High-temperature behavior of dicesium molybdate Cs2MoO4: Implications for fast neutron reactors

Dicesium molybdate (Cs2MoO4)׳s thermal expansion and crystal structure have been investigated herein by high temperature X ray diffraction in conjunction with Raman spectroscopy. This first crystal-chemical insight at high temperature is aimed at predicting the thermostructural and thermomechanical behavior of this oxide formed by the accumulation of Cs and Mo fission products at the periphery of nuclear fuel rods in sodium-cooled fast reactors. Within the temperature range of the fuel׳s rim, Cs2MoO4 becomes hexagonal P63/mmc, with disordered MoO4 tetrahedra and 2D distribution of Cs–O bonds that makes thermal axial expansion both large (50≤αl≤70 10−6 °C−1, 500–800 °C) and highly anisotropic (αc−αa=67×10−6 °C−1, hexagonal form). The difference with the fuel׳s expansion coefficient is of potential concern with respect to the cohesion of the Cs2MoO4 surface film and the possible release of cesium radionuclides in accidental situations.