The density of uranium metal changes markedly with temperature. Table 5.6 summarizes densities inferred from x-ray diffraction data. The large density change at 662°C accompanying transition to the beta phase makes it undesirable to operate uranium metal reactor fuel above this temperature. Even at the lower temperatures at which the alpha phase is stable, its large density change with temperature, and its unequal temperature coefficient of thermal expansion along the three crystal axes (+19, —0.8, and +20 X 10’6/°C at 25°C) cause severe distortion and elongation of fuel assemblies during temperature cycling unless the fuel is given special treatment prior to irradiation.

Table 5.5 Phases of uranium metal

Transition

temperature, °С Phase Crystal system

Solid a Orthorhombic Solid jS Tetragonal

Solid 7 Cubic

Liquid Vapor