Nuclear graphite is any grade of graphite, usually electro-graphite, specifically manufactured for use as a moderator or reflector within nuclear reactors. Graphite is an important material for the construction of both historical and modern nuclear reactors, due to its extreme purity and its ability to withstand extremely high temperatures.
In 1942 Enrico Fermi decided to use graphite as a moderator in the first attempt to create a self-sustaining nuclear chain reaction. The initial attempt was not successful as the graphite was not sufficiently pure. A second attempt was made with both AGX graphite, manufactured by the National Carbon Company, and Speer Carbon Company graphite, which successfully went critical. The Pile constructed was called the Chicago Pile-1.
Unlike the promising results in the US and in the USSR, German investigators came to the conclusion that graphite could not be used with natural uranium to produce a nuclear chain reaction. The purest graphite available to them at that time was a product from the Siemens Plania company, which exhibited a neutron absorption cross section of about 7.5 mb. Compared to that the graphites used in CP-1 exhibited average thermal absorption cross sections of 6.68 mb (AGX, National Carbon Company), 5.51 mb (Speer Carbon Company), and 4.97 mb (AGOT, National Carbon Company). (Haag )
Already in December 1942, Eugene P. Wigner first called attention to the effects of fast particles on solids. It was suggested that neutrons from fission would displace atoms from their lattice sites by momentum transfer with effects on most of the properties of graphite, which has become known as the Wigner effect. The possibility was even suggested that graphite bars might fuse together as chemical bonds at the surface of the bars are opened and closed again, and during operation of a reactor enough energy would be deposited in the graphite to open all the chemical bonds several times. Even the possibility that the graphite parts might very quickly break into small pieces could not be ruled out. However, the first power-producing reactors (X-10 Graphite Reactor and Hanford B Reactor) had to be built without such knowledge. Cyclotrons, which were the only fast neutron sources available, would need several months of irradiation to produce damaging effects equivalent to one day in a Hanford reactor.
This was the starting point for large-scale research programmes to investigate the property changes due to fast particle radiation and to predict their influence on the safety and the lifetime of graphite reactors to be built. Influences of fast neutron radiation on strength, electrical and thermal conductivity, thermal expansitivity, dimensional stability, on the storage of internal energy (Wigner energy) and on many other properties have been observed many times and in many countries after the first results emerged from the X-10 reactor in 1944.
Although catastrophic behaviour such as fusion or crumbling of graphite pieces, has never occurred, large changes in many properties do result from fast neutron irradiation which need to be taken into account when graphite components of nuclear reactors are designed. Although not all effects are well understood yet, more than 100 graphite reactors have successfully operated in the last 60 years. A few severe accidents in graphite reactors can in no case be attributed to a lack of knowledge or insufficient properties of the graphite in use.
Care must be taken that reactor-grade graphite is free of neutron absorbing materials such as boron, widely used as the seed electrode in commercial graphite deposition systems-- this caused the failure of the Germans' World War II graphite-based nuclear reactors. Boron and or equivalent boron content should be less than 5 PPM.
Behaviour under irradiation
This describes the behavior of nuclear graphite, specifically when exposed to fast neutron irradiation.
Specific phenomena addressed:
- Dimensional change (shrinkage and Neutron-induced swelling)
- Change in elastic modulus (measured by Impulse excitation technique)
- Change in coefficient of thermal expansion
- Change in thermal conductivity
- Change in electrical resistivity
- Irradiation induced creep
Nuclear graphite for the UK Magnox reactors were manufactured from petroleum coke mixed with coal-based binder pitch heated and extruded into bricks, and then baked at 1,000 °C for several days. To reduce porosity and increase density the bricks were impregnated with coal tar at high temperature and pressure before a final bake at 2,800 °C. Individual bricks were then machined into the final required shapes.
Accidents in graphite-moderated reactors
In the Windscale fire, an untested annealing process for the graphite was used, causing overheating in unmonitored areas of the core and leading directly to the ignition of the fire; however, the material that caught fire was not, in fact, the graphite moderator itself, but rather the canisters of metallic uranium fuel within the reactor. When the fire was extinguished, it was found that the only areas of the graphite moderator to have incurred thermal damage were those that had been in immediate close proximity to the actively burning fuel canisters.
In the Chernobyl disaster, the moderator was not primarily responsible for the primary event; instead, a massive power excursion during a mishandled test caused the catastrophic failure of the reactor vessel and a near-total loss of coolant supply, and as a result, the fuel rods rapidly melted and flowed together while in an extremely-high-power state, causing a small portion of the core to reach a state of runaway prompt criticality and leading to a massive energy release, resulting in catastrophic disassembly of the reactor core and the destruction of the reaction building. However, the massive energy release during the primary event superheated the graphite of the moderator, and the disruption of the reactor vessel and building allowed the superheated graphite to come into contact with atmospheric oxygen; as a result, the graphite moderator caught fire, sending a plume of highly radioactive fallout into the atmosphere and over a very widespread area.
- Haag, G. 2005, Properties of ATR-2E Graphite and Property Changes due to Fast Neutron Irradiation, FZ-Juelich, Juel-4813.
- Gareth B. Neighbour (2007). Management of ageing in graphite reactor cores. Royal Society of Chemistry. ISBN 978-0-85404-345-3. Retrieved 2009-06-15.
- "Meeting of RG2 with Windscale Pile 1 Decommissioning Project Team" (PDF). Nuclear Safety Advisory Committee. 2005-09-29. NuSAC(2005)P 18. Retrieved 2008-11-26.
- Marsden, B.J.; Preston, S.D.; Wickham, A.J. (8–10 September 1997). "Evaluation of graphite safety issues for the British production piles at Windscale]". AEA Technology (IAEA). IAEA-TECDOC—1043. Retrieved 13 November 2010.
- Pakhomov, Sergey A.; Dubasov, Yuri V. (2009). "Estimation of Explosion Energy Yield at Chernobyl NPP Accident". Pure and Applied Geophysics 167 (4–5): 575. doi:10.1007/s00024-009-0029-9.
- "Frequently Asked Chernobyl Questions". International Atomic Energy Agency – Division of Public Information. May 2005. Retrieved 23 March 2011.