Nuclear graphite

Core graphite from the Molten-Salt Reactor Experiment

Nuclear graphite is any grade of graphite, usually electro-graphite, specifically manufactured for use as a moderator or reflector within a nuclear reactor. 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.

History

The potential for creating a chain reaction in uranium became apparent in 1939 following the nuclear fission experiments of Otto Hahn and Fritz Strassman, and the interpretation of these results by Lise Meitner and Otto Frisch. The exciting possibilities that this presented rapidly spread throughout the world physics community. In order for the fission process to chain react, the neutrons created by uranium fission must be slowed down by interacting with a neutron moderator (an element with a low atomic weight, that will "bounce", when hit by a neutron) before they will be captured by other uranium atoms. It became well known by late 1939 that the two most promising moderators were heavy water and graphite.

In February 1940, using funds that were allocated partly as a result of the Einstein-Szilard letter to President Roosevelt, Leo Szilard purchased several tons of graphite from the Speer Carbon Company and from the National Carbon Company (the National Carbon Division of the Union Carbide and Carbon Corporation in Cleveland Ohio) for use in Enrico Fermi's first fission experiments, the so-called exponential pile. Fermi writes that "The results of this experiment was [sic] somewhat discouraging" presumably due to the absorption of neutrons by some unknown impurity. So, in December 1940 Fermi and Szilard met with Herbert G. MacPherson and V. C. Hamister at National Carbon to discuss the possible existence of impurities in graphite. During this conversation it became clear that minute quantities of boron impurities were the source of the problem.

As a result of this meeting, over the next two years, MacPherson and Hamister developed thermal and gas extraction purification techniques at National Carbon for the production of boron-free graphite. The resulting product was designated AGOT Graphite ("Atcheson Graphite Ordinary Temperature") by National Carbon, and it was "the first true nuclear grade graphite".

During this period, Fermi and Szilard purchased graphite from several manufacturers with various degrees of neutron absorption cross section: AGX graphite from National Carbon Company with 6.68 mb (millibarns) cross section, US graphite from United States Graphite Company with 6.38 mb cross section, Speer graphite from the Speer Carbon Company with 5.51 mb cross section, and when it became available, AGOT graphite from National Carbon, with 4.97 mb cross section. (See also Haag [2005].) By November 1942 National Carbon had shipped 250 tons of AGOT graphite to the University of Chicago where it became the primary source of graphite to be used in the construction of Fermi's Chicago Pile-1, the first nuclear reactor to generate a sustained chain reaction (December 2, 1942). AGOT graphite was used to build the X-10 graphite reactor in Oak Ridge TN (early 1943) and the first reactors at the Hanford Site in Washington (mid 1943), for the production of plutonium during and after World War II. The AGOT process and its later refinements became standard techniques in the manufacture of nuclear graphite.

The cross section of graphite was also investigated during the second world war in Germany by Walter Bothe, P. Jensen, and Werner Heisenberg. The purest graphite available to them was a product from the Siemens Plania company, which exhibited a neutron absorption cross section of about 6.4 mb to 7.5 mb (Haag 2005). Heisenberg therefore decided that graphite would be unsuitable as a moderator in a reactor design using natural uranium, due to this apparently high rate of neutron absorption. Consequently, the German effort to create a chain reaction involved attempts to use heavy water, an expensive and scarce alternative, made all the more difficult to acquire as a consequence of the Norwegian heavy water sabotage by Norwegian and Allied forces. Writing as late as 1947, Heisenberg still did not understand that the only problem with graphite was the boron impurities.

Wigner effect

In December 1942 E. Wigner suggested that neutron bombardment might introduce dislocations and other damage in the molecular structure of materials such as the graphite moderator in a nuclear reactor. The resulting buildup of energy in the material became a matter of concern The possibility was suggested that graphite bars might fuse together as chemical bonds at the surface of the bars are opened and closed again. 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.

Purity

Reactor-grade graphite must be free of neutron absorbing materials, in particular boron, which has a large neutron capture cross section. Boron sources in graphite include the raw materials, the packing materials used in baking the product, and even the choice of soap (for example, borax) used to launder the clothing worn by workers in the machine shop. Boron concentration in thermally purified graphite (such as AGOT graphite) can be less than 0.4 ppm and in chemically purified nuclear graphite it is less than 0.06 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

Manufacture

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

There have been two major accidents in graphite moderated reactors, the Windscale fire and the Chernobyl disaster.

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.

References

• Haag, G. 2005, Properties of ATR-2E Graphite and Property Changes due to Fast Neutron Irradiation, FZ-Juelich, Juel-4813.