in the fuel may undergo fission and be changed, each one to two intermed iate mass fission oduct atoms Uranium carbide, UC, is another ceramic fuel of possible interest, but it has not been developed or used to anything like the same extent as UO2. It may have some advantages over UO, principally its higher thermal conductivity and higher density which leads to mor uranium atoms per un it volume of fuel, which is an advantage in a reactor. Uranium carbide reacts with water which makes it unsuitable for use in water cooled reactors but it does not react with sodium below 500 C, so it might be used in fast reactors. Its melting point, 2380C is rather lower than that of UO2, but this is compensated for by it higher thermal conductivity The development of uranium carbide so far has been principally as the fuel for high temperature gas-cooled reactors 1.2 Plutonium Pure plutonium metal is not suitable as reactor fuel due to the large number of crystall ine phases which exist up to its melting point of 640oC. The thermal conductivity is also very low, about 4.2W/mK at room temperature. Plutonium metal is highly reactive in moist air, but it can be stored in dry air at low temperature. It is a very dangerous material, being rad ioactive, toxic and an essential component of nuclear weapons, and is potentially a serious health hazard, particularly if it exists as dust in the atmosphere and is taken into the lungs b inhalation As a reactor fuel plutonium is used as the oxide Puo2. Its melting point is 2400oC Plutonium dioxide is mixed with uranium dioxide to form mixed oxide fuel (MOX)which for fast reactors ty lly contains 20 to 25 per cent of PuO2. The properties of this mixed oxide fuel are similar to those ofuo2 alone 1.3 Thorium Thorium has not been used as a reactor fuel to any great extent yet except in a few high temperature gas-cooled reactors. Thorium 232 is the fertile isotope from which uranium 233 is produced, and it is theoretically possible to obtain high breed ing ratios in thermal as well as fast reactors using this combination. Pure metallic thorium has a melting point of about 1700oC, It is superior to uranium due to its better stability, but it is not used as a fuel in its pure form. Instead it is used either as thorium dioxide Tho2 or thorium carbide ThC2. To date these compounds have only been used to a very small extent in a few high temperature gas-cooled reactors Thorium dioxide is similar in many respects to uranium dioxide. It is produced by the same methods of powder metallurgy, and it is chemically inert and has a good resistance to radiation damage. Thorium carbide has been used in the form of coated particle fuel in HTGRS. Very small spherical particles less than 1 mm diameter of mixed ThC2 and UC2 (highly enriched in 235U)are coated with thin layers of pyrolitic carbon and silicon carbide to retain fission products. These particles are dispersed in graphite to form a homogeneous mixture of fuel and moderator which has a very high operating temperature and good resistance to rad iation damage 2 Moderators The requirements of the moderator for a thermal reactor, namely low mass number, very low neutron capture cross-section and high scattering cross-section, limit the choice to only a few materials. Hydrogen and its isotope deuterium, carbon and beryllium are the onlyin the fuel may undergo fission and be changed, each one to two intermediate mass fission product atoms. Uranium carbide, UC, is another ceramic fuel of possible interest, but it has not been developed or used to anything like the same extent as UO2. It may have some advantages over UO2, principally its higher thermal conductivity and higher density which leads to more uranium atoms per unit volume of fuel, which is an advantage in a reactor. Uranium carbide reacts with water, which makes it unsuitable for use in water cooled reactors, but it does not react with sodium below 500oC, so it might be used in fast reactors. Its melting point, 2380oC, is rather lower than that of UO2, but this is compensated for by it higher thermal conductivity. The development of uranium carbide so far has been principally as the fuel for high temperature gas-cooled reactors. 1.2 Plutonium Pure plutonium metal is not suitable as reactor fuel due to the large number of crystalline phases which exist up to its melting point of 640oC. The thermal conductivity is also very low, about 4.2W/mK at room temperature. Plutonium metal is highly reactive in moist air, but it can be stored in dry air at low temperature. It is a very dangerous material, being radioactive, toxic and an essential component of nuclear weapons, and is potentially a serious health hazard, particularly if it exists as dust in the atmosphere and is taken into the lungs by inhalation. As a reactor fuel plutonium is used as the oxide PuO2. Its melting point is 2400oC. Plutonium dioxide is mixed with uranium dioxide to form mixed oxide fuel (MOX) which for fast reactors typically contains 20 to 25 per cent of PuO2. The properties of this mixed oxide fuel are similar to those of UO2 alone. 1.3 Thorium Thorium has not been used as a reactor fuel to any great extent yet except in a few high temperature gas-cooled reactors. Thorium 232 is the fertile isotope from which uranium 233 is produced, and it is theoretically possible to obtain high breeding ratios in thermal as well as fast reactors using this combination. Pure metallic thorium has a melting point of about 1700oC. It is superior to uranium due to its better stability, but it is not used as a fuel in its pure form. Instead it is used either as thorium dioxide ThO2 or thorium carbide ThC2. To date these compounds have only been used to a very small extent in a few high temperature gas-cooled reactors. Thorium dioxide is similar in many respects to uranium dioxide. It is produced by the same methods of powder metallurgy, and it is chemically inert and has a good resistance to radiation damage. Thorium carbide has been used in the form of coated particle fuel in HTGRs. Very small spherical particles less than 1 mm diameter of mixed ThC2 and UC2 (highly enriched in 235U) are coated with thin layers of pyrolitic carbon and silicon carbide to retain fission products. These particles are dispersed in graphite to form a homogeneous mixture of fuel and moderator which has a very high operating temperature and good resistance to radiation damage. 2 Moderators The requirements of the moderator for a thermal reactor, namely low mass number, very low neutron capture cross-section and high scattering cross-section, limit the choice to only a few materials. Hydrogen and its isotope deuterium, carbon and beryllium are the only