Dopant solubility and lattice contraction in gadolinia and gadolinia-chromia doped UO2 fuels

Gadolinia doped UO2 fuel is widely used as burnable neutron absorber in Light Water Reactors to reduce power peaking and excess reactivity during the first reactor cycle of fresh fuel assemblies. The thermal conductivity of gadolinia doped fuel is substantially lower than that of standard UO2. To maintain safety margins later in life, some design or operating restrictions can be defined, for example to compensate higher fission gas release levels. Development of large grain U/Gd fuel by suitable doping, e.g. Cr2O3, could offer a solution to such restrictions, but solid state information about the double doped (U1-x-yGdxCry)O2 system is very scarce. In the present paper, we present X-ray diffraction and microstructure results of standard U/Gd fuel and chromia doped U/Gd fuel manufactured by powder metallurgy. The dissolution of chromium in (U1-xGdx)O2 as a function of Gd content, the role of free UO2 and the lattice contraction at different Gd and Cr doping levels of (U1-x-yGdxCry)O2 is studied both for the single doped and double doped system. On the basis of lattice contraction and precise measurements of the composition of the solid solution phases, the evolution of theoretical density with dopant concentration is derived.