Dissolution study of spent PWR fuel: Dissolution behavior and chemical properties of insoluble residues

The dissolution behavior of PWR spent nuclear fuels of 7000 to 39000 MWd/t and the chemical properties of fission product insoluble residues obtained by the nitric acid dissolution of the fuels were investigated. UO 2 pellets in the irradiated fuel rods (10.7 mm in diameter) sliced to 3 to 5 mm in length were completely dissolved within 2 h in 3M nitric acid at about 100°C, regardless of their burnups. The amount of insoluble residue remaining after dissolution increased linearly with burnup from 7000 to 30000 MWd/t, and above 30000 MWd/t it increased steeply. About 70% of the insoluble residue was composed of fission products such as molybdenum, technetium, ruthenium, rhodium and palladium. The remainder was fine chips of cladding, etc. The relative ratio of these elements in the insoluble residue was different from that in the spent fuel based on calculation. In insoluble residues only hexagonal ruthenium alloy (ϵ-phase) was identified.