Metallic phases precipitated in UO 2 fuel : II. Insoluble residue in simulated fuel

Abstract

The formation and the chemical properties of the insoluble residue in simulated spent fuels were studied. Simulated fuels corresponding to burnups of 5 to 30% FIMA were prepared by adding non-radioactive FP elements to uranium in nitrate solution, evaporating the mixed solution to dryness and heating the pellet at 1273 to 2273 K under various oxygen potentials. The insoluble residue was obtained at the dissolution of the simulated fuel with 3M HNO 3. The chemical composition of the residue was determined by chemical analysis and the phases in the residue were identified by X-ray diffraction. The insoluble residue was composed mainly of Mo and Ru with minor amounts of Pd and Rh. At some conditions Zr and Sn were also included. The amount of residue was less than 1 wt% up to a 10% FIMA burnup, but increased markedly at a higher burnup. The amount of residue decreased at a temperature higher than 1873 K. This was attributed to the fact that the metallic α-phase formed at high temperature was apt to be easily dissolved in HNO 3. The amount of metallic residue and Mo/Ru ratio decreased with the increase in oxygen potential. The oxygen potential for Mo-MoO 2 equilibrium may be related to this result. Formation of the Zr-molybdate hydrated oxide compound, ZrMo 2O 7 (OH) 2(H 2O) 2, was observed only in the residue at high burnups and high oxygen potentials. This phase was possibly formed during the dissolution process.