Anodic oxidation and dissolution of CANDU fuel (UO 2) in slightly alkaline sodium perchlorate solutions

Abstract

The anodic dissolution of UO 2 in aqueous sodium perchlorate solutions at pH∼9.5 shows two distinct regions of oxidation/dissolution behavior. In the potential ( E) range 0.10 E<0.35 V (vs SCE) currents decay continuously with time owing to the formation of oxidized surface films which gradually block further oxidation of the electrode. An analysis of anodic and cathodic charges ( Q A, Q C, respectively) obtained by integration of the anodic current–time plots ( Q A) and cathodic 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 UO 2.67 or, more probably, UO 3· xH 2O. These may be located primarily at grain boundaries, although this has not been confirmed. For E>0.35 V, steady-state currents are obtained and measurements of 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 uranyl species leads to the development of acidic conditions in the grain boundaries. At these lower pH values UO 3·2H 2O is soluble and does not accumulate. Analyses of Tafel slopes suggest that the observed behavior is consistent with dissolution underacidic conditions, although other possibilities have not been excluded.