ANODIC-OXIDATION AND DISSOLUTION OF CANDU FUEL (UO2) IN SLIGHTLY ALKALINE SODIUM-PERCHLORATE SOLUTIONS

The anodic dissolution of UO2 in aqueous sodium perchlorate solutions at pH similar to 9.5 shows two distinct regions of oxidation/dissoluti on behavior. In the potential (E) range 0.10E < 0.35 V (vs SCE) curren ts decay continuously with time owing to the formation of oxidized sur face films which gradually block further oxidation of the electrode. A n analysis of anodic and cathodic charges (Q(A), Q(C), respectively) o btained by integration of the anodic current-time plots (Q(A)) and cat hodic potential scans to reduce accumulated oxidized surface films (Q( C)) in this range, shows that 90% of the anodic oxidation current goes to produce these films. The films blocking anodic dissolution appear to be either UO2.67 Or, more probably, UO3 . xH(2)O. These may be loca ted primarily at grain boundaries, although this has not been confirme d. For E>0.35 V, steady-state currents are obtained and measurements o f Q(A) and Q(C) show that the majority of the current goes to produce soluble species. It is proposed that, at the higher potentials, rapid oxidation and dissolution followed by the hydrolysis of dissolved uran yl species leads to the development of acidic conditions in the grain boundaries. At these lower pH values UO3 . 2H(2)O is soluble and does not accumulate. Analyses of Tafel slopes suggest that the observed beh avior is consistent with dissolution under acidic conditions, although other possibilities have not been excluded. Crown copyright (C) 1998 Published by Elsevier Science Ltd. All rights reserved.