Nuclear graphite

The highest-purity graphite then commercially available (so called electro-graphite) was dismissed by the Germans and the British as a possible moderator because it contained boron and cadmium impurities.

[3] However, graphite of high enough purity was developed in the early 1940's in the United States, and this then was utilized in the first and subsequent nuclear reactors for the Manhattan Project.

[7]: 40  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.

[4][9] 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.

[18] In December 1942 Eugene Wigner suggested[19] that neutron bombardment might introduce dislocations and other damage in the molecular structure of materials such as the graphite moderator in a nuclear reactor.

This was the starting point for large-scale research programmes to investigate the property changes from fast particle radiation and to predict their influence on the safety and the lifetime of graphite reactors to be built.

[12]: 47 Nuclear graphite for the UK Magnox reactors was manufactured from petroleum coke mixed with coal-based binder pitch heated and extruded into billets, and then baked at 1,000 °C for several days.

To reduce porosity and increase density, the billets were impregnated with coal tar at high temperature and pressure before a final bake at 2,800 °C.

Instead, a massive power excursion (exacerbated by the high and positive void coefficient of the RBMK as it was designed and used at the time) during a mishandled test caused the catastrophic failure of the reactor vessel and a near-total loss of coolant supply.

As a result, the graphite moderator caught fire, sending a plume of highly radioactive fallout into the atmosphere and over a very widespread area.