Graphite is the carbon allotrope used as a neutron moderator in several nuclear reactors.
A moderator slows fast neutrons released by fission, making them more likely to cause further fission in fuels such as uranium-235. Graphite works because carbon nuclei can reduce neutron speeds efficiently while absorbing relatively few neutrons. Its high-temperature stability and useful mechanical properties also make it suitable for reactor cores.
Graphite moderation made it possible for some reactor designs to use natural uranium, which contains only about 0.7% uranium-235. The first artificial nuclear reactor, Chicago Pile-1, used graphite blocks and achieved a self-sustaining chain reaction under Enrico Fermi’s team in 1942. Later graphite-moderated designs included gas-cooled reactors and the Soviet RBMK family.
Graphite is not the same as amorphous carbon, diamond, or fullerene: all are carbon forms, but their structures and properties differ greatly. Diamond is extremely hard and is not the standard moderator. Graphite can also burn under the wrong conditions, a fact that became significant during the 1986 Chernobyl disaster, when exposed graphite contributed to the spread of radioactive material.