The first gas-cooled reactors with CARBON-DI-OXIDE (at a pressure of 16 bar) as coolant and GRAPHITE as moderator were developed in Britain during 1956-69. The fuel was a NATURAL URANIUM, clad with an alloy of magnesium called MAGNOX.
Several types of gas-cooed reactor have been designed and built, with England developing an ADVANCED GAS-COOLED REACTOR (AGR) SYSTEM, and GERMANY and the USA developing HELIUM-COLED, GRAPHITE-MODERATOR SYSTEM (HTGH). The AGR uses URANIUM OXIDE as the fuel clad in stainless steel tubes with CARBON-DI-OXIDE gas as coolant and graphite as moderator.
The graphite moderator HELLIUM-COOLED HTGR is designed to use U-233 as the fissile material and thorium as fertile material. Initially, the system would have to be fueled with U-235, until sufficient U-233 is available for makeup fuel. Because of the very high melting point of graphite, these fuel elements can operate at very high temperatures, and it is possible to generate steam at conditions equivalent to those in modern coal-fired power plant. The basic fuel forms are small spheres of fissile and fertile material as carbide, URANIUM CARBIDE AND THORIUM CARBIDE. The fissile spheres are 0.35 to 0.5 mm in diameter and the fertile spheres are 0.6 to 0.7 mm in diameter. Each spheres is coated with two or three layers of CARBON and SILICON CARBIDE to prevent fission products from escaping from the particles. HELIUM is a suitable coolant in sense that it is chemically INERT, as good heat transfer characteristic and low neutron absorption. Being an MONOATOMIC gas, it can produce for given temperature in the BRAYTON CYCLE and HIGH EFFICIENCY.
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