Many of the properties that make graphite attractive for a particular application have been discussed above. However, the following characteristics have been ascribed to synthetic, polygranular graphite6 and are those properties that make graphite suitable for its many applications: chemical stability; corro­sion resistance (in a nonoxidizing atmosphere); non­reactive with many molten metals and salts; nontoxic; high electrical and thermal conductivity; small ther­mal expansion coefficient and consequently high thermal shock resistance; light weight (low bulk den­sity); high strength at high temperature; high lubric­ity; easily dissolved in iron, and highly reductive; biocompatible; low neutron absorption cross-section and high neutron-moderating efficiency; resistance to radiation damage. The latter properties are what make graphite an attractive choice for a solid moder­ator in nuclear reactor applications.

Nuclear applications, both fission and fusion (of keen interest the reader), are described in detail in Chapter 4.10, Radiation Effects in Graphite, and Chapter 4.18, Carbon as a Fusion Plasma­Facing Material. Accounts of nuclear applications have also been published elsewhere.9, — 0 Graphite is used in fission reactors as a nuclear moderator because of its low neutron absorption cross-section and high neutron moderating efficiency, its resistance to radia­tion damage, and high-temperature properties. In fusion reactors, where it has been used as plasma facing components, advantage is taken of its low atomic number and excellent thermal shock characteristics.

The largest applications of nuclear graphite involve its use as a moderator and in the fuel forms of many thermal reactor designs. These have included the early, air-cooled experimental and weapons materials producing reactors; water-cooled graphite-moderated reactors of the former Soviet Union; the CO2-cooled reactors built predominantly in the United Kingdom, but also in Italy and Japan; and helium-cooled
high-temperature reactors, built by many nations, which are still being operated in Japan and designed and constructed in China and the United States. All of the high-temperature reactor designs utilize the ceramic Tri-isotropic (TRISO) type fuel (see Chapter 3.07, TRISO-Coated Particle Fuel Performance), which incorporates two pyrolytic graphite layers in its form. Graphite-moderated reactors that were molten-salt cooled have also been operated.