Nuclear materials I Fuels 1.1 Uranium Uranium in one form or another is by far the most common fuel materials for nuclear reactors. (By comparison, the use of thorium and plutonium has so far been on a very small scale. It can be used either as pure uranium, a metal, or as compound such as uranium dioxide UO or uranium carbide UC Uranium is a rather soft and ductile metal which oxidizes readily in air and water at high temperature. Its melting point is 1133.C. It exists in one of three allotropic forms, depending on its temperature. These three different forms are called the alpha, beta and gamma phases and changes from one phase to another due to temperature changes are accompanied by density changes. Alpha phase uranium has a density of 19g/cmand a thermal conductivity which varies from 25W/mK at 25C to 42W/mK at 665]C. The transition from alpha to the beta phase takes place at 665C and is accompanied by dimensional changes in crystalline structure of the uranium, expansion along one axis and contraction along the others. To avoid distortion due to these anisotropic dimensional changes 665 C is considered to be the maximum operating temperature of uranium Metallic uranium is also very susceptible to radiation damage which produces dimensional changes and swelling above about 450.C. Consequently high burnups of metallic uranium fuel are not possible. In the British gascooled Magnox Reactors, which are the principal users of this type fuel, the burnup is limited to about 3500MWd/t To summarize, the low operating temperature, susceptible to radiation damage and low permissible burnup of uranium are serious disadvantages to its choice as a reactor fuel, and account for its very limited use Uranium dioxide UO2 is a black powder which can be fabricated by cold pressing and sintering at high temperature to produce small cylindrical pellets, and in this form it is by far the most common material for the fuel of commercial reactors In this ceramic form uO2 has good stability at high temperature and good resistance to rad iation damage which enables it to be used to high burnups. The melting point is 2865C and the theoretical density is 10.96g/cm, although in practice the density of UO2 pellets produced as described above is about 10g/cm. The thermal conductivity is low, being about 2. 5W/mK in the temperature range from 1000 to 2000C, however this low thermal conductivity is compensated for by the very high melting point which permits high maximum fuel temperatures Uranium dioxide does not react with water at high temperature, a very valuable characteristic as otherwise cladding failures in water cooled reactors would lead to serious reactions. It can retain a large fraction of the gaseous fission products at temperatures below 1000 C, but as the fuel temperature at the center of a pellet is likely to be greatly in excess of this value, prov ision must made for fission product gas release. This is usually done by having an empty space at the top of each fuel tube into which the gases can diffuse 4. During operation in reactor UO2 pellets suffer structural changes, principally as a result of ne high operating temperatures and high temperature gradients, but also as a result of prolonged irradiation. The effects may include swelling, formation of cracks and voids in the pellet and changes in the grain structure of the UO2. This type of fuel is normally subjected to much higher burnups than pure uranium, and 5 per cent or more of the orig inal uranium atomsNuclear Materials 1 Fuels 1.1 Uranium Uranium in one form or another is by far the most common fuel materials for nuclear reactors. (By comparison, the use of thorium and plutonium has so far been on a very small scale.) It can be used either as pure uranium, a metal, or as compound such as uranium dioxide UO2 or uranium carbide UC. Uranium is a rather soft and ductile metal which oxidizes readily in air and water at high temperature. Its melting point is 1133oC. It exists in one of three allotropic forms, depending on its temperature. These three different forms are called the alpha, beta and gamma phases, and changes from one phase to another due to temperature changes are accompanied by density changes. Alpha phase uranium has a density of 19g/cm3 and a thermal conductivity which varies from 25W/mK at 25oC to 42W/mK at 665oC. The transition from alpha to the beta phase takes place at 665oC and is accompanied by dimensional changes in crystalline structure of the uranium, expansion along one axis and contraction along the others. To avoid distortion due to these anisotropic dimensional changes 665oC is considered to be the maximum operating temperature of uranium. Metallic uranium is also very susceptible to radiation damage which produces dimensional changes and swelling above about 450oC. Consequently high burnups of metallic uranium fuel are not possible. In the British gascooled Magnox Reactors, which are the principal users of this type fuel, the burnup is limited to about 3500MWd/t. To summarize, the low operating temperature, susceptible to radiation damage and low permissible burnup of uranium are serious disadvantages to its choice as a reactor fuel, and account for its very limited use. Uranium dioxide UO2 is a black powder which can be fabricated by cold pressing and sintering at high temperature to produce small cylindrical pellets, and in this form it is by far the most common material for the fuel of commercial reactors. In this ceramic form UO2 has good stability at high temperature and good resistance to radiation damage which enables it to be used to high burnups. The melting point is 2865oC and the theoretical density is 10.96g/cm3 , although in practice the density of UO2 pellets produced as described above is about 10g/cm3 . The thermal conductivity is low, being about 2.5W/mK in the temperature range from 1000 to 2000oC, however this low thermal conductivity is compensated for by the very high melting point which permits high maximum fuel temperatures. Uranium dioxide does not react with water at high temperature, a very valuable characteristic as otherwise cladding failures in water cooled reactors would lead to serious reactions. It can retain a large fraction of the gaseous fission products at temperatures below 1000oC, but as the fuel temperature at the center of a pellet is likely to be greatly in excess of this value, provision must made for fission product gas release. This is usually done by having an empty space at the top of each fuel tube into which the gases can diffuse. During operation in reactor UO2 pellets suffer structural changes, principally as a result of the high operating temperatures and high temperature gradients, but also as a result of prolonged irradiation. The effects may include swelling, formation of cracks and voids in the pellet and changes in the grain structure of the UO2. This type of fuel is normally subjected to much higher burnups than pure uranium, and 5 per cent or more of the original uranium atoms