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.
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.
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.:80 Boron concentration in thermally purified graphite (such as AGOT graphite) can be less than 0.4 ppm:81 and in chemically purified nuclear graphite it is less than 0.06 ppm.