Windscale fire

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On October 10, 1957, the graphite core of a British nuclear reactor at Windscale, Cumbria, caught fire, releasing substantial amounts of radioactive contamination into the surrounding area. The event, known as the Windscale fire, was considered the world's worst nuclear accident until Three Mile Island in 1979. Both were dwarfed by the Chernobyl accident in 1986.

Contents

[edit] Background

The design of Windscale Pile no. 1.
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The design of Windscale Pile no. 1.

After the Second World War, in 1946, in spite of the participation of many British scientists in the Manhattan Project, the United States government passed legislation that closed its nuclear weapons program to all other countries.

The British government, not wanting to be left behind as a world power in an emerging arms race, embarked on a program to build its own atomic bomb as quickly as possible. Because of the American decision to exclude Britain from its weapons program, the British had no source of the fissile element plutonium, which can be used as a key component of a nuclear weapon. It was therefore decided to build two nuclear reactors, or atomic piles as they were known at the time, to convert unenriched uranium into plutonium.

[edit] The Windscale Piles

[edit] Location and filters

The reactors were built in a short time near the tiny village of Seascale, Cumbria, and were known as Windscale Pile 1 and Windscale Pile 2, housed in large, concrete buildings a few hundred feet from one another. The reactors were graphite-moderated and air-cooled. Because nuclear fission produces large amounts of heat, it was necessary to cool the reactor cores by blowing cold air through channels in the graphite. Hot air was then exhausted out of the back of the core and up the chimney. It is important to note that filters were added late into construction at the insistence of Sir John Cockcroft and these were housed in galleries at the very top of the discharge stacks. They were deemed unnecessary, a waste of money and time and presented something of an engineering headache, being added very late in construction in vast concrete houses at the muzzles of the 400-ft chimneys. Due to this, they were known as 'Cockcroft's Folly' by workers and engineers. As it was, 'Cockcroft's Folly' probably prevented a disaster from becoming a catastrophe.

[edit] Core design

The reactors themselves were an octagonal arrangement of graphite blocks, with vertical shafts for control rods cut into them and horizontal channels through which cans of uranium and Isotope cartridges could be passed, to expose them to neutron radiation and produce plutonium and radioisotopes, respectively. Fuel and isotopes were fed into the channels in the front of the reactor, the 'charge face', and spent fuel was then pushed all the way through the core and out of the back - the 'discharge face' - into a water duct for initial cooling prior to retrieval and processing to extract the plutonium.

[edit] Fuel

Unenriched Uranium metal in aluminium cans with fins to improve cooling was used for the production of plutonium. As a result it is likely that the burnup of the fuel would have been kept low to prevent the generation of large amounts of the heavier plutonium isotopes (240Pu, 241Pu etc).

[edit] Isotope cartridges

The following substances were placed inside metal cans and subjected to neutron irradation to create radioisotopes. Both the target material and some of the product isotopes are listed below. Of these, the Polonium-210 release made the most significant contribution to the collective dose on the general population.[1]

[edit] Wigner energy

When the reactors were being built, little was known about the behavior of graphite when exposed to neutrons. Hungarian physicist Eugene Wigner discovered that graphite, when bombarded by neutrons, suffers dislocations in its crystalline structure causing a build up of potential energy. This energy, if allowed to accumulate, could escape spontaneously in a powerful rush of heat. Once commissioned and settled into operations, Windscale Pile 2 experienced a mysterious rise in core temperature and this was attributed to a sudden Wigner energy release. This worried British scientists and a means of safely releasing the stored energy was sought. The only viable solution was also extremely simple: an annealing process, in which the graphite core was heated to 250 degrees Celsius and the crystalline structure of the graphite expanded enough to allow the displaced molecules to slip back into place and gradually release their stored energy (as heat) as they did so, causing a uniform release which spread throughout the core. Annealing succeeded in preventing the buildup of Wigner energy, but the monitoring equipment, and indeed the reactor itself and all of its ancillaries (such as the cooling system) were never designed for this. Each annealing cycle was slightly different and were also growing more difficult as time went on; many of the later cycles had to be repeated and higher and higher temperatures were required to start the annealing process. It was also found that some pockets of Wigner energy remained that had not been released on previous occasions. The annealing was performed with the reactor charged; fully fueled.

Because they were built hastily and during a time when little was known about reactor design, the reactors had a number of serious design flaws that contributed to the disaster. Graphite is flammable in air, and air was being fed into the reactors constantly for cooling, so there was a constant fire hazard. The direct venting of the cooling air to the atmosphere meant any radioactive material released by the core that slipped through the filters would be released into the countryside. The annealing phases were not part of the original plan, so what thermocouples there were, were placed in positions in the reactor to monitor the normal operations, but not to monitor the annealing process. This allowed unknown hot-spots to form. The reactor's fuel, metallic uranium, will also catch fire if it becomes too hot unlike the uranium dioxide used in modern reactors.

[edit] The accident

[edit] Ignition

On October 7, 1957, operators began an annealing cycle for Windscale Pile no. 1 by switching the cooling fans to low power and stabilizing the reactor at low power. The next day, to carry out the annealing, the operators increased the power to the reactor. When it appeared that the annealing process was taking place, control rods were lowered back into the core to shut down the reactor, but it soon became apparent that the Wigner energy release was not spreading through the core, but dwindling prematurely. The operators withdrew the control rods again to apply a second nuclear heating and complete the annealing process. As already explained, some thermocouples were not in the hottest parts of the core and it is now known that some areas were considerably hotter than others. This and the second heating are suspected to be the deciding factors behind the fire, although the precise cause is not known. The official report suggests that a can of uranium ruptured and oxidized causing further overheating and the fire, but a more recent report suggests that it may actually have been a magnesium/lithium isotope cartridge. All that was visible on the instruments was a gentle increase in temperature, which was to be expected during the Wigner release.

Early in the morning on October 10, it was suspected that something unusual was going on. The temperature in the core was supposed to gradually fall as Wigner release ended, but the monitoring equipment showed something more ambiguous was going on and one thermocouple indicated that core temperature was instead rising. It was then realised that the radiation monitoring devices in the filter galleries the top of the discharge stack were at full scale reading and the shift foreman declared a site emergency. No-one at Windscale was now in any doubt that Pile Number 1 was in serious trouble.

[edit] The fire

Operators tried to examine the pile with a remote scanner but it had jammed. Tom Hughes, second in command to the Reactor Manager, suggested examining the reactor personally and so he and another operator marched down to the charge face of the reactor, clad in protective gear. A fuel channel inspection plug was taken out close to a thermocouple registering high temperatures and it was then that the operators saw that the fuel was red hot.

"An inspection plug was taken out," said Tom Hughes in a later interview, "and we saw, to our complete horror, four channels of fuel glowing bright cherry red."

There was no doubt that the reactor was now on fire; and had been for almost 48 hours. Reactor Manager Tom Tuohy donned full protective equipment and breathing apparatus and scaled the 80 feet to the top of the reactor building, where he stood atop the reactor lid to examine the rear of the reactor, the discharge face. Here he reported a dull red luminescence visible, lighting up the void between the back of the reactor and the rear containment: red hot fuel cartridges glowing in the fuel channels on the discharge face. He returned to the reactor upper containment several times throughout the incident, at the height of which a fierce conflagration was raging from the discharge face and playing on the back of the reinforced concrete containment - concrete which specifications insisted that it must be kept below a certain temperature to prevent its disintegration and collapse.

[edit] Initial fire fighting attempts

Operators were unsure what to do about the fire. First, they tried to blow the flames out by putting the blowers onto full power and increasing the cooling, but predictably this simply fueled the fire. Tom Hughes and his colleague had already ejected some undamaged fuel cartridges from around the blaze and Tom Tuohy suggested trying to eject some from the heart of the fire, by bludgeoning them through the reactor and into the cooling pond behind it with scaffolding poles. This proved impossible and the fuel rods refused to budge, no matter how much force was applied. The poles were withdrawn with their ends red hot and, once, a pole was returned red hot and dripping with molten metal. Hughes knew this had to be molten irradiated uranium and this caused serious radiation problems on the charge hoist itself.

"It [the exposed fuel channel] was white hot," said Hughes' colleague on the charge hoist with him, "it was just white hot. Nobody, I mean, nobody, can believe how hot it could possibly be."

[edit] Carbon dioxide

Next, the operators tried to extinguish the fire using carbon dioxide. The new gas-cooled Calder Hall reactors next door had just received a delivery of 25 tonnes of liquid carbon dioxide and this was rigged up to the charge face of Windscale Pile 1, but the heat generated by the fire was so extreme that the oxygen was stripped from the carbon atoms and added to the blaze. It is likely that the carbon dioxide reacted to form carbon monoxide in a reaction similar to that used to form producer gas (see Town gas for more details).

CO2 + C → 2 CO

[edit] The use of water

On the morning of Friday October 11 and at its peak, 11 tonnes of uranium were ablaze. Temperatures were becoming extreme (one thermocouple registered 1,300 degrees Celsius) and the biological containment around the stricken reactor was now in severe danger of collapse. Faced with this crisis, the operators decided to use water. This was incredibly risky: molten metal oxidises in contact with water, stripping oxygen from the water molecules and leaving free hydrogen, which could mix with incoming air and explode, tearing open the weakened containment. But there was no other choice. About a dozen hoses were hauled to the charge face of the reactor; their nozzles were cut off and the lines themselves connected to scaffolding poles and fed into fuel channels about a meter above the heart of the fire.

Tom Tuohy then ordered everyone out of the reactor building except himself and the Fire Chief and all cooling and ventilating air entering the reactor was shut off. Tuohy once again hauled himself atop the reactor shielding and ordered the water to be turned on, listening carefully at the inspection holes for any sign of a hydrogen reaction as the pressure was increased. Tuohy climbed up several times and reported watching the flames leaping from the discharge face slowly dying away. During one of the inspections, Tuohy found that the inspection plates - which are removed with a metal hook to facilitate viewing of the discharge face of the core - were stuck fast. This, Tuohy reported, was the fire trying to suck air in from wherever it could.

"I have no doubt it was even sucking air in through the chimney at this point to try and maintain itself," he remarked in interview.

Finally he managed to pull the inspection plate away and was greeted with the unfathomable sight of the fire dying away.

"First the flames went, then the flames reduced and the glow began to die down," he described, "I went up to check several times until I was satisfied that the fire was out. I did stand to one side, sort of hopefully," he went on to say, "but if you're staring straight at the core of a shut down reactor you're going to get quite a bit of radiation."

Water was kept flowing through the pile for a further 24 hours until it was completely cold.

[edit] The aftermath

The fire itself released an estimated 20,000 curies (700 terabecquerels) of radioactive material into the nearby countryside. Of particular concern was the radioactive isotope iodine-131, which has a half-life of only 8 days but is taken up by the human body and stored in the thyroid. As a result, consumption of iodine-131 often leads to cancer of the thyroid.

No one was evacuated from the surrounding area, but there was concern that milk might be dangerously contaminated. Milk from about 500km² of nearby countryside was destroyed (by dumping in local rivers) for about a month.

Public reactions varied. Locally, few people panicked. Reported comments were overwhelmingly calm and downplayed the seriousness of the accident. Many were involved with the plant, while others could see no ill effects and assumed they were safe. Several of those interviewed by reporters said that they were unhappy about how the media was accusing them of panic. The most serious sign of local distress was a 15% drop in milk sales in nearby Carlisle.

Nationally, the accident was generally reported with fear in the tabloids and with restraint by the broadsheets. The News Chronicle described it as "the accident experts said could not happen", and boasted an interview with "Britain's first atom-dust casualty". The Manchester Guardian, on the other hand, emphasized the strength of safety measures already in place. Coverage of the milk ban tended to stress how little damage it could do to an adult. In spite of occasionally hyperbolic coverage, the accident had no appreciable long term impact on British attitudes to nuclear power.

The reactor was unsalvageable; all the fuel rods that could be removed were, and the reactor bioshield was sealed and left intact. Approximately 6,700 fire-damaged fuel elements and 1,700 fire-damaged isotope canisters remain in the pile. The damaged reactor core is still slightly warm as a result of continuing nuclear reactions. Details of the levels and nature of the radioactivity remaining in the core can be seen at [2]. Windscale Pile no. 2, though undamaged by the fire, was considered too unsafe for continued use. It was shut down shortly afterward. No air-cooled reactors have been built since.

The Windscale site was decontaminated and is still in use; several more modern nuclear reactors are there now. Part of the site was later renamed Sellafield, after being transferred to BNFL in a part-privatization.

In the 1990s, the UKAEA began plans to decommission, disassemble and clean up, both piles; the decommissioning is now partially complete. Plans are being explored to safely remove the fire-damaged core, which is still highly radioactive and laced with radioisotopes and irradiated fuel.

[edit] Comparison with other accidents

The fire has been described as the worst nuclear accident until Three Mile Island (TMI) in 1979. However since neither event resulted in immediate casualties this assertion is dependent upon epidemiological assessments. In contrast some accidents did result in immediate casualties, such as the 1961 incident at the SL-1 plant in Idaho which killed three operators, or the criticality accident which killed Louis Slotin in 1946. All of these nuclear accidents were dwarfed by the Chernobyl accident of 1986.

[edit] Further reading

  • Windscale, 1957: Anatomy of a Nuclear Accident, Lorna Arnold
  • An Assessment of the Radiological Impact of the Windscale Reactor Fire, Oct., 1957, Nov., 1982 (NRPB Reports) M J Crick, G.S. Linsley
  • An airborne radiometric survey of the Windscale area, October 19-22nd,1957 (A.E.R.E. reports;no.R2890) Atomic Energy Research Establishment
  • The deposition of strontium 89 and strontium 90 on agricultural land and their entry into milk after the reactor accident at Windscale in October, 1957 (A.H.S.B) United Kingdom Atomic Energy Authority
  • Accident at Windscale No.1 Pile on 10 October,1957 (Cmnd.302)

[edit] See also

Nuclear and radiation accidents

[edit] External links

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