Atomic Energy Agency (IAEA) has also expressed concern about the ability of Japan's nuclear plants to withstand seismic activity. At a meeting of the G8's Nuclear Safety and Security Group, held in Tokyo in 2008, an IAEA expert warned that a strong earthquake with a magnitude above 7.0 could pose a "serious problem" for Japan's nuclear power stations.[53]
On request of the Japan Broadcasting Corporation, on 2 October 2011 the Japanese Government released a report of TEPCO to NISA. These papers proved that TEPCO was well aware of the possibility that the plant could be hit by a tsunami with waves far higher than the 5.7 meters which the plant was designed to withstand. Simulations done in 2008, based on the destruction caused by the 1896-earthquake in this area, made it clear that waves between 8.4 and 10.2 meters could overflow the plant. Three years later the report was sent to NISA, where it arrived on the 7 March 2011, just 4 days before the plant was hit by the tsunami. Further studies by scientists and an examination of the plant's tsunami resistance measures were not planned by TEPCO before April 2011, and no further actions were planned to deal with this subject before October 2012. TEPCO official Junichi Matsumoto said that the company did not feel the need to take prompt action on the estimates, which were still tentative calculations in the research stage. An official of NISA said that these results should have been made public by TEPCO, and that the firm should have taken measures right away.[54][55]
This all was in sharp contrast with the events at the Tōkai Nuclear Power Plant, there the dike around the plant was raised to 6.1 meters after evaluations showed the possibility of tsunami-waves higher than previously expected. Although the dike was not completely finished at 11 March 2011, the plant could ride out the tsunami, even though the external power-sources in Tokai were lost too. With two (of three) functioning sea-water-pumps and the emergency diesel-generator the reactor could be kept safely in cold shutdown.[56]
On 26 November a TEPCO spokes-man mentioned that TEPCO would have been better prepared to cope with the tsunami in March 2011, if it had taken the 2008-study more seriously. TEPCO was also willing to use the estimates of renewed study done by a national civil engineering society for its facility management. [57]
The 9.0 MW Tōhoku earthquake occurred at 14:46 JST on Friday, 11 March 2011 with epicenter near the island of Honshu.[58] It resulted in maximum ground accelerations of 0.56, 0.52, 0.56 g (5.50, 5.07 and 5.48 m/s2) at Units 2, 3 and 5 respectively, above their designed tolerances of 0.45, 0.45 and 0.46 g (4.38, 4.41 and 4.52 m/s2), but values within the design tolerances at Units 1, 4 and 6.[29] The Fukushima I facility had not initially been designed for a tsunami of the size that struck the plant,[59][60] nor had the reactors been modified when later concerns were raised in Japan and by the IAEA.[61] When the earthquake occurred, the reactors on Units 1, 2, and 3 were operating, but those on Units 4, 5, and 6 had already been shut down for periodic inspection.[28][62] Units 1, 2 and 3 underwent an automatic shutdown (called SCRAM) when the earthquake struck.[63][64]
When the reactors shut down, the plant stopped generating electricity, stopping the normal source of power for the plant.[65] TEPCO reported that one of the two connections to off-site power for Reactors 1–3 also failed[65] so 13 on-site emergency diesel generators began powering the plant's cooling and control systems.[66] There are two emergency diesel generators for each of the Units 1–5 and three for Unit 6.[67]
The earthquake was followed by a 13–15 m (43–49 ft) maximum height tsunami arriving approximately 50 minutes later which topped the plant's 5.7 m (19 ft) seawall,[68][69][70] flooding the basement of the Turbine Buildings and disabling the emergency diesel generators[71][72] located there[67] at approximately 15:41.[65][73] At this point, TEPCO notified authorities, as required by law, of a "First level emergency".[63] The Fukushima II plant, which was also struck by the tsunami, incorporated design changes which improved its resistance to flooding and it sustained less damage. Generators were located in the watertight reactor building rather than the turbine building which was flooded. Seawater pumps for cooling were given protection from flooding, and although 3 of 4 failed in the tsunami, they were able to be restored to operation.[74]
In the late 1990s to comply with new regulatory requirements, three additional backup generators for reactors Nos. 2 and 4 were placed in new buildings located higher on the hillside. All six reactors were given access to these generators, however the switching stations that sent power from these backup generators to the reactors' cooling systems for Units 1 through 5 were still in the poorly protected turbine buildings. All three of the generators added in the late 1990s were operational after the tsunami. If the switching stations had been moved to inside the reactor buildings or to other flood-proof locations, power would have been provided by these generators to the reactors' cooling systems.[75]
After the diesel generators located in the turbine buildings failed, emergency power for control systems was supplied by batteries that were designed to last about eight hours.[76] Further batteries and mobile generators were dispatched to the site, delayed by poor road conditions with the first not arriving until 21:00 JST 11 March,[66][77] almost six hours after the tsunami struck.
Attempts to connect portable generating equipment to power water pumps were eventually discontinued after numerous attempts, as the connection point in the Turbine Hall basement was flooded and because of difficulties finding suitable cables.[71] TEPCO switched its efforts to provide power to the facility to re-stringing high voltage cables to the grid.[78] One plant generator at Unit 6 was restored to operation on 17 March, and external power returned to Units 5 and 6, on 20 March, allowing cooling equipment to be restarted.[79]
F. Tanabe has estimated that the core contained the following materials:[80]
On 11 March at 14:46 JST, Unit 1 scrammed successfully in response to the earthquake[65] though evacuated workers reported violent shaking and burst pipes within the reactor building.[81] At 15:37 all generated electrical power was lost following the tsunami leaving only emergency batteries, able to run some of the monitoring and control systems. It was later learned that Unit 1's batteries were damaged and unavailable following the tsunami. At 15:42, TEPCO declared a "Nuclear Emergency Situation" for Units 1 and 2 because "reactor water coolant injection could not be confirmed for the emergency core cooling systems."[65] The alert was temporarily cleared when water level monitoring was restored for Unit 1 but it was reinstated at 17:07 JST.[65] Potentially radioactive steam was released from the primary circuit into the secondary containment area to reduce mounting pressure.[82]
After the loss of site power and reactor shutdown, Unit 1 was initially cooled using the isolation condenser system. About 10 minutes after the earthquake, TEPCO operators removed both of Unit 1's isolation condensers from service, and instead chose to activate the HPCI (High Pressure Coolant Injection) systems to cool the reactor and the core spray system was activated at 15:07 to cool the suppression pool. The core spray system was disabled with AC power loss at 15:37 (The tsunami) and the HPCI system failed following DC power loss.
Operators were unable to restart the isolation condensers for an extended period of time after the tsunami (greater than 30 minutes). After that, the isolation condensers were operated intermittently, for unknown reasons. The isolation condensers were designed to successfully cool Unit 1 for at least 8 hours, and it is unknown how effective they were. After that, refill would have been required to the isolation condenser tanks which are under atmospheric pressure (low pumping head requirements). By design, isolation condensers would have removed the heat from the reactor transferring it out of the primary containment and into the atmosphere, but with limited and non-existent operation, core and containment cooling was not successful. By midnight water levels in the reactor were falling and TEPCO gave warnings of the possibility of radioactive releases.[83] In the early hours of 12 March, TEPCO reported that radiation levels were rising in the turbine building for Unit 1[84] and that it was considering venting some of the mounting pressure into the atmosphere, which could result in the release of some radioactivity.[85] Chief Cabinet Secretary Yukio Edano stated later in the morning the amount of potential radiation would be small and that the prevailing winds were blowing out to sea.[86] At 02:00 JST, the pressure inside the reactor containment was reported to be 600 kPa (6 bar or 87 psi), 200 kPa higher than under normal conditions.[72] At 05:30 JST, the pressure inside Reactor 1 was reported to be 2.1 times normal levels, 820 kPa.[87] Isolation cooling ceased to operate between midnight and 11:00 JST 12 March, at which point TEPCO started relieving pressure and injecting water.[88] One employee working inside Unit 1 at this time received a radiation dose of 106 mSv and was later sent to a hospital to have his condition assessed.[89]
Rising heat within the containment area led to increasing pressure. Electricity was needed for both the cooling water pumps and ventilation fans used to drive gases through heat exchangers within the containment.[90] Releasing gases from the reactor is necessary if pressure becomes too high and has the benefit of cooling the reactor as water boils off but this also means cooling water is being lost and must be replaced.[71] If there was no damage to the fuel elements, water inside the reactor should be only slightly radioactive.
In a press release at 07:00 JST 12 March, TEPCO stated, "Measurement of radioactive material (iodine, etc.) by monitoring car indicates increasing value compared to normal level. One of the monitoring posts is also indicating higher than normal level."[91] Dose rates recorded on the main gate rose from 69 nGy/h (for gamma radiation, equivalent to 69 nSv/h) at 04:00 JST, 12 March, to 866 nGy/h 40 minutes later, before hitting a peak of 0.3855 mSv/h at 10:30 JST.[91][92][93][94] At 13:30 JST, workers detected radioactive caesium-137 and iodine-131 near Reactor 1,[3] which indicated some of the core's fuel had been damaged.[95] Cooling water levels had fallen so much that parts of the nuclear fuel rods were exposed and partial melting might have occurred.[96][97] Radiation levels at the site boundary exceeded the regulatory limits.[98]
On 14 March, radiation levels had continued to increase on the premises, measuring at 02:20 an intensity of 0.751 mSv/h on one location and at 02:40 an intensity of 0.650 mSv/h at another location on the premises.[99] On 16 March, the maximum readings peaked at 10.850 mSv/h.[100]
At 07:00 JST on 12 March, Prime Minister Naoto Kan asked Daiichi director Masao Yoshida why his workers were not opening the valves to release rising steam pressure within the reactor. Yoshida answered that they could not open the electrical valves because of the power failure and the radiation was too high to send workers to manually open the valves. Nevertheless, with the pressure and temperatures continuing to rise, at 09:15, TEPCO sent workers to begin manually opening the valves. The high radiation slowed the work and the valves were not opened until 14:30.[101]
At 15:36 JST on 12 March, there was an explosion in the reactor building at Unit 1. The side walls of the upper level were blown away, leaving in place only the vertical steel framed gridworks. The roof collapsed, covering the floor and some machinery on the south side. The walls were relatively intact compared to later explosions at Units 3 and 4.[102][103] Video of the explosion shows that it was primarily directed sideways.
The roof of the building was designed to provide ordinary weather protection for the areas inside, not to withstand the high pressure of an explosion. In the Fukushima I reactors the primary containment consists of "drywell" and "wetwell" concrete structures below the top level, immediately surrounding the reactor pressure vessel. The secondary containment includes the top floor with water-filled pools for storing fresh or irradiated fuel and for storage of irradiated tools and structures.[87][104]
Experts soon agreed that the cause was a hydrogen explosion.[105][106][107] Almost certainly the hydrogen was formed inside the reactor vessel[105] because of falling water levels exposing zircaloy structures/fuel assembly cladding, which then reacted with steam and produced hydrogen,[108] with the hydrogen subsequently vented into the containment building.[105]
Officials indicated that reactor primary containment had remained intact and that there had been no large leaks of radioactive material,[87][105] although an increase in radiation levels was confirmed following the explosion.[109][110] However, the report[111] of the fact-finding commission states that "There is a possibility that the bottom of the RPV was damaged and some of the fuel might have dropped and accumulated on the D/W floor (lower pedestal)." The Fukushima prefectural government reported radiation dose rates at the plant reaching 1.015 mSv/h.[112] The IAEA stated on 13 March that four workers had been injured by the explosion at the Unit 1 reactor, and that three injuries were reported in other incidents at the site. They also reported one worker was exposed to higher-than-normal radiation levels but the level fell below their guidance for emergency situations.[113]
At 20:05 JST on 12 March, the Japanese government ordered seawater to be injected into Unit 1 in a new effort to cool the reactor core.[114] The treatment had been held as a last resort since it ruins the reactor.[115] TEPCO started seawater cooling at 20:20, adding boric acid as a neutron absorber to prevent a criticality accident.[116][117] The water would take five to ten hours to fill the reactor core, after which the reactor would cool down in around ten days.[105] The injection of seawater into the reactor pressure vessel was done by fire trucks of the fire department.[118][119][120] At 01:10 on 14 March, injection of seawater was halted for two hours because all available water in the plant pools had run out (similarly, feed to Unit 3 was halted).[118] NISA news reports stated 70% of the fuel rods had been damaged when uncovered.[121]
On 18 March, a new electrical distribution panel was installed in an office adjacent to Unit 1 to supply power via Unit 2 when it was reconnected to the transmission grid two days later.[119] On 21 March, injection of seawater continued, as did repairs to the control instrumentation.[3] On 23 March, it became possible to inject water into the reactor using the feed water system rather than the fire trucks, raising the flow rate from 2 to 18 m3/h (later reduced to 11m3/h,[122][123] and even further to reduce the build up of contaminated water), and on 24 March, lighting was restored to the central operating room.[124]
As of 24 March, the spent fuel pool was "thought to be fully or partially exposed", according to CNN.[125] Pressure in the reactor had increased owing to the seawater injection, resulting in steam being vented, later alleviated by reducing the water flow. Temperature increases were also reportedly temporary. TEPCO condensed some of the steam to water in the spent fuel pool.
It was estimated[126] that as much as 26 tonnes of sea salt may have accumulated in reactor Unit 1, and twice that amount in Units 2 and 3. As salt clogs cooling pipes and erodes zirconium oxide layer of the fuel rods, switching to the use of freshwater for cooling was a high priority.
By 24 March, electrical power (initially from temporary sources, but off-site power used from 3 April) was being restored to parts of the Unit, with the Main Control Room lighting being restored.[127]
On 25 March, fresh water became available again to be added to the reactor instead of salt water,[128] while work continued to repair the unit's cooling systems.[129] A volume of 1890 m3 (500,000 USgal) of fresh water was brought to the plant by a barge provided by US Navy.[130] On 29 March, the fire trucks which had been used to inject water into the reactor were replaced by electrically-driven pumps.[124]
On 28 March, pumping began to remove water contaminated with radioactive 137Cs and 131I from basement areas, storing it in the condenser system.[124] By 29 March, pumping was halted because condensate reservoirs were almost full and plans were being considered to transfer water to the suppression pool surge tanks.[131]
On 7 April, TEPCO began injecting nitrogen into the containment vessel, which was expected to reduce the likelihood of further hydrogen explosions.[132] The injection has been ongoing since then, and has been repeated on the other units at Fukushima.[133] Later on 7 April, but prior to a large aftershock, temperatures in the reactor core unexpectedly "surged in temperature to 260 °C", the cause was unknown, but the temperature dropped to 246 °C by 8 April.[134] On 27 April, TEPCO revised its estimate of damaged fuel in Unit 1 from 55% to 70%.[135]
On 17 April, remote control robot was used to enter the Reactor Building and performed a series of inspections, which confirmed on 29 April that there was no significant water leakage coming from the containment vessel.[127]
On 23 and 26 April, concerns that Unit 1 fuel rods may be exposed to air caused TEPCO to consider filling the "containment vessel with water to cool the reactor" despite concerns for building integrity.[136][137] However, efforts were slowed by Unit 1 radiation measurements "as high as 1,120 mSv/hr".[138] On 13 May, TEPCO announced it would proceed with a plan to fill the containment vessel despite the possibility of holes caused by melting fuel elements existing in the pressure vessel.[138][139] TEPCO had expected to increase the amount of water pumped to Unit 1 to compensate for any leakage from the holes,[140] but decided on 15 May to abandon the plan after finding the Unit 1 basement was already half flooded.[141]
On 5 May, ventilation systems were installed in the Reactor Building, to clean the highly radioactive air encapsulated within the Reactor Building.[127]
On 12 May, the water level gauge for the Reactor was calibrated, and it was subsequently identified that the water level was lower than previously thought (as the water level went off the lower side of the gauge).[127]
On 13 May, preparatory work started on the installation of the Reactor Building covers. Construction work started on 28 June.[142]
On 20 May, staff entered the Reactor Building, confirming Reactor water level and radioactivity.[142]
Since 2 July, the Reactor has been cooled using fresh water treated by the on site water treatment plant.[142]
On 21 August, TEPCO reported that all of the temperature sensors of Unit 1 were recording temperatures less than 100 degrees Celsius on Friday 19 August. Once other goals are met, Unit 1 will have achieved Cold Shutdown state.[143]
On 28 October, TEPCO reported the completion of cover construction at reactor building of unit 1 of Fukushima Daiichi nuclear power station.[144]
Reports of 13 observations of neutron beams 1.5 km "southwest of the plant's No. 1 and 2 reactors" from 13 to 16 March raised the possibility that nuclear fission could have occurred after the initial SCRAMing of the reactors at Fukushima Daiichi.[145] 16 March reports that fuel rods in the spent fuel pool at Unit 4 could have been exposed to air appeared to indicate that fission may have occurred in that fuel pool.[146] Later reports of exceptionally high iodine-134 levels appeared to confirm this theory because very high levels of iodine-134 would be indicative of fission reactions.[147] The same report also showed high measurements of chlorine-38,[148] which some nuclear experts used to calculate that fission must be occurring in Unit 1.[149][150] Despite TEPCO suggesting the iodine-134 report was inaccurate, the IAEA appeared to accept the chlorine-based analysis as a valid theory suggesting fission when it stated at a press conference that "melted fuel in the No. 1 reactor building may be causing isolated, uncontrolled nuclear chain reactions".[151] However, TEPCO confirmed its concern about the accuracy of the high iodine and chlorine report by formally retracting the report on 21 April,[152] which eliminated both the exceptionally high iodine-134 and chlorine-38 levels as proof of criticality. TEPCO did not appear to comment on the criticality concern when withdrawing its report,[153][154] but the IAEA has not withdrawn its comments, and some off-site experts find the currently-measured iodine-134 levels higher than expected.[155][156]
On 12 May, TEPCO engineers confirmed that a meltdown occurred, with molten fuel having fallen to the bottom of the reactor's containment vessel.[157] The utility said that fuel rods of the No. 1 reactor are fully exposed, with the water level 1 meter (3.3 feet) below the base of the fuel assembly. According to a Japanese press report, there are holes in the base of the pressure vessel, and most of the fuel has probably melted. The nuclear fuel has possibly leaked into the containment vessel, which was damaged in an explosion during the crisis. This caused both the Japanese government and TEPCO to be criticized for consistently underestimating the severity of the situation.[158] The operator found the basement flooded with 4.2 meters of water. Workers were unable to observe the flooding situation due to high levels of radiation from the water.
TEPCO estimates the nuclear fuel was exposed to the air less than five hours after the earthquake struck. Fuel rods melted away rapidly as the temperature inside the core reached 2,800 °C within six hours. In less than 16 hours, the reactor core melted and dropped to the bottom of the pressure vessel, burning a hole through the vessel. By that time, water was pumped into the reactor in an effort to prevent the worst-case scenario – overheating fuel melting its way through the containment and discharging large amounts of radionuclides in the environment.[159] In June the Japanese government confirmed that Unit 1 reactor vessel containment was breached, and pumped cooling water continues to leak months after the disaster.[160]
From 31 March, additional sea water was added to the spent fuel pool, initially by using a concrete pump. Fresh water was used from 14 May. However, by 29 May water was able to be injected using a temporary pump and the Spent Fuel Pool Cooling (FPC) line.[127][142]
On 10 August the spent fuel pool was switched from the water-injection system - that functioned some 5 months - to a circulatory cooling system. For the first time since the 11 March disaster, all four damaged reactors at the plant were using circulatory cooling systems with heat-exchangers.[161]
Unit 2 was operating at the time of the earthquake and experienced the same controlled initial shutdown as the other units.[87] As with unit 1, the reactor scrammed following the earthquake. The two diesel generators came online and initially all cooling systems were available. Initially the high pressure coolant injection (HPCI) system was primary cooling the core and at 15:00 operators activated the residual heat removal system main pump and the containment vessel spray pump at 15:07 to cool the suppression pool however all these systems failed following both AC power and DC power loss after the tsunami as the diesel generators and other systems failed when the tsunami overran the plant. The reactor core isolation cooling (RCIC) system was manually activated by operators at 15:39 following power loss, but by midnight the status of the reactor was unclear; some monitoring equipment was still operating on temporary power.[83] The coolant level was stable and preparations were underway to reduce pressure in the reactor containment vessel should it become necessary, though TEPCO did not state in press releases what these preparations were, and the government had been advised that this might happen.[162] The RCIC was reported by TEPCO to have shut down around 19:00 JST on 12 March, but reported to be operating again as of 09:00 JST 13 March.[163] The pressure reduction of the reactor containment vessel commenced before midnight on 12 March[164] although the IAEA reported that as of 13:15 JST 14 March, that according to information supplied to them, no venting had taken place at the plant.[3] A report in The New York Times suggested that plant officials initially concentrated efforts on a damaged fuel storage pool at Unit 2, diverting attention from problems arising at the other reactors, but that incident was not reported in official press releases.[165] The IAEA reported that on 14 March at 09:30, the RCIC was still operating and that power was being provided by a mobile generator.[3] By midday on 19 March grid power had been connected to the existing transformer at Unit 2 and work continued to connect the transformer to the new distribution panel installed in a nearby building.[166] Outside electricity became available at 15:46 JST on 20 March, but equipment still had to be repaired and reconnected.[119]
On 14 March, TEPCO reported the shutdown of the RCIC system presumably due to low reactor pressure. Operators had for days taken measures to prevent the reactor pressure from dropping below the level at which the RCIC can operate to keep it running as long as possible. The system was never designed to be used for an extended period.[167] Fuel rods had been fully exposed for 140 minutes and there was a risk of a core meltdown.[168] Reactor water level indicators were reported to be showing minimum-possible values at 19:30 JST on 14 March.[169]
At 22:29 JST, workers had succeeded in refilling half the reactor with water but parts of the rods were still exposed, and technicians could not rule out the possibility that some had melted. It was hoped that holes blown in the walls of reactor building 2 by the earlier blast from Unit 3 would allow the escape of hydrogen vented from the reactor and prevent a similar explosion.[168] At 21:37 JST, the measured dose rates at the gate of the plant reached a maximum of 3.13 mSv/h, which was enough to reach the annual limit for non-nuclear workers in twenty minutes,[168] but had fallen back to 0.326 mSv/h by 22:35.[170]
It was believed that around 23:00 JST, the 4 m long fuel rods in the reactor were fully exposed for the second time.[168][171] At 00:30 JST on 15 March, NHK ran a live press conference with TEPCO stating that the water level had sunk under the rods once again and pressure in the vessel was raised. The utility said that the hydrogen explosion at Unit 3 might have caused a glitch in the cooling system of Unit 2: Four out of five water pumps being used to cool the Unit 2 reactor had failed after the explosion at Unit 3. In addition, the last pump had briefly stopped working when its fuel ran out.[172][173] To replenish the water, the contained pressure would have to be lowered first by opening a valve of the vessel. The unit's air flow gauge was accidentally turned off and, with the gauge turned off, flow of water into the reactor was blocked leading to full exposure of the rods. As of 04:11 JST on 15 March, water was being pumped into the reactor of Unit 2 again.[174]
At Thursday 23 June Tepco-workers entered the building of reactor 2, to install a provisional gauge for measuring the water level inside the reactor. The original device was damaged in March. Next Saturday 25 June Tepco reported, that is was still not possible to obtain accurate data on the water level and pressure of this reactor. The temperature near the containment vessel is very high, because of this the gauge did not function properly: the water inside the tubes of the gauge was evaporated.[175]
It was later revealed that workers were minutes from restoring power to the standby liquid control (SLC) system pumps in unit 2 as a way to inject borated water once the RCIC shut down and had spent hours laying cable from a generator truck to the unit 2 power center when the unit 1 explosion occurred. This damaged the cable preventing this method from being used. It is possible this system could have prevented a complete meltdown as it took hours after the explosion until injection using fire trucks was able to be started.[176]
An explosion was heard after 06:14 JST[177] on 15 March in Unit 2, possibly damaging the pressure-suppression system, which is at the bottom part of the containment vessel.[178][179] The radiation level was reported to exceed the legal limit and the plant's operator started to evacuate all non-essential workers from the plant.[180] Only a minimum crew of 50 men, also referred to as the Fukushima 50, was left at the site.[181] Soon after, radiation equivalent dose rates had risen to 8.2 mSv/h[182] around two hours after the explosion and again down to 2.4 mSv/h, shortly after.[183] Three hours after the explosion, the rates had risen to 11.9 mSv/h.[184]
While admitting that the suppression pool at the bottom of the containment vessel had been damaged in the explosion, causing a drop of pressure there, Japanese nuclear authorities emphasized that the containment had not been breached as a result of the explosion and contained no obvious holes.[185] In a news conference on 15 March the director general of the IAEA, Yukiya Amano, said that there was a "possibility of core damage" at Unit 2 of less than 5%.[186] Japan's Nuclear and Industrial Safety Agency (NISA) stated 33% of the fuel rods were damaged, in news reports the morning of 16 March.[121] On 30 March, NISA reiterated concerns about a possible Unit 2 breach at either the suppression pool, or the reactor vessel.[187] NHK World reported the NISA's concerns as "air may be leaking", very probably through "weakened valves, pipes and openings under the reactors where the control rods are inserted", but that "there is no indication of large cracks or holes in the reactor vessels".[187]
On 8 November workers did enter reactor-building no. 4, and inspected the place to determine the cause of the hydrogen-blast on 15 March 2011. They found the 5th floor more severely damaged compared with the 4th floor, where the spent fuel pool was located. The fuel itself was found undamaged. The workers also found a severely damaged air conditioning duct on floor 5. These findings did not support earlier assumptions that the hydrogen in the blast originated from the spent fuel pool of reactor 4, but instead proved that the explosion was caused by hydrogen from the number 3 reactor, after the valves were opened. The hydrogen reached the fifth floor of reactor building 4 through the air-condition pipe.[188] [189]
From 20 March, seawater was added to the spent fuel pool[119] via the Fuel Pool Cooling (FPC) line.[142] Fresh water was used from 29 March.[142]
On 31 May, the spent fuel pool was switched from the water-injection system, to a circulatory cooling system.[142]
Unit 2 was considered the most likely unit to have a damaged reactor containment vessel, as of 24 March.[125] On 27 March, TEPCO reported measurements of very high radiation levels, over 1000 mSv/h, in the basement of the Unit 2 turbine building, which officials reported was 10 million times higher than what would be found in the water of a normally functioning reactor. Hours into the media frenzy, the company retracted its report and stated that the figures were not credible.[190] "because the level was so high the worker taking the reading had to evacuate before confirming it with a second reading."[191] Shortly following the ensuing wave of media retractions that discredited the report worldwide, TEPCO clarified its initial retraction; the radiation from the pool surface in the basement of the Unit 2 turbine building was found to be "more than 1,000 millisieverts per hour", as originally reported, but the concentration of radioactive substances was 100,000 times higher than usual, not 10 million.[192]
At 20:05 JST on 14 March, the Japanese government ordered seawater to be injected into Unit 2 in a new effort to cool the reactor core. The treatment had been held as a last resort since it ruins the reactor. TEPCO started seawater cooling at 16:34.[127] From 26 March, freshwater was used to cool the core.[142]
By 26 March, electrical power (initially from temporary sources, but off-site power used from 3 April) was being restored to parts of the Unit, with the Main Control Room lighting being restored.[142]
On 28 March, the Nuclear Safety Commission announced its suspicion that radioactive materials had leaked from Unit 2 into water in trenches connecting Unit 2's buildings, leading TEPCO to reduce the amount of water pumped into the reactor because of fears that the water could leak into the sea.[193] The reduction in water pumping could have raised reactor temperatures.[194]
On 27 March, the IAEA reported temperatures at the bottom of the Reactor Pressure Vessel (RPV) at Unit 2 fell to 97 °C from 100 °C on Saturday. Operators attempted to pump water from the turbine hall basement to the condenser.[195][196] However, "both condensers turned out to be full."[197] Therefore, condenser water was first attempted to be pumped to storage tanks, freeing condenser storage for water currently in the basement of Unit 2.[197] The pumps now being used can move 10 to 25 tons of water per hour.[197] On 19 April 2011, TEPCO began transferring excess, radioactive cooling water from the reactor's basement and maintenance tunnels to a waste processing facility.[198]
On 18 April, remote control robot was used to enter the Reactor Building and performed a series of inspections.[127]
On 18 May, staff entered the Reactor Building for the first time since 15 March.[142]
On 11 June, ventilation systems were installed in the Reactor Building, to clean the highly radioactive air encapsulated within the Reactor Building.[142]
On 28 June, TEPCO began injecting nitrogen into the containment vessel, which was expected to reduce the likelihood of further hydrogen explosions.[142]
Since 2 July, the Reactor has been cooled using fresh water treated by the on site water treatment plant.[142]
On 14 September at 11AM (JST) TEPCO changed began injecting water to the No. 2 reactor using the core spray system piping in addition to the feed water piping already being used as this method seemed to be effective in reducing the temperature in the no. 3 reactor. At that time the temperature at the bottom of the No.2 reactor was still 114.4 degrees Celsius, compared to the 84.9 degrees at the No.1 reactor and the 101.3 at the No. 3 reactor. The new method has led to some temperature decrease, but not as significant as the decrease that occurred in the no. 3 reactor.[199]
After some positive effect was noticed of this change, TEPCO decided on 16 September to increase the amount of water pumped into the no.2 reactor with 1000 kilo, in an attempt to lower the temperature inside, to a total of 7 tons per hour. The same was done for reactor no.3, here 5 tons were added to a total of 12 tons per hour. TEPCO added, that the cooling of the no.1 reactors could be increased also, when necessary.[200]
On 21 September 2011, Masanori Naitoh, director in charge of nuclear safety analysis at the Institute of Applied Energy, an expert commenting on the plan to contain the crisis at the Fukushima Daiichi nuclear plant, mentioned that the interior temperatures of the damaged reactors had to to be checked to confirm cold-shutdown. Naitoh said that TEPCO was only measuring temperatures outside the reactors. Naitoh said, that the temperatures inside should be confirmed through simulation that they had fallen below 100 degrees, and that there was no risks of a recurrence of nuclear reactions.[201]
On 15 May, TEPCO revealed that the pressure vessel that holds nuclear fuel "is likely to be damaged and leaking water at Units 2 and 3", which means most of the thousands of tons of water pumped into the reactors had leaked.[159]
On 29 March, Richard Lahey, former head of safety research for boiling-water reactors at General Electric, speculated that the reactor core may have melted through the reactor containment vessel onto a concrete floor, raising concerns of a major release of radioactive material, while failing to divulge the report by Dale G. Bridenbaugh which condemned the design as "unsafe".[202] On 27 April, TEPCO revised its estimate of damaged fuel in Unit 2 from 30% to 35%.[135] TEPCO reported on 23 May that Reactor 2 suffered a meltdown about 100 hours after the earthquake.[203]
On 1 November 2011 TEPCO said that Xenon-133 and Xenon-135 was detected in gas-samples taken from the containment vessel of reactor 2, in a concentration of 6 to 10 (or more) parts per million becquerels per cubic centimeter. Xenon-135 was also detected in gas samples collected on 2 November. These isotopes are the result of nuclear fission-reaction of uranium. Because the short half-lifes of these gases: (Xe-133: 5 days Xe-135: 9 hours), the presence could only mean that nuclear fissions were occurring at some places in the reactor. Boric-acid was poured into the reactor in an attempt to stop the fission-reactions. No significant change in temperature or pressure was found by TEPCO, so there was no sign of large-scale criticallity. The reactor-cooling was continued, but TEPCO would examine the situation at reactor 1 and 3 also.[204][205][206][207] Professor Koji Okamoto of the University of Tokyo Graduate School made the comment that localized and temporary fission might still happen, and that the melted fuel could undergo fission, but the fuel was probably scattered around. However, neutrons from radioactive materials could react with the uranium fuel and other substances. Self-sustaining chain reactions were unlikely, thanks to the huge amounts of boric acid that have been poured into the reactor. According to Okamoto, these neutrons should be closely monitored to make sure fission did not happen, because when the fission-reactions were not controlled, it would be impossible to reach a state of "cold-shutdown". Therefore it was needed to locate all molten fuel in and outside the reactor-vessel.[205] [208]
On 3 November 2011 TEPCO said that the tiny amounts of xenon-135 detected in the reactor's containment vessel atmosphere came from spontaneous nuclear fission with curium-242 and curium-244, substances that were present in the nuclear fuel. A critical fission would have caused much higher concentrations of xenon-isotopes. These reactions would occur constantly, and did not lead to criticality in the melted fuel of reactor 2. All assessments would be sent to NISA for reevaluation.[209][210] [211]
The detection of xenon on the afternoon of 1 November by TEPCO was reported to NISA in the night. The next day, 2 November just past 7 a.m., NISA informed the Prime Minister Yoshihiko Noda's secretary about the possibility of critical reactions in reactor 2. Two hours later at 9 a.m. prime minister Edano learned the news. At a press-conference, the Chief Cabinet Secretary Osamu Fujimura revealed that Minister of Economy, Trade and Industry Yukio Edano sent a strong reprimand to Hiroyuki Fukano, the chief of NISA, because NISA failed to report the incident immediately to both himself and the Prime Minister's Office, and that NISA waited almost a day after the find was done. Fujimura said, "I have been told that NISA decided not to report the incident until the following morning because the agency didn't believe it was a dangerous situation." [212]
Unlike the other five reactor units, reactor 3 ran on mixed core, containing both uranium fuel and mixed uranium and plutonium oxide, or MOX fuel (with the core comprising ~6% MOX fuel[214]), during a loss of cooling accident in a subcritical reactor MOX fuel will not behave differently to UOX fuel. The key difference between plutonium-239 and uranium-235 is that plutonium emits fewer delayed neutrons than uranium when it undergoes fission.
While water-insoluble forms of plutonium such as plutonium dioxide are very harmful to the lungs, this toxicity is not relevant during a Loss Of Coolant Accident (LOCA) because plutonium is very involatile and unlikely to leave the reactor in large amounts. Plutonium dioxide has a very high boiling point. The toxic effect of the plutonium to the public under these conditions is much less than that of iodine-131 and cesium. A key difference between the Fukushima accident and the Chernobyl accident was that the Chernobyl explosion shattered the fuel and flung it out of the reactor building, while at Fukushima there was no steam explosion driven by the release of fission energy. During a loss of cooling accident, the fuel is not subject to such intense mechanical stresses, so the release of radioactivity is controlled by the boiling-point of the different elements present.[215]
Following the reactor SCRAM, operators activated the reactor core isolation cooling system (RCIC) and the residual heat removal system and core spray systems were made available to cool the suppression pool, however whether they were activated prior to the tsunami has not been made clear. The RHRS and CS pumps were knocked out of commission by the tsunami. With DC battery power remaining, the RCIC continued to keep the water level stable, however the operators chose to switch to the high pressure coolant injection (HPCI) system when water level began to drop. On 13 March, the HPCI system failed, the reason for which is not completely clear due to instrumentation not being available however it is believed to be either due to loss of DC power due to depletion of batteries or to reactor pressure dropping below the level at which it can operate. Operators were unable to restart it as batteries were exhausted. After this the operators were unable to start the RCIC system and began injecting seawater. Although it was not clear at the time, some of the fuel in Reactor 3 apparently melted around sixty hours after the earthquake (the night of the 12th to 13th).[203]
Early on 13 March an official of the Japan Nuclear and Industrial Safety Agency (NISA) told at a news conference that the emergency cooling system of Unit 3 had failed, spurring an urgent search for a means to supply cooling water to the reactor vessel in order to prevent a meltdown of its reactor core.[216] At 05:38 there was no means of adding coolant to the reactor, owing to loss of power. Work to restore power and to vent excessive pressure continued.[217] At one point, the top three meters of the uranium/mixed oxide (MOX) fuel rods were not covered by coolant.[218]
At 07:30 JST, TEPCO prepared to release radioactive steam, indicating that "the amount of radiation to be released would be small and not of a level that would affect human health"[219] and manual venting took place at 08:41 and 09:20.[220] At 09:25 JST on 13 March, operators began injecting water containing boric acid into the primary containment vessel (PCV) via the pump of a fire truck.[221][222] When water levels continued to fall and pressure to rise, the injected water was switched to seawater at 13:12.[217] By 15:00 it was noted that despite adding water the level in the reactor did not rise and radiation had increased.[223] A rise was eventually recorded but the level stuck at 2 m below the top of reactor core. Other readings suggested that this could not be the case and the gauge was malfunctioning.[220]
Injection of seawater into the primary containment vessel (PCV) was discontinued at 01:10 on 14 March because all the water in the reserve pool had been used up. Supplies were restored by 03:20 and injection of water resumed.[222] On the morning of 15 March, Secretary Edano announced that according to TEPCO, at one location near reactor Units 3 and 4, radiation at an equivalent dose rate of 400 mSv/h was detected.[3][224][225] This might have been due to debris from the explosion in Unit 4.[226]
At 12:33 JST on 13 March, the chief spokesman of the Japanese government, Yukio Edano said hydrogen gas was building up inside the outer building of Unit 3 just as had occurred in Unit 1, threatening the same kind of explosion.[227] At 11:15 JST on 14 March, the envisaged explosion of the building surrounding Reactor 3 of Fukushima 1 occurred, owing to the ignition of built up hydrogen gas.[228][229] The Nuclear and Industrial Safety Agency of Japan (NISA) reported, as with Unit 1, the top section of the reactor building was blown apart, but the inner containment vessel was not breached. The explosion was larger than that in Unit 1 and felt 40 kilometers away. Pressure readings within the reactor remained steady at around 380 kPa at 11:13 and 360 kPa at 11:55 compared to nominal levels of 400 kPa and a maximum recorded of 840 kPa. Water injection continued. Dose rates of 0.05 mSv/h were recorded in the service hall and of 0.02 mSv/h at the plant entrance.[230]
Eleven people were reported injured in the blast.[231] TEPCO and NISA announced that four TEPCO employees, three subcontractor employees, and four Self-Defence-Force soldiers were injured.[232][233][234] Six military personnel from the Ground Self Defense Force's Central Nuclear Biological Chemical Weapon Defense Unit, led by Colonel Shinji Iwakuma, had just arrived outside the reactor to spray it with water and were exiting their vehicles when the explosion occurred. Iwakuma later said that TEPCO had not informed them that there was a danger of a hydrogen explosion in the reactor, adding, "Tokyo Electric was desperate to stabilize (the plant), so I am not angry at them. If there is a possibility of an explosion, I would be reluctant to send my men there."[235]
TEPCO claimed that there was a small but non-zero probability that the exposed fuel assemblies could reach criticality.[236][237] The BBC commented that criticality would never mean a nuclear explosion, but could cause a sustained release of radioactive materials.[236] Criticality is usually considered highly unlikely, owing to the low enrichment level used in light water reactors.[238][239][240]
American nuclear engineer Arnold Gundersen, noting the much greater power and vertical debris ejection compared to the Unit 1 hydrogen blast, has theorized that the Unit 3 explosion involved a prompt criticality in the spent fuel pool material, triggered by the mechanical disruption of an initial, smaller hydrogen gas explosion in the building.[241] Low-dose radiation researcher and anti-nuclear activist Christopher Busby speculated on Russia Today that the explosion that destroyed the Reactor 3 building was a "nuclear explosion" of some kind in the spent fuel pool.[242]¨
On 11 May, TEPCO released underwater robotic video from the spent fuel pool. The video appears to show large amounts of debris contaminating the pool. However, based on water samples analysed, unnamed experts and TEPCO reported that the fuel rods were left "largely undamaged",[243][244] and that it appears that the Unit 3 explosion was entirely related to hydrogen buildup within the building from venting of the reactor.
Around 10:00 JST on 16 March, NHK helicopters flying 30 km away videotaped white fumes rising from the Fukushima I facility. Officials suggested that the Reactor 3 building was the most likely source, and said that its containment systems may have been breached.[245] The control room for Reactors 3 and 4 was evacuated at 10:45 JST but staff were cleared to return and resume water injection into the reactor at 11:30 JST.[246] At 16:12 JST, Self Defence Force (SDF) Chinook helicopters were preparing to pour water on Unit 3, where white fumes rising from the building was believed to be water boiling away from the fuel rod cooling pond on the top floor of the reactor building, and on Unit 4 where the cooling pool was also short of water. The mission was cancelled when helicopter measurements reported radiation levels of 50 mSv.[247][248] At 21:06 pm JST, the government reported that major damage to Reactor 3 was unlikely but that it nonetheless remained their highest priority.[249]
Early on 17 March, TEPCO requested another attempt by the military to put water on the reactor using a helicopter[250] and four helicopter drops of seawater took place around 10:00 JST.[251] The riot police used a water cannon to spray water onto the top of the reactor building and then were replaced by members of the SDF with spray vehicles. On 18 March, a crew of firemen took over the task with six fire engines each spraying 6 tons of water in 40 minutes. 30 further hyper rescue vehicles were involved in spraying operations.[252] Spraying continued each day to 23 March because of concerns the explosion in Unit 3 may have damaged the pool (total 3,742 tonnes of water sprayed up to 22 March) with changing crews to minimise radiation exposure.[3] Lighting in the control room was restored on 22 March after a connection was made to a new grid power supply and by 24 March it was possible to add 35 tonnes of seawater to the spent fuel pool using the cooling and purification system.[123] On 21 March grey smoke was reported to be rising from the southeast corner of Unit 3 - where the spent fuel pool is located. Workers were evacuated from the area. TEPCO claimed no significant change in radiation levels and the smoke subsided later the same day.[253]
On 23 March, black smoke billowed from Unit 3, prompting another evacuation of workers from the plant, though Tokyo Electric Power Co. officials said there had been no corresponding spike in radiation at the plant. "We don't know the reason for the smoke", Hidehiko Nishiyama of the Nuclear Safety Agency said.[254]
On 24 March three workers entered the basement of the turbine building and were exposed to radiation when they stepped into contaminated water. Two of them were not wearing high boots and received beta ray burns. They were hospitalized but their injuries were not life-threatening.[255]
From 25 March, the source of water being injected into the core was switched from seawater to freshwater.[142]
In August, TEPCO began considering changing the core injection method for the no. 3 reactor as it was requiring a much larger quantity of water to cool and the temperatures remained relatively high compared to the nos. 1 and 2 reactors which required far less water. TEPCO has hypothesized that this may be because some fuel is still present above the core support plate inside the pressure vessel of the no. 3 reactor in addition to the fuel that has fallen to the bottom of the pressure vessel. The fuel on the bottom would be easily cooled by the existing method, however as the pressure vessel is leaking, any fuel located on the support plate was likely only being cooled due to the steam generated by the cooling of the melted fuel at the bottom. TEPCO began considering utilizing the reactor's core spray system pipes as an additional path of water injection and then reduce the amount of water through the existing feedwater piping system. A team of workers were sent inside the reactor building to inspect the core spray system pipes and it was found that the piping was undamaged. Hoses were then run from the temporary injection pumps located outside the building and connected to the core spray system piping. On 1 September, TEPCO began injecting water using the new route. The new injection method has been considerably effective in lowering the temperature of the reactor to below 100 degrees celsius. As of 27 September, most of the no. 3 reactor's temperature readings are between 70-80 degrees celsius. Later, TEPCO began utilizing the same method in the no. 2 reactor, however it has not has as significant effect on the no. 2 reactor as it did on the no. 3.[256]
On 25 March, officials announced the reactor vessel might be breached and leaking radioactive material. High radiation levels from contaminated water prevented work.[257] Japan Nuclear and Industrial Safety Agency (NISA) reiterated concerns about a Unit 3 breach on 30 March.[187] NHK World reported the NISA's concerns as "air may be leaking", very probably through "weakened valves, pipes and openings under the reactors where the control rods are inserted", but that "there is no indication of large cracks or holes in the reactor vessels".[187] As with the other reactors, water was transferred from condenser reservoirs to the suppression pool surge tanks so that condensers could be used to hold radioactive water pumped from the basement.[131]
On 17 April, remote control robots were used to enter the Reactor Building and performed a series of inspections.[142]
On 27 April, TEPCO revised its estimate of damaged fuel in Unit 3 from 25% to 30%.[135] Radiation measurements of the water in the Unit 3 spent fuel pool were reported at 140 kBq of radioactive cesium-134 per cubic centimeter, 150 kBq of cesium-137 per cubic centimeter, and 11 kBq per cubic centimeter of iodine-131 on 10 May.[244]
On 15 May, TEPCO revealed that the pressure vessel that holds nuclear fuel "is likely to be damaged and leaking water at Units 2 and 3", which means most of the thousands of tons of water pumped into the reactors had leaked.[159]
On 23 May, TEPCO reported that Reactor 3 had suffered a meltdown some sixty hours after the earthquake.[203]
On 9 June, staff entered the Reactor Building to conduct radiation surveying.[142]
On 25 June and the following day boric acid dissolved in 90 tons of water was pumped into the spent fuel pool of Reactor 3. Concrete debris from the March hydrogen explosion of the reactor building have been detected in the spent fuel pool. In June TEPCO discovered that the water in the pool was strongly alkaline: the pH had reached a value of 11.2. Leaching of calcium hydroxide (portlandite) or calcium silicate hydrate (CSH) from the concrete could have caused this. The alkaline water could accelerate the corrosion of the aluminium racks holding the spent fuel rods. If the fuel assemblies would fall, this could lead to re-criticality. In the mean time preparative works began to install a recirculation cooling system at the fuel pool, that should be operational in the first weeks of July.[258]
On 14 July, TEPCO began injecting nitrogen into the containment vessel, which was expected to reduce the likelihood of further hydrogen explosions.[142]
On 1 July the spent fuel pool was switched from the water-injection cooling system, to a circulatory cooling system.[142]
Since 2 July, the Reactor has been cooled using fresh water treated by the on site water treatment plant.[142]
When the Fukushima Daiichi nuclear disaster began on 11 March 2011, reactor unit 4 was shut down and all fuel rods had been transferred to the spent fuel pool on an upper floor of the reactor building. On 15 March, an explosion damaged the fourth floor rooftop area of the unit 4 reactor. Japan's nuclear safety agency NISA reported two large holes in a wall of the outer building of unit 4 after the explosion. It was reported that water in the spent fuel pool might be boiling. Radiation inside the unit 4 control room prevented workers from staying there permanently. Visual inspection of the spent fuel pool of reactor 4 on 30 April showed that there was no significant visible damage to the fuel rods in the pool. A radiochemical examination of the water from the pond confirms that little of the fuel in the pond has been damaged.[259]
Reactors 5 and 6 were also not operating when the earthquake struck although, unlike reactor 4, they were still fueled. The reactors have been closely monitored, as cooling processes were not functioning well.
Used fuel assemblies taken from reactors are initially stored for at least 18 months in the pools adjacent to their reactors. They can then be transferred to the central fuel storage pond.[3] This contains 6375 fuel assemblies and was reported "secured" with a temperature of 55 °C. After further cooling, fuel can be transferred to dry cask storage, which has shown no signs of abnormalities.[260] On 21 March, temperatures in the fuel pond had risen a little to 61 °C and water was sprayed over the pool.[3] Power was restored to cooling systems on 24 March and by 28 March temperatures were reported down to 35 °C.[124]
Japan's response to the crisis at Fukushima Daiichi was flawed by "poor communication and delays in releasing data on dangerous radiation leaks at the facility", a government-appointed investigative panel has found. The panel was led by University of Tokyo Professor Yotaro Hatamura and the panel's Investigation Committee on the Accident at the Fukushima Nuclear Power Stations of Tokyo Electric Power Company report attaches blame to Japan's central government as well as Tokyo Electric Power Co., "depicting a scene of harried officials incapable of making decisions to stem radiation leaks as the situation at the coastal plant worsened in the days and weeks following the disaster".[261] The 507-page interim report, the product of hundreds of interviews with utility workers and government officials, said poor planning also worsened the disaster response, noting that authorities had "grossly underestimated tsunami risks" that followed the magnitude 9.0 earthquake. The 40-foot-high tsunami that struck the plant was twice as tall as the highest wave predicted by officials, and the erroneous assumption that the plant's cooling system continued to work after the tsunami struck worsened the disaster. "Plant workers had no clear instructions on how to respond to such a disaster, causing miscommunication, especially when the disaster destroyed backup generators. Ultimately, the series of failures led to the worst nuclear catastrophe since Chernobyl".[261]
Regulatory capture may have contributed to the cascade of failures which were revealed after the tsunami receded. Regulatory capture may have also contributed to the current situation. Critics argue that the government shares blame with regulatory agency for not heeding warnings, for not ensuring the independence of the nuclear industry's oversight while encouraging the expansion of nuclear energy domestically and internationally.[262] World media have argued that the Japanese nuclear regulatory system tends to side with and promote the nuclear industry because of amakudari (roughly translated as descent from heaven), in which senior regulators accept high paying jobs at the companies they once oversaw. To protect their potential future position in the industry, regulators seek to avoid taking positions that upset or embarrass the utilities they regulate. TEPCO's position as the largest electrical utility in Japan led it to be the most desirable position for retiring regulators, typically the "most senior officials went to work at Tepco, while those of lower ranks ended up at smaller utilities" according to the New York Times.[263]
In August 2011, several top energy officials were fired by the Japanese government; affected positions included the Vice-minister for Economy, Trade and Industry; the head of the Nuclear and Industrial Safety Agency, and the head of the Agency for Natural Resources and Energy.[264]
The severity of the nuclear accident is provisionally[265] rated 7 on the International Nuclear Event Scale (INES). This scale runs from 0, indicating an abnormal situation with no safety consequences, to 7, indicating an accident causing widespread contamination with serious health and environmental effects. Prior to Fukushima, the Chernobyl disaster was the only level 7 accident on record, while the Three Mile Island accident was a level 5 accident. Arnold Gundersen, a former nuclear power industry executive who served as an expert witness in the investigation of the Three Mile Island accident, said that "Fukushima is the biggest industrial catastrophe in the history of mankind."[266][267]
The Japan Atomic Energy Agency initially rated the situation at Unit 1 below both of these previous accidents; on 13 March it announced it was classifying the event at level 4, an "accident with local consequences".[268] On 18 March it raised its rating on Units 1, 2 and 3 to Level 5, an "accident with wider consequences". It classified the situation at Unit 4 as a level 3 "serious incident".[269]
Several parties disputed the Japanese classifications, arguing that the situation was more severe than they were admitting at the time. On 14 March, three Russian experts stated that the nuclear accident should be classified at Level 5, perhaps even Level 6.[270] One day later, the French nuclear safety authority ASN said that the Fukushima plant could be classified as a Level 6.[271] as of 18 March[update], the French nuclear authority—and as of 15 March, the Finnish nuclear safety authority—estimated the accidents at Fukushima to be at Level 6 on the INES.[272][273] On 24 March, a scientific consultant for noted anti-nuclear environmental group Greenpeace, working with data from the Austrian ZAMG[274] and French IRSN, prepared an analysis in which he rated the total Fukushima accident at INES level 7.[275]
The Asahi Shimbun newspaper reported on 26 March that the accident might warrant level 6, based on its calculations.[276] The Wall Street Journal stated that Japan's NISA would make any decision on raising the level.[277] INES level 6, or "serious accident", had only been applied to the Kyshtym disaster (Soviet Union, 1957), while the only level 7 was Chernobyl (Soviet Union, 1986). Previous level 5 accidents included the Windscale fire (United Kingdom, 1957); the Lucens reactor (Switzerland, 1969); Three Mile Island (United States, 1979); and the Goiânia accident (Brazil, 1987).
Assessing "seriousness" as partial or full meltdown at a civilian plant, The New York Times reported on 3 April that based on remote sensing, computer "simulations suggest that the number of serious accidents has suddenly doubled, with three of the reactors at the Fukushima Daiichi complex in some stage of meltdown." The Times counted three previous civilian meltdowns, from World Nuclear Association information: Three Mile Island; Saint-Laurent Nuclear Power Plant (France, 1980, INES level 4); and Chernobyl.[278]
On 11 April, the Japanese Nuclear and Industrial Safety Agency (NISA) temporarily raised the disaster at Fukushima Daiichi to Level 7 on the INES scale, by considering the whole event and not considering each reactor as an individual event per se (rated between 3 and 5). This would make Fukushima the second Level 7 "major accident" in the history of the nuclear industry; having said that, radiation released as a result of the events at Fukushima was, as of 12 April, only approximately 10% of that released as a result of the accident at Chernobyl (1986), also rated as INES Level 7.[265][279] However, the largest study, as of 21 October 2011, on Fukushima fallout concludes that Fukushima was "the largest radioactive noble gas release in history not related to nuclear bomb testing. The release is a factor of 2.5 higher than the Chernobyl 133Xe source term." [280][281] Arnold Gundersen said Fukushima has 20 times the potential to be released than Chernobyl. Hot spots are being found 60 to 70 kilometres away from the reactor (further away than they were found from Chernobyl), and the amount of radiation in many of them is the amount that caused areas to be declared no-man's-land for Chernobyl.[282]
In off-the-record-interviews with Japanese newspapers like the Tokyo Shimbun, Naoto Kan, former premier minister of Japan, revealed that there were moments he believed the disaster could have surpassed Chernobyl, many times. At first Tepco denied that fuel-cores were melted, after all cooling functions were lost. Trade minister, Banri Kaieda, mentioned that Tepco seriously considered pulling away all staff-members from the plant and leaving it abandoned. Kan could not accept this: "Withdrawing from the plant was out of the question, If that had happened, Tokyo would be deserted by now. It was a critical moment for Japan's survival. It could have been a led to leaks of dozens of times more radiation than Chernobyl." That might have "compromised the very existence of the Japanese nation".
Tepco's president at that time, Masataka Shimizu, was never clear in his answers, and TEPCO failed to obey the orders to vent one of the overheating reactors, In an interview to the Asahi Shimbun newspaper. Kan revealed, that he went to the plant itself and inspected the plant from above in a helicopter because: "I felt I had to go there in person and speak to the people in charge or I would never have known what was going on." The American Government was seriously concerned about the Japanese response to the accident: Kan said: "We were not told straight out, but it was obvious that they questioned whether we were really taking this seriously."
Kan did defend his changed attitude to a non-nuclear energy policy with the following remarks: "If there is a risk of accidents that could make half the land mass of our country uninhabitable, then we cannot afford to take that risk."[283]
Major news source reporting at least 2 TEPCO employees confirmed dead from "disaster conditions" following the earthquake.[284] "The two workers, aged 21 and 24, sustained multiple external injuries and were believed to have died from blood loss, TEPCO said. Their bodies were decontaminated as radiation has been spewing from the plant for three weeks."[285]
45 patients were reported dead after the evacuation of a hospital in Futaba. Some of them "were suffering from dehydration because they had not eaten anything for three days".[286]
The Associated Press has reported that fourteen senior citizens died after being moved from their hospital which was in the Fukushima plant evacuation zone.[287]
On April 14, 2011 it was reported that the oldest resident of Iitate, a 102-year-old, committed suicide rather than to leave following the announcement of his village's evacuation[288].
According to the Japanese Government, over 160,000 people in the general population were screened in March 2011 for radiation exposure and no case was found which affects health.[289] Thirty workers conducting operations at the plant had exposure levels greater than 100 mSv.[290]
In April 2011, the United States Department of Energy published projections of the radiation risks over the next year for people living in the neighborhood of the plant. Potential exposure could exceed 20 mSv/year (2 rems/year) in some areas up to 50 kilometers from the plant. That is the level at which relocation would be considered in the USA, and it is a level that could cause roughly one extra cancer case in 500 young adults. However, natural radiation levels are higher in some part of the world than the projected level mentioned above, and about 4 people out of 10 can be expected to develop cancer without exposure to radiation.[291][292] Further, the radiation exposure resulting from the accident for most people living in Fukushima is so small compared to background radiation that it may be impossible to find statistically significant evidence of increases in cancer.[293]
As of August 31, 2011, a man in his 40s who had worked for a week on the Fukushima Daiichi site has died of acute leukemia not long after passing a physical test, but it was not caused by exposure to fallout, according to Tepco offcials.[294][295]
As of September 2011, six workers at the Fukushima Daiichi site have exceeded lifetime legal limits for radiation and more than 300 have received significant radiation doses.[296]
As of September 2011, there were no deaths or serious injuries due to direct radiation exposures. Cancer deaths due to accumulated radiation exposures cannot be ruled out, and according to one expert, might be in the order of 100 cases.[24]
Frank N. von Hippel, a U.S. scientist, has estimated that “on the order of 1,000” people will die from cancer as a result of their exposure to radiation from the Fukushima Daiichi disaster, that is, an increase of 0.1 percent in the incidence of cancer, and much less than the approximately 20,000 people killed directly by the earthquake and tsunami. Because contaminated milk was “interdicted in Japan” the number of (mostly non-fatal) thyroid cancer cases will probably be less than 1 percent of similar cases at Chernobyl. Von Hippel added that “fear of ionizing radiation could have long-term psychological effects on a large portion of the population in the contaminated areas”.[20]
On 7 June 2011 a government-appointed committee of 10 people convened to investigate the accident. The panel was headed by Yotaro Hatamura, professor emeritus of the University of Tokyo, and included Yukio Takasu, Michio Furukawa, the mayor of Kawamata, Fukushima, and author Kunio Yanagida, considered an expert on crisis management.[297][298]
As part of the government inquiry, the House of Representatives of Japan's special science committee directed TEPCO to submit to them its manuals and procedures for dealing with reactor accidents. TEPCO responded by submitting manuals with most of the text blotted out. In response, the Nuclear and Industrial Safety Agency ordered TEPCO to resubmit the manuals by 28 September 2011 without hiding any of the content. TEPCO replied that it would comply with the order.[299]
On 24 October NISA published a large portion of Tokyo Electric Power Company's procedural manuals for nuclear accidents. These were the manuals that the operator of the Fukushima Daiichi nuclear power plant earlier did send to the Lower House with most of the contents blacked out, saying that this information should be kept secret to protect its intellectual property rights, and that disclosure would offer information to possible terrorists. NISA ordered TEPCO to send the manuals without any redaction, as the law orders. 200 pages were released from the accident procedural manuals used for Fukushima Daiichi nuclear power plant. All their contents were published, only the names of individuals were left out.
From these documents could be concluded:
The agency said, the decision to publish the manuals was taken, for transparency in the search what caused the nuclear accident in Fukushima and also to establish better safety measures for the future.[300]
On 24 October 2011 the first meeting was held by a group of 6 nuclear energy specialists invited by NISA to dicuss the lessons to be learned from the accidents in Fukushima. Their first remarks were:
According to professor Tadashi Narabayashi of the Hokkaido University Graduate School, plant operators should arrange emergency power supplies with other utilities. These discussion should be completed in March 2012, in order to be able to implement their conclusions into the new safety-regulations by the new nuclear safety agency to be launched in April 2012.[301]
The Investigation Committee on the Accident at the Fukushima Nuclear Power Stations of Tokyo Electric Power Company was formed June 7, 2011 by the Japanese government as an independent body to investigate the Fukushima Daiichi disaster.[302] The Investigation Committee issued an interim report in December, 2011, and is expected to issue its final report summer, 2012. The interim report was "a scathing assessment of the response to the Fukushima disaster", in which the investigative panel "blamed the central government and the Tokyo Electric Power Co., saying both seemed incapable of making decisions to stem radiation leaks as the situation at the coastal plant worsened in the days and weeks after the disaster".[303]
According to Munich Re, a major reinsurer, the private insurance industry will not be significantly affected by the accidents at the Fukushima nuclear power plant.[304] Swiss Re similarly states "Coverage for nuclear facilities in Japan excludes earthquake shock, fire following earthquake and tsunami, for both physical damage and liability. Swiss Re believes that the incident at the Fukushima nuclear power plant is unlikely to result in a significant direct loss for the property & casualty insurance industry."[305]
Radioactive material has been released from the Fukushima containment vessels as the result of deliberate venting to reduce gaseous pressure, deliberate discharge of coolant water into the sea, and accidental or uncontrolled events. Concerns about the possibility of a large scale radiation leak resulted in 20 km exclusion zone being set up around the power plant and people within the 20–30 km zone being advised to stay indoors. Later, the UK, France and some other countries told their nationals to consider leaving Tokyo, in response to fears of spreading radioactive contamination.[306] The Fukushima accident has led to trace amounts of radiation, including iodine-131, caesium-134 and caesium-137, being observed around the world (New York State, Alaska, Hawaii, Oregon, California, Montreal, and Austria).[307][308][309] Large amounts of radioactive isotopes have also been released into the Pacific Ocean.
According to one expert, the release of radioactivity is about one-tenth that from the Chernobyl disaster and the contaminated area is also about one-tenth that that of Chernobyl.[20] But according to a study conducted by Norwegian Institute for Air Research, radioactive material released is reaching 40 percent of the total from Chernobyl.[25]
In March 2011, Japanese officials announced that "radioactive iodine-131 exceeding safety limits for infants had been detected at 18 water-purification plants in Tokyo and five other prefectures".[310] As of July 2011, the Japanese government has been unable to control the spread of radioactive material into the nation’s food. Radioactive material has been detected in a range of produce, including spinach, tea leaves, milk, fish and beef, up to 200 miles from the nuclear plant. Inside the 12-mile evacuation zone around the plant, all farming has been abandoned.[311][312]
As of August 2011, the crippled Fukushima nuclear plant is still leaking low levels of radiation and areas surrounding it could remain uninhabitable for decades due to high radiation. It could take “more than 20 years before residents could safely return to areas with current radiation readings of 200 millisieverts per year, and a decade for areas at 100 millisieverts per year”.[313]
Based on worldwide measurements of iodine-131 and caesium-137, early estimates in April 2011 suggested that the releases of those isotopes from Fukushima could be of the same order of magnitude as those from Chernobyl in 1986,[314][315] while other studies gave an estimate of more than 10% of the Chernobyl emissions.[316][317]
On 24 August 2011, the Nuclear Safety Commission (NSC) of Japan published the results of the recalculation of the total amount of radioactive materials released into the air during the accident at the Fukushima Daiichi Nuclear Power Station. The total amounts released between 11 March and 5 April were revised downwards to 1.3 × 1017 Bq for iodine-131 and 1.1 × 1016 Bq for caesium-137, which is about 11% of Chernobyl emissions. Earlier estimations were 1.5 × 1017 Bq and 1.2 × 1016 Bq.[318][319]
On 8 September 2011 a group of Japanese scientists working for the Japan Atomic Energy Agency, the Kyoto University and other institutes, published the results of a recalculation of the total amount of radioactive material released into the ocean: between late March through April they found a total of 15,000 TBq for the combined amount of iodine-131 and caesium-137. This was more than triple the figure of 4,720 TBq estimated by the plant-owner. TEPCO made only a calculation about the releases from the plant in April and May into the sea. The new calculations were needed because a large portion of the airborne radioactive substances would enter the seawater when it came down as rain.[320]
In the first half of September 2011 the amount of radioactive substances released from the plant was about 200 million becquerels per hour, according to TEPCO, this was approximately one four-millionth of the level of the initial stages of the accident in March.[321]. Traces of iodine-131 are still detected in several Japanese prefectures in the months of November[322] and December 2011[323].
According to a report published in October 2011 by the French Institute for Radiological Protection and Nuclear Safety, between 21 March and mid-July around 2.7 × 1016 Bq of cesium-137 entered the ocean, about 82 percent having flowed into the sea before 8 April.[324] This emission of radioactivity into the sea represens the most important individual emission of artificial radioactivity into the sea ever observed. However, the Fukushima coast has one of the world's strongest currents and these transported the contaminated waters far into the Pacific Ocean, thus causing a high dispersion of the radioactive elements. The results of measurements of both the seawater and the coatal sediments lead to suppose that the consequences of the accident, for what concerns radioactivity, will be minor for marine life as of autumn 2011 (weak concentration of radioactivity in the water and limited accumulation in sediments). On the other hand, significant pollution of sea water along the coast near the nuclear plan might persist, because of the continuing arrival of radioactive material transported towards the sea by surface water running over contaminated soil. Further, some coastal areas might have less favorable dilution or sedimentation characteristics than those observed so far. Finally, the possible presence of other persistent radioactive substances, such as strontium 90 or plutonium, has not been sufficiently studied. Recent measurments show persistent contamination of some marine species (mostly fish) caught along the coast of Fukushima district. Organisms that filter water and fish at the top of the food chain are, over time, the most sensitive to cesium pollution. It is thus justified to maintain surveillance of marine life that is fished in the coastal waters off Fukushima.
A nuclear emergency was declared by the government of Japan on 11 March 2011. Later Prime Minister Naoto Kan issued instructions that people within a 20 km (12 mi) zone around the Fukushima Daiichi nuclear plant must leave, and urged that those living between 20 km and 30 km from the site to stay indoors.[325][326] The latter groups were also urged to evacuate on 25 March.[327]
Japanese authorities have admitted that lax standards and poor oversight contributed to the nuclear disaster.[328] They have come under fire for their handling of the emergency, and have engaged in a pattern of withholding damaging information and denying facts of the accident.[328][329][330] Authorities apparently wanted to "limit the size of costly and disruptive evacuations in land-scarce Japan and to avoid public questioning of the politically powerful nuclear industry". There has been public anger about an "official campaign to play down the scope of the accident and the potential health risks".[329][330] The accident is the second biggest nuclear accident after the Chernobyl disaster, but more complex as all reactors are involved.[331]
Once a proponent of building more reactors, Prime Minister Naoto Kan took an increasingly anti-nuclear stance in the months following the Fukushima disaster. In May, he ordered the aging Hamaoka Nuclear Power Plant be closed over earthquake and tsunami fears, and said he would freeze plans to build new reactors. In July 2011, Mr. Kan said that "Japan should reduce and eventually eliminate its dependence on nuclear energy ... saying that the Fukushima accident had demonstrated the dangers of the technology".[332]
On 22 August 2011 a spokesman of the Japanese Government mentioned the possibility, that some areas of the evacuation zone around the nuclear plant for "could stay for some decades a forbidden zone". According to the Japanese newspaper Yomiuri Shimbun the Japanese government was planning to buy some properties from civilians to store radioactive waste and materials that had become radioactive after the accidents.[333][334]
Due to frustration with Tokyo Electric Power Company (TEPCO) and the Japanese government "providing differing, confusing, and at times contradictory, information on critical health issues"[335] a citizen's group called "Safecast" has been recording detailed radiation level data in Japan.[336] The Japanese government "does not consider non-government readings to be authentic". The group uses off-the-shelf Geiger counter equipment. Members of the Air Monitoring station facility at the Department of Nuclear Engineering at the University of Berkeley, California have been doing extensive tests of environmental samples in Northern California.[337]
The international reaction to the 2011 Fukushima Daiichi nuclear disaster has been diverse and widespread. Many inter-governmental agencies are responding to the Fukushima Daiichi nuclear disaster, often on an ad hoc basis. Responders include International Atomic Energy Agency, World Meteorological Organization and the Preparatory Commission for the Comprehensive Nuclear Test Ban Treaty Organization, which has radiation detection equipment deployed around the world.[338]
Many countries have advised their nationals to leave Tokyo, citing the risk associated with the nuclear plants' ongoing accident. International experts have said that a workforce in the hundreds or even thousands would take years or decades to clean up the area.[339] Stock prices of many energy companies reliant on nuclear sources have dropped.
There has been a significant re-evaluation of existing nuclear power programs in many countries. What had been growing acceptance of nuclear power in the United States was eroded sharply following the 2011 Japanese nuclear accidents.[340] World-wide, a study by UBS, reported on 12 April 2011, suggests that around 30 nuclear plants may be closed as a result of Fukushima, with those located in seismic zones or close to national boundaries being the most likely to shut. Events at Fukushima "cast doubt on the idea that even an advanced economy can master nuclear safety".[341] Increased anti-nuclear sentiment has been evident in India, Italy, Germany, Spain, Switzerland, Taiwan, and the United States.
| No immediate concern | Concern | Severe Condition |
| Status of Fukushima I at 22 August 06:00 JST | Unit 1 | Unit 2 | Unit 3 | Unit 4 | Unit 5 | Unit 6 |
|---|---|---|---|---|---|---|
| Connected to grid | 1970/11/17 | 1973/12/24 | 1974/10/26 | 1978/02/24 | 1977/09/22 | 1979/05/04 |
| Rated Electrical Power output (MWe) | 460 | 784 | 784 | 784 | 784 | 1,100 |
| Rated Thermal Power output (MWt)[342] | 1,380 | 2,381 | 2,381 | 2,381 | 2,381 | 3,293 |
| Type of reactor | BWR-3 | BWR-4 | BWR-4 | BWR-4 | BWR-4 | BWR-5 |
| Containment type | Mark I | Mark I | Mark I | Mark I | Mark I | Mark II |
| Core fuel assemblies[343] | 400 | 548 | 548 | 0[344] | 548 | 764 |
| Spent fuel assemblies[344] | 292 | 587 | 514 | 1,331 | 946 | 876 |
| Spent fuel residual decay heat[345][346] | 60 kW | 400 kW | 200 kW | 2,000 kW | 700 kW | 600 kW |
| Fuel type | Low-enriched uranium | Low-enriched uranium | Mixed-oxide (MOX) and low-enriched uranium | Low-enriched uranium | Low-enriched uranium | Low-enriched uranium |
| Status at earthquake | In service | In service | In service | Outage (scheduled) | Outage (scheduled) | Outage (scheduled) |
| Fuel integrity | Fully melted[347] | Major damage,[348] possibly fully melted[349] | Major damage,[348] possibly fully melted[349] | Some spent fuel damaged[350] | Not damaged | Not damaged |
| Reactor pressure vessel integrity | Damage and leakage suspected[351] | Damage and leakage suspected[348] | Damage and leakage suspected[348] | Not damaged (defueled) | Not damaged | Not damaged |
| Containment integrity | Damage and leakage suspected[351] | Damaged at torus | Damage and leakage suspected[352] | Not damaged | Not damaged | Not damaged |
| Core cooling system 1 (ECCS/RHR) | Not functional | Not functional | Not functional | Not necessary (defueled) | Functional | Functional |
| Core cooling system 2 (RCIC/MUWC) | Not functional | Not functional | Not functional | Not necessary (defueled) | Functional (in cold shutdown) | Functional (in cold shutdown) |
| Building integrity | Severely damaged owing to hydrogen explosion | Slightly damaged, also panel removed to prevent hydrogen explosion | Severely damaged owing to hydrogen explosion | Severely damaged owing to hydrogen explosion | Panel removed to prevent hydrogen explosion | Panel removed to prevent hydrogen explosion |
| Pressure vessel, water level | Lower than the bottom of fuel | Fuel exposed partially or fully | Fuel exposed partially or fully | Safe (defueled) | Safe (in cold shutdown) | Safe (in cold shutdown) |
| Pressure vessel, pressure (Two instrument trains) | Stable at 0.016 MPa (gauge) (Train A) / (Train B not in service – suspected faulty) at 22 August 06:00 JST[353] | Stable at 0.013 MPa (gauge) (Train A) / (Train B not in service) at 22 August 06:00 JST[353] | Stable at −0.176 MPa (gauge) (Train A) / −0.104 MPa (gauge) (Train B) at 22 August 06:00 JST[353] | Safe (defueled) | Safe (in cold shutdown) | Safe (in cold shutdown) |
| Pressure vessel, temperature (not direct measurements) |
Decreased to 93.2 °C on 22 August[353] | Decreased to 107.0 °C by 22 August[353] | Decreased to 109.4 °C on 22 August[353] | Safe (defueled) | Safe (in cold shutdown) | Safe (in cold shutdown) |
| Containment (Drywell) pressure | Stable at 0.1268 MPa (absolute) at 22 August 06:00 JST[353] | Stable at 0.115 MPa (absolute) at 22 August 06:00 JST[353] | Stable at 0.1015 MPa (absolute) on 22 August 06:00 JST[353] | Safe | Safe | Safe |
| Water injection into core | Seawater injection started 12 March, with freshwater injection started 25 March and continuing[354] | Seawater injection started 14 March, with freshwater injection started 26 March and continuing[354] | Seawater injection started 13 March, with freshwater injection started 25 March and continuing[354] | Not necessary (defueled) | Not necessary | Not necessary |
| Seawater injection into containment vessel | Not necessary | Not necessary | Not necessary | Not necessary | Not necessary | Not necessary |
| Containment venting | Temporarily stopped | Temporarily stopped | Temporarily stopped | Not necessary | Not necessary | Not necessary |
| INES (individual) | Level 5 | Level 5 | Level 5 | Level 3 | – | – |
| INES (combined) | Level 7[197][355] | |||||
| Evacuation radius | 20 km from Nuclear Power Station (NPS), but 30 km should consider leaving as of 25 March 11:30 JST[327] | |||||
| General status from all sources regarding reactor cores | Stabilized by injecting water and boron[356] | Stabilized by injecting water and boron[356] | Stabilized by injecting water and boron[356] | Defueled | Cold shutdown on 20 March 14:30 JST[356][357] | Cold shutdown on 20 March 19:27 JST[356][357] |
| General status from all sources regarding Spent Fuel Pools (SFP) | Cooling system restored 10 August, 32.0 °C on 20 August 11:00 JST[358] | Cooling system restored 31 May, 36.0 °C on 20 August 11:00 JST[358] | Cooling system restored 1 July, 32.8 °C on 20 August 11:00 JST[358] | Cooling system restored 31 July, 42.0 °C on 20 August 11:00 JST[358] | Cooling system restored, 26.4 °C on 22 August 06:00 JST[353] | Cooling system restored, 28.2 °C on 22 August 06:00 JST[353] |
International experts have said that a workforce in the hundreds or even thousands would take years or decades to clean up Japan's Fukushima Daiichi nuclear disaster. On 10 April 2011, TEPCO began using remote-controlled, unmanned heavy equipment to remove debris from around nuclear reactor Units 1 to 4.
When the monsoon season began in June, a light fabric cover was used to protect the damaged reactor buildings from storms and heavy rainfall. On 1 August 2011, TEPCO said that very high radiation levels were found outside the building of reactors 1 and 2 from an exhaust pipe. On 16 August, TEPCO announced the installation of devices in the spent fuel pools of reactors 2, 3 and 4, which used special membranes and electricity to desalinate the water. These pools were cooled with seawater for some time, and TEPCO feared the salt would corrode stainless steel pipes and the pool walls. Burying the reactors in sand and concrete is considered to be a last resort.
TEPCO has announced the steps that will be needed for long-term core fuel removal although admits that new technologies may have to be invented to accomplish the many steps of the plan. The first step calls for decontaminating the outdoor areas and insides of the reactor buildings and lowering radiation levels as much as possible to allow better worker access to the areas around the containment vessels. This likely will require the use of robots and remote controlled machinery. Once the reactor buildings are safe enough to work in, the second step will begin. In the second step the primary containment vessels will be inspected and the water leakage paths will be identified and sealed. Once the containment vessels are air and water tight again, the next step calls for the containment vessels and pressure vessels to be be filled with water and a more permenent cooling system utilizing heat exchangers and water circulation will be installed. The final step calls for the containment vessel and pressure vessels to be opened from the top (as is done during normal refueling) and for special robotic arms to descend and remove the melted fuel from the bottom of the pressure vessels as well as any fuel that leaked to the floor of the outer containment vessels. Once the fuel has been removed the reactors can be demolished and the plant grounds can be fully decontaminated. TEPCO admits that the plan goes far into "uncharted territory" and that new technologies and methods will need to be invented to accomplish these steps and that this may involve multiple decades of work. In the more immediate future TEPCO hopes to begin removal of all fuels located in the spent fuel pools of Units 1 to 4 and transfer the damaged rods to the common spent fuel pool.
MP Yasuhiro Sonoda became internationally known for drinking a glass of water, collected from puddles under the reactor buildings at the plant, during a press conference to prove the safety of the installed decontamination procedures.[359][360]
An energy white paper, approved by the Japanese Cabinet in October 2011, says "public confidence in safety of nuclear power was greatly damaged" by the Fukushima disaster, and calls for a reduction in the nation’s reliance on nuclear power. It also omits a section on nuclear power expansion that was in last year’s policy review.[361]
Benjamin K. Sovacool has said that, with the benefit of hindsight, the Fukushima disaster was entirely avoidable in that Japan could have chosen to exploit the country's extensive renewable energy base. Japan has a total of "324 GW of achievable potential in the form of onshore and offshore wind turbines (222 GW), geothermal power plants (70 GW), additional hydroelectric capacity (26.5 GW), solar energy (4.8 GW) and agricultural residue (1.1 GW)."[362]
One result of the Fukushima Daiichi nuclear disaster could be renewed public support for the commercialization of renewable energy technologies.[363] In August 2011, the Japanese Government passed a bill to subsidize electricity from renewable energy sources. The legislation will become effective on 1 July 2012, and require utilities to buy electricity generated by renewable sources including solar power, wind power and geothermal energy at above-market rates.[364]
In September 2011, Mycle Schneider said that the Fukushima disaster can be understood as a unique chance "to get it right" on energy policy. "Germany – with its nuclear phase-out decision based on a highly successful renewable energy program – and Japan – having suffered a painful shock but possessing unique technical capacities and societal discipline – can be at the forefront of an authentic paradigm shift toward a truly sustainable, low-carbon and nuclear-free energy policy".[365]
As of September 2011[update], Japan plans to build a pilot floating wind farm, with six 2-megawatt turbines, off the Fukushima coast.[366] After the evaluation phase is complete in 2016, "Japan plans to build as many as 80 floating wind turbines off Fukushima by 2020."[366]
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