Corrosion of used nuclear fuel in aqueous perchlorate and carbonate solutions

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