Liquid Water Graphite moderated reactor
A graphite reactor is a nuclear reactor that uses carbon as a neutron moderator, which allows un-enriched uranium to be used as nuclear fuel.
The very first artificial nuclear reactor, the Chicago Pile-1, used graphite as a moderator. Graphite moderated reactors were involved in two of the best known nuclear disasters: An untested graphite annealing process contributed to the Windscale fire (but the graphite itself did not catch fire), and a graphite fire during the Chernobyl disaster contributed to the spread of radioactive material (but was not a cause of the accident itself).
There have been several major accidents in graphite moderated reactors, with the Windscale fire and the Chernobyl disaster probably the best known.
In the Windscale fire, an untested annealing process for the graphite was used, and that contributed to the accident – however it was the uranium fuel rather than the graphite in the reactor that caught fire. The only graphite moderator damage was found to be localized around burning fuel elements.
In the Chernobyl disaster the graphite was a contributing factor to the cause of the accident. Due to overheating from lack of adequate cooling the fuel rods began to deteriorate. After the SCRAM (AZ5) button was pressed to shut down the reactor, the control rods jammed in the middle of the core causing a positive loop since the nuclear fuel reacted to graphite. This is what has been dubbed the “final trigger” of events before the rupture. A graphite fire after the main event contributed to the spread of radioactive material. The massive power excursion in Chernobyl during a mishandled test led to the rupture of the reactor vessel and a series of steam explosions, which destroyed the reactor building. Now exposed to both air and the heat from the reactor core, the graphite moderator in the reactor core caught fire, and this fire sent a plume of highly radioactive fallout into the atmosphere and over an extensive geographical area.
In addition, the French Saint-Laurent Nuclear Power Plant and the Spanish Vandellòs Nuclear Power Plant – both UNGG graphite-moderated natural uranium reactors – suffered major accidents. Particularly noteworthy is a partial core meltdown on 17. October 1969 and an heat excursion during graphite annealing on 13. March 1980 in Saint-Laurent, which were both classified as INES 4. The Vandellòs NPP was damaged on 19. October 1989, and a repair was considered not economic.
Breeder/ Fast Neutron Reactors
Fast neutron reactors are a technological step beyond conventional power reactors, but are poised to become mainstream. They offer the prospect of vastly more efficient use of uranium resources and the ability to burn actinides which are otherwise the long-lived component of high-level nuclear wastes.
Some 400 reactor-years experience has been gained in operating them. Generation IV reactor designs are largely FNRs, and international collaboration on FNR designs is proceeding with high priority.
About 20 fast neutron reactors (FNR) have already been operating, some since the 1950s, and some supplying electricity commercially. About 400 reactor-years of operating experience have been accumulated to the end of 2010. Fast reactors more deliberately use the uranium-238 as well as the fissile U-235 isotope used in most reactors. If they are designed to produce more plutonium than the uranium and plutonium they consume, they are called fast breeder reactors (FBRs). But many designs are net consumers of fissile material including plutonium.* Fast neutron reactors also can burn long-lived actinides which are recovered from used fuel out of ordinary reactors.
If the ratio of final to initial fissile content is less than 1 they are burners, consuming more fissile material (U-235, Pu and minor actinides) than they produce (fissile Pu), if more than 1 they are breeders. This is the burn ratio or breeding ratio. If the ratio is 1 they are iso-breeders, producing the same amount of fuel as they consume during operation.
Several countries have research and development programs for improved fast neutron reactors, and the IAEA’s INPRO program involving 22 countries (see later section) has fast neutron reactors as a major emphasis, in connection with closed fuel cycle. For instance one scenario in France is for half of the present nuclear capacity to be replaced by fast neutron reactors by 2050 (the first half being replaced by 3rd-generation EPR units).
An agreement between Japan’s Atomic Energy Agency (JAEA), France’s CEA and the US Department of Energy was signed in October 2010. This expanded previous FNR collaboration towards the joint design and development of reliable world-class FNRs and getting private manufacturers involved. JAEA is working on the design of a demonstration reactor to succeed the prototype FBR Monju, France is developing the Advanced Sodium Technical Reactor for Industrial Demonstration (ASTRID) with Japan, and wanted Japan to test its fuel in Monju. The USA is standing back from new plants and is focused on systems, materials and safety analysis but has an extensive base of information and experiences as a result of past efforts to develop FNRs, notably FFTF and EBR-II. GE Hitachi is taking forward some of this work with its new PRISM, which is under serious consideration in the UK for burning its reactor-grade plutonium stockpile while producing electricity. Both pool-type and loop-type FNR designs are seen to have potential, though most larger designs are pool-type. The work will include FNR fuel cycles.