CORROSION OF USED NUCLEAR-FUEL IN AQUEOUS PERCHLORATE AND CARBONATE SOLUTIONS

The corrosion of used fuel was investigated using electrodes construct ed from fuel pins discharged from the Pickering, Bruce and Darlington CANDU reactors, and compared to the corrosion behaviour observed on un irradiated UO2 and SIMFUEL. Experiments were carried out in solutions of NaClO4 (pH similar to 9.5) in the presence and absence of (a) subst antial concentrations of sodium carbonate, and (b) additional external gamma fields. Used fuel electrodes reached oxidizing corrosion potent ials (E(CORR)) rapidly compared with unirradiated UO2 electrodes. Howe ver, optical and SEM examinations showed no evidence for rapid oxidati ve dissolution. This reaction,expected to be fast since high values of E(CORR) are observed, appears to be blocked by the accumulation of se condary phases in grain boundaries. The oxidation and dissolution beha viour of used fuel is determined predominantly by (i) the dose rate in solution near the fuel surface, (ii) the extent of burnup (which dete rmines the degree of fission product doping), and (iii) the degree of non-stoichiometry.