Nuclear reactor technology

Core of CROCUS, a small nuclear reactor used for research at the EPFL in Switzerland.
This article is a subarticle of Nuclear power.

A nuclear reactor is a device to initiate, and control, a sustained nuclear chain reaction. The most common use of nuclear reactors is for the generation of electrical power (see Nuclear power) and for the power in some ships (see Nuclear marine propulsion). This is usually accomplished by methods that involve using heat from the nuclear reaction to power steam turbines. There are also other less common uses as discussed below.

Contents

How it works

An induced nuclear fission event. A neutron is absorbed by the nucleus of a uranium-235 atom, which in turn splits into fast-moving lighter elements (fission products) and free neutrons. Though both reactors and nuclear weapons rely on nuclear chain reactions, the rate of reactions in a reactor is much slower than in a bomb.

Just as conventional power stations generate electricity by harnessing the thermal energy released from burning fossil fuels, nuclear reactors convert the thermal energy released from nuclear fission.

Fission

When a large fissile atomic nucleus such as uranium-235 or plutonium-239 absorbs a neutron, it may undergo nuclear fission. The heavy nucleus splits into two or more lighter nuclei, releasing kinetic energy, gamma radiation and free neutrons; collectively known as fission products.[1] A portion of these neutrons may later be absorbed by other fissile atoms and trigger further fission events, which release more neutrons, and so on. This is known as a nuclear chain reaction.

The reaction can be controlled by using neutron poisons, which absorb excess neutrons, and neutron moderators which reduces the velocity of fast neutrons, thereby turning them into thermal neutrons, which are more likely to be absorbed by other nuclei. Increasing or decreasing the rate of fission has a corresponding effect on the energy output of the reactor.

Commonly used moderators include regular (light) water (75% of the world's reactors) solid graphite (20% of reactors) and heavy water (5% of reactors). Beryllium has also been used in some experimental types, and hydrocarbons have been suggested as another possibility.[2]

Heat generation

The reactor core generates heat in a number of ways:

Cooling

A nuclear reactor coolant — usually water but sometimes a gas or a liquid metal or molten salt — is circulated past the reactor core to absorb the heat that it generates. The heat is carried away from the reactor and is then used to generate steam. Most reactor systems employ a cooling system that is physically separated from the water that will be boiled to produce pressurized steam for the turbines, like the pressurized water reactor. But in some reactors the water for the steam turbines is boiled directly by the reactor core, for example the boiling water reactor.[5]

Reactivity control

The power output of the reactor is controlled by controlling how many neutrons are able to create more fissions.

Control rods that are made of a nuclear poison are used to absorb neutrons. Absorbing more neutrons in a control rod means that there are fewer neutrons available to cause fission, so pushing the control rod deeper into the reactor will reduce its power output, and extracting the control rod will increase it.

In some reactors, the coolant also acts as a neutron moderator. A moderator increases the power of the reactor by causing the fast neutrons that are released from fission to lose energy and become thermal neutrons. Thermal neutrons are more likely than fast neutrons to cause fission, so more neutron moderation means more power output from the reactors. If the coolant is a moderator, then temperature changes can affect the density of the coolant/moderator and therefore change power output. A higher temperature coolant would be less dense, and therefore a less effective moderator.

In other reactors the coolant acts as a poison by absorbing neutrons in the same way that the control rods do. In these reactors power output can be increased by heating the coolant, which makes it a less dense poison. Nuclear reactors generally have automatic and manual systems to insert large amounts of poison (often boron in the form of boric acid) into the reactor to shut the fission reaction down if unsafe conditions are detected or anticipated.[6]

Electrical power generation

The energy released in the fission process generates heat, some of which can be converted into usable energy. A common method of harnessing this thermal energy is to use it to boil water to produce pressurized steam which will then drive a steam turbine that generates electricity.[6]

History - Early reactors

The concept of a nuclear chain reaction was first realized by Hungarian scientist Leó Szilárd in 1933. He filed a patent for his idea of a simple nuclear reactor the following year.[7]

The first artificial nuclear reactor, Chicago Pile-1, was constructed at the University of Chicago by a team led by Enrico Fermi in 1942. It achieved criticality on December 2, 1942[8] at 3:25 PM. The reactor support structure was made of wood, which supported a pile of graphite blocks, embedded in which was natural Uranium-oxide 'pseudospheres' or 'briquettes'. Inspiration for such a reactor was provided by the discovery by Lise Meitner, Fritz Strassman and Otto Hahn in 1938 that bombardment of Uranium with neutrons (provided by an Alpha-on-Beryllium fusion reaction, a "neutron howitzer") produced a Barium residue, which they reasoned was created by the fissioning of the Uranium nuclei. Subsequent studies revealed that several neutrons were also released during the fissioning, making available the opportunity for a chain reaction. Shortly after the discovery of fission, Hitler's Germany invaded Poland in 1939, starting World War II in Europe, and all such research became militarily classified. On August 2, 1939 Albert Einstein wrote a letter to President Franklin D. Roosevelt suggesting that the discovery of Uranium's fission could lead to the development of "extremely powerful bombs of a new type", giving impetus to the study of reactors and fission.

Soon after the Chicago Pile, the U.S. military developed nuclear reactors for the Manhattan Project starting in 1943. The primary purpose for these reactors was the mass production of plutonium (primarily at the Hanford Site) for nuclear weapons. Fermi and Leo Szilard applied for a patent on reactors on 19 December 1944. Its issuance was delayed for 10 years because of wartime secrecy.[9]

"World's first nuclear power plant" is the claim made by signs at the site of the EBR-I, which is now a museum near Arco, Idaho. This experimental LMFBR operated by the U.S. Atomic Energy Commission produced 0.8 kW in a test on December 20, 1951[10] and 100 kW (electrical) the following day,[11] having a design output of 200 kW (electrical).

Besides the military uses of nuclear reactors, there were political reasons to pursue civilian use of atomic energy. U.S. President Dwight Eisenhower made his famous Atoms for Peace speech to the UN General Assembly on December 8, 1953. This diplomacy led to the dissemination of reactor technology to U.S. institutions and worldwide.

The first nuclear power plant built for civil purposes was the AM-1 Obninsk Nuclear Power Plant, launched on June 27, 1954 in the Soviet Union. It produced around 5 MW (electrical).

After World War II, the U.S. military sought other uses for nuclear reactor technology. Research by the Army and the Air Force never came to fruition; however, the U.S. Navy succeeded when they steamed the USS Nautilus (SSN-571) on nuclear power January 17, 1955.

The first commercial nuclear power station, Calder Hall in Sellafield, England was opened in 1956 with an initial capacity of 50 MW (later 200 MW).[12][13]

The first portable nuclear reactor "Alco PM-2A" used to generate electrical power (2 MW) for Camp Century from 1960.[14]

Components

The control room of NC State's Pulstar Nuclear Reactor.

The key components common to most types of nuclear power plants are:

The people in a nuclear power plant

Nuclear power plants typically employ just under a thousand people per reactor (including security guards and engineers associated with the plant but possibly working elsewhere).

In the United States and Canada, workers except for management, professional (such as engineers) and security personnel are likely to be members of either the International Brotherhood of Electrical Workers (IBEW) or the Utility Workers Union of America (UWUA).

Reactor types

NC State's PULSTAR Reactor is a 1 MW pool-type research reactor with 4% enriched, pin-type fuel consisting of UO2 pellets in zircaloy cladding.

Classifications

Nuclear Reactors are classified by several methods; a brief outline of these classification schemes is provided.

Classification by type of nuclear reaction

Classification by moderator material

Used by thermal reactors:

Classification by coolant

In thermal nuclear reactors (LWRs in specific), the coolant acts as a moderator that must slow down the neutrons before they can be efficiently absorbed by the fuel.

Classification by generation

The "Gen IV"-term was dubbed by the United States Department of Energy (DOE) for developing new plant types in 2000.[18] In 2003, the French Commissariat à l'Énergie Atomique (CEA) was the first to refer to Gen II types in Nucleonics Week; .[19] First mentioning of Gen III was also in 2000 in conjunction with the launch of the Generation IV International Forum (GIF) plans.

Classification by phase of fuel

Classification by use

Current technologies

There are two types of nuclear power in current use:

Pressurized Water Reactors (PWR)
These reactors use a pressure vessel to contain the nuclear fuel, control rods, moderator, and coolant. They are cooled and moderated by high pressure liquid water. The hot radioactive water that leaves the pressure vessel is looped through a steam generator, which in turn heats a secondary (non-radioactive) loop of water to steam that can run turbines. They are the majority of current reactors, and are generally considered the safest and most reliable technology currently in large scale deployment. This is a thermal neutron reactor design, the newest of which are the VVER-1200, Advanced Pressurized Water Reactor and the European Pressurized Reactor. United States Naval reactors are of this type.
Laguna Verde nuclear power plant — a BWR
Boiling Water Reactors (BWR)
A BWR is like a PWR without the steam generator. A boiling water reactor is cooled and moderated by water like a PWR, but at a lower pressure, which allows the water to boil inside the pressure vessel producing the steam that runs the turbines. Unlike a PWR, there is no primary and secondary loop. The thermal efficiency of these reactors can be higher, and they can be simpler, and even potentially more stable and safe. This is a thermal neutron reactor design, the newest of which are the Advanced Boiling Water Reactor and the Economic Simplified Boiling Water Reactor.
The CANDU Qinshan Nuclear Power Plant
Pressurized Heavy Water Reactor (PHWR)
A Canadian design (known as CANDU), these reactors are heavy-water-cooled and -moderated Pressurized-Water reactors. Instead of using a single large pressure vessel as in a PWR, the fuel is contained in hundreds of pressure tubes. These reactors are fueled with natural uranium and are thermal neutron reactor designs. PHWRs can be refueled while at full power, which makes them very efficient in their use of uranium (it allows for precise flux control in the core). CANDU PHWRs have been built in Canada, Argentina, China, India (pre-NPT), Pakistan (pre-NPT), Romania, and South Korea. India also operates a number of PHWRs, often termed 'CANDU-derivatives', built after the Government of Canada halted nuclear dealings with India following the 1974 Smiling Buddha nuclear weapon test.
The Ignalina Nuclear Power Plant — a RBMK type (closed 2009)
Reaktor Bolshoy Moschnosti Kanalniy (High Power Channel Reactor) (RBMK)
A Soviet design, built to produce plutonium as well as power. RBMKs are water cooled with a graphite moderator. RBMKs are in some respects similar to CANDU in that they are refuelable during power operation and employ a pressure tube design instead of a PWR-style pressure vessel. However, unlike CANDU they are very unstable and large, making containment buildings for them expensive. A series of critical safety flaws have also been identified with the RBMK design, though some of these were corrected following the Chernobyl accident. Their main attraction is their use of light water and un-enriched uranium. As of 2010, 11 remain open, mostly due to safety improvements and help from international safety agencies such as the DOE. Despite these safety improvements, RBMK reactors are still considered one of the most dangerous reactor designs in use. RBMK reactors were deployed only in the former Soviet Union.
The Magnox Sizewell A nuclear power station
The Torness nuclear power station — an AGR
Gas Cooled Reactor (GCR) and Advanced Gas Cooled Reactor (AGR)
These are generally graphite moderated and CO2 cooled. They can have a high thermal efficiency compared with PWRs due to higher operating temperatures. There are a number of operating reactors of this design, mostly in the United Kingdom, where the concept was developed. Older designs (i.e. Magnox stations) are either shut down or will be in the near future. However, the AGCRs have an anticipated life of a further 10 to 20 years. This is a thermal neutron reactor design. Decommissioning costs can be high due to large volume of reactor core.
Liquid Metal Fast Breeder Reactor (LMFBR)
This is a reactor design that is cooled by liquid metal, totally unmoderated, and produces more fuel than it consumes. They are said to "breed" fuel, because they produce fissionable fuel during operation because of neutron capture. These reactors can function much like a PWR in terms of efficiency, and do not require much high pressure containment, as the liquid metal does not need to be kept at high pressure, even at very high temperatures. BN-350 and BN-600 in USSR and Superphénix in France were a reactor of this type, as was Fermi-I in the United States. The Monju reactor in Japan suffered a sodium leak in 1995 and is pending restart earliest in February 2010. All of them use/used liquid sodium. These reactors are fast neutron, not thermal neutron designs. These reactors come in two types:
The Superphenix, one of the few FBRs
Lead cooled
Using lead as the liquid metal provides excellent radiation shielding, and allows for operation at very high temperatures. Also, lead is (mostly) transparent to neutrons, so fewer neutrons are lost in the coolant, and the coolant does not become radioactive. Unlike sodium, lead is mostly inert, so there is less risk of explosion or accident, but such large quantities of lead may be problematic from toxicology and disposal points of view. Often a reactor of this type would use a lead-bismuth eutectic mixture. In this case, the bismuth would present some minor radiation problems, as it is not quite as transparent to neutrons, and can be transmuted to a radioactive isotope more readily than lead. The Russian Alfa class submarine uses a lead-bismuth-cooled fast reactor as its main power plant.
Sodium cooled
Most LMFBRs are of this type. The sodium is relatively easy to obtain and work with, and it also manages to actually prevent corrosion on the various reactor parts immersed in it. However, sodium explodes violently when exposed to water, so care must be taken, but such explosions wouldn't be vastly more violent than (for example) a leak of superheated fluid from a SCWR or PWR. EBR-I, the first reactor to have a core meltdown, was of this type.
Pebble Bed Reactors (PBR)
These use fuel molded into ceramic balls, and then circulate gas through the balls. The result is an efficient, low-maintenance, very safe reactor with inexpensive, standardized fuel. The prototype was the AVR.
Molten Salt Reactors
These dissolve the fuels in fluoride salts, or use fluoride salts for coolant. These have many safety features, high efficiency and a high power density suitable for vehicles. Notably, they have no high pressures or flammable components in the core. The prototype was the MSRE, which also used Thorium's fuel cycle to produce 0.1% of the radioactive waste of standard reactors.
Aqueous Homogeneous Reactor (AHR)
These reactors use soluble nuclear salts dissolved in water and mixed with a coolant and a neutron moderator.

Future and developing technologies

Advanced reactors

More than a dozen advanced reactor designs are in various stages of development.[22] Some are evolutionary from the PWR, BWR and PHWR designs above, some are more radical departures. The former include the Advanced Boiling Water Reactor (ABWR), two of which are now operating with others under construction, and the planned passively safe ESBWR and AP1000 units (see Nuclear Power 2010 Program).

Generation IV reactors

Generation IV reactors are a set of theoretical nuclear reactor designs currently being researched. These designs are generally not expected to be available for commercial construction before 2030. Current reactors in operation around the world are generally considered second- or third-generation systems, with the first-generation systems having been retired some time ago. Research into these reactor types was officially started by the Generation IV International Forum (GIF) based on eight technology goals. The primary goals being to improve nuclear safety, improve proliferation resistance, minimize waste and natural resource utilization, and to decrease the cost to build and run such plants.[24]

Generation V+ reactors

Designs which are theoretically possible, but which are not being actively considered or researched at present. Though such reactors could be built with current or near term technology, they trigger little interest for reasons of economics, practicality, or safety.

Fusion reactors

Controlled nuclear fusion could in principle be used in fusion power plants to produce power without the complexities of handling actinides, but significant scientific and technical obstacles remain. Several fusion reactors have been built, but as yet none has 'produced' more thermal energy than electrical energy consumed. Despite research having started in the 1950s, no commercial fusion reactor is expected before 2050. The ITER project is currently leading the effort to commercialize fusion power.

Nuclear fuel cycle

Thermal reactors generally depend on refined and enriched uranium. Some nuclear reactors can operate with a mixture of plutonium and uranium (see MOX). The process by which uranium ore is mined, processed, enriched, used, possibly reprocessed and disposed of is known as the nuclear fuel cycle.

Under 1% of the uranium found in nature is the easily fissionable U-235 isotope and as a result most reactor designs require enriched fuel. Enrichment involves increasing the percentage of U-235 and is usually done by means of gaseous diffusion or gas centrifuge. The enriched result is then converted into uranium dioxide powder, which is pressed and fired into pellet form. These pellets are stacked into tubes which are then sealed and called fuel rods. Many of these fuel rods are used in each nuclear reactor.

Most BWR and PWR commercial reactors use uranium enriched to about 4% U-235, and some commercial reactors with a high neutron economy do not require the fuel to be enriched at all (that is, they can use natural uranium). According to the International Atomic Energy Agency there are at least 100 research reactors in the world fueled by highly enriched (weapons-grade/90% enrichment uranium). Theft risk of this fuel (potentially used in the production of a nuclear weapon) has led to campaigns advocating conversion of this type of reactor to low-enrichment uranium (which poses less threat of proliferation).[25]

Fissile U-235 and non-fissile but fissionable and fertile U-238 are both used in the fission process. U-235 is fissionable by thermal (i.e. slow-moving) neutrons. A thermal neutron is one which is moving about the same speed as the atoms around it. Since all atoms vibrate proportionally to their absolute temperature, a thermal neutron has the best opportunity to fission U-235 when it is moving at this same vibrational speed. On the other hand, U-238 is more likely to capture a neutron when the neutron is moving very fast. This U-239 atom will soon decay into plutonium-239, which is another fuel. Pu-239 is a viable fuel and must be accounted for even when a highly enriched uranium fuel is used. Plutonium fissions will dominate the U-235 fissions in some reactors, especially after the initial loading of U-235 is spent. Plutonium is fissionable with both fast and thermal neutrons, which make it ideal for either nuclear reactors or nuclear bombs.

Most reactor designs in existence are thermal reactors and typically use water as a neutron moderator (moderator means that it slows down the neutron to a thermal speed) and as a coolant. But in a fast breeder reactor, some other kind of coolant is used which will not moderate or slow the neutrons down much. This enables fast neutrons to dominate, which can effectively be used to constantly replenish the fuel supply. By merely placing cheap unenriched uranium into such a core, the non-fissionable U-238 will be turned into Pu-239, "breeding" fuel.

Fueling of nuclear reactors

The amount of energy in the reservoir of nuclear fuel is frequently expressed in terms of "full-power days," which is the number of 24-hour periods (days) a reactor is scheduled for operation at full power output for the generation of heat energy. The number of full-power days in a reactor's operating cycle (between refueling outage times) is related to the amount of fissile uranium-235 (U-235) contained in the fuel assemblies at the beginning of the cycle. A higher percentage of U-235 in the core at the beginning of a cycle will permit the reactor to be run for a greater number of full-power days.

At the end of the operating cycle, the fuel in some of the assemblies is "spent" and is discharged and replaced with new (fresh) fuel assemblies, although in practice it is the buildup of reaction poisons in nuclear fuel that determines the lifetime of nuclear fuel in a reactor. Long before all possible fission has taken place, the buildup of long-lived neutron absorbing fission byproducts impedes the chain reaction. The fraction of the reactor's fuel core replaced during refueling is typically one-fourth for a boiling-water reactor and one-third for a pressurized-water reactor. The disposition and storage of this spent fuel is one of the most challenging aspects of the operation of a commercial nuclear power plant. This nuclear waste is highly radioactive and its toxicity presents a danger for thousands of years.[26]

Not all reactors need to be shut down for refueling; for example, pebble bed reactors, RBMK reactors, molten salt reactors, Magnox, AGR and CANDU reactors allow fuel to be shifted through the reactor while it is running. In a CANDU reactor, this also allows individual fuel elements to be situated within the reactor core that are best suited to the amount of U-235 in the fuel element.

The amount of energy extracted from nuclear fuel is called its burnup, which is expressed in terms of the heat energy produced per initial unit of fuel weight. Burn up is commonly expressed as megawatt days thermal per metric ton of initial heavy metal.

Safety

Natural nuclear reactors

Although nuclear fission reactors are often thought of as being solely a product of modern technology, the first nuclear fission reactors were in fact naturally occurring. A natural nuclear fission reactor can occur under certain circumstances that mimic the conditions in a constructed reactor.[27] Fifteen natural fission reactors have so far been found in three separate ore deposits at the Oklo mine in Gabon, West Africa. First discovered in 1972 by French physicist Francis Perrin, they are collectively known as the Oklo Fossil Reactors. Self-sustaining nuclear fission reactions took place in these reactors approximately 1.5 billion years ago, and ran for a few hundred thousand years, averaging 100 kW of power output during that time.[28] The concept of a natural nuclear reactor was theorized as early as 1956 by Paul Kuroda at the University of Arkansas.[29][30]

Such reactors can no longer form on Earth: radioactive decay over this immense time span has reduced the proportion of U-235 in naturally occurring uranium to below the amount required to sustain a chain reaction.

The natural nuclear reactors formed when a uranium-rich mineral deposit became inundated with groundwater that acted as a neutron moderator, and a strong chain reaction took place. The water moderator would boil away as the reaction increased, slowing it back down again and preventing a meltdown. The fission reaction was sustained for hundreds of thousands of years.

These natural reactors are extensively studied by scientists interested in geologic radioactive waste disposal. They offer a case study of how radioactive isotopes migrate through the Earth's crust. This is a significant area of controversy as opponents of geologic waste disposal fear that isotopes from stored waste could end up in water supplies or be carried into the environment.

See also

Wikinews has related news:
  • Canada pursues new nuclear research reactor to produce medical isotopes
  • Canadian nuclear reactor shutdown causes worldwide medical isotope shortage

References

  1. ↑ "Neutrons and gammas from Cf-252". Health Physics Society. http://www.hps.org/publicinformation/ate/q6333.html. Retrieved September 24, 2008. 
  2. ↑ "DOE Fundamentals Handbook: Nuclear Physics and Reactor Theory" (PDF). US Department of Energy. http://www.hss.energy.gov/NuclearSafety/techstds/standard/hdbk1019/h1019v2.pdf. Retrieved September 24, 2008. 
  3. ↑ http://bioenergy.ornl.gov/papers/misc/energy_conv.html
  4. ↑ http://www.cambridge.org/us/catalogue/catalogue.asp?isbn=9780521884082&ss=fro
  5. ↑ "How nuclear power works". HowStuffWorks.com. http://science.howstuffworks.com/nuclear-power3.htm. Retrieved September 25, 2008. 
  6. ↑ 6.0 6.1 "Reactor Protection & Engineered Safety Feature Systems". The Nuclear Tourist. http://www.nucleartourist.com/systems/rp.htm. Retrieved September 25, 2008. 
  7. ↑ L. Szilárd, "Improvements in or relating to the transmutation of chemical elements," British patent number: GB630726 (filed: 28 June 1934; published: 30 March 1936). esp@cenet document view
  8. ↑ The First Reactor, U.S. Atomic Energy Commission, Division of Technical Information
  9. ↑ U.S. Patent 2,708,656 issued 17 May 1955-"Neutronic Reactor "
  10. ↑ Experimental Breeder Reactor 1 factsheet, Idaho National Laboratory
  11. ↑ Fifty years ago in December: Atomic reactor EBR-I produced first electricity American Nuclear Society Nuclear news, November 2001
  12. ↑ Kragh, Helge (1999). Quantum Generations: A History of Physics in the Twentieth Century. Princeton NJ: Princeton University Press. p. 286. ISBN 0691095523. 
  13. ↑ "On This Day: 17 October". BBC News. 1956-10-17. http://news.bbc.co.uk/onthisday/hi/dates/stories/october/17/newsid_3147000/3147145.stm. Retrieved 2006-11-09. 
  14. ↑ http://gombessa.tripod.com/scienceleadstheway/id9.html Frank J. Leskovitz (inc images)
  15. ↑ Golubev, V. I.; et al. (January 1993). "Fast-reactor actinide transmutation". Atomic Energy (New York: Springer) 74 (1): 83–84. doi:10.1007/BF00750983. ISSN 1063-4258. 
  16. ↑ Nuclear Energy Institute. N.p., n.d. Web. 3 Oct. 2009. <http://www.nei.org/ resourcesandstats/nuclear_statistics/usnuclearpowerplants/>.
  17. ↑ Lipper, Ilan, and Jon Stone. "Nuclear Energy and Society." University of Michigan. N.p., n.d. Web. 3 Oct. 2009. <http://www.umich.edu/~gs265/ society/nuclear.htm>.
  18. ↑ http://www.euronuclear.org/info/generation-IV.htm
  19. ↑ Nucleonics Week, Vol. 44, No. 39; Pg. 7, September 25, 2003 Quote: "Etienne Pochon, CEA director of nuclear industry support, outlined EPR's improved performance and enhanced safety features compared to the advanced Generation II designs on which it was based."
  20. ↑ A Technology Roadmap for Generation IV Nuclear Energy Systems PDF (4.33 MB); see "Fuel Cycles and Sustainability"
  21. ↑ World Nuclear Association Information Brief -Research Reactors
  22. ↑ "Advanced Nuclear Power Reactors". World Nuclear Association. http://world-nuclear.org/info/inf08.html. Retrieved January 29, 2010. 
  23. ↑ Dr. Charles Till. "Nuclear Reaction: Why Do Americans Fear Nuclear Power?". Public Broadcasting Service (PBS). http://www.pbs.org/wgbh/pages/frontline/shows/reaction/interviews/till.html. Retrieved 2006-11-09. 
  24. ↑ "Generation IV Nuclear Reactors". World Nuclear Association. http://world-nuclear.org/info/inf77.html. Retrieved January 29, 2010. 
  25. ↑ IAEA, Improving Security at World's Nuclear Research Reactors: Technical and Other Issues Focus of June Symposium in Norway (7 June 2006).
  26. ↑ Lipper, Ilan, and Jon Stone. "Nuclear Energy and Society." University of Michigan. N.p., n.d. Web. 3 Oct. 2009. <http://www.umich.edu/~gs265/ society/nuclear.htm>.
  27. ↑ Video of physics lecture - at Google Video; a natural nuclear reactor is mentioned at 42:40 mins into the video
  28. ↑ Meshik, Alex P. "The Workings of an Ancient Nuclear Reactor." Scientific American. November, 2005. Pg. 82.
  29. ↑ "Oklo: Natural Nuclear Reactors". Office of Civilian Radioactive Waste Management. http://www.ocrwm.doe.gov/factsheets/doeymp0010.shtml. Retrieved June 28, 2006. 
  30. ↑ "Oklo's Natural Fission Reactors". American Nuclear Society. http://www.ans.org/pi/np/oklo. Retrieved June 28, 2006. 

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