The effects of alpha-radiolysis on UO2 dissolution determined from electrochemical experiments with 238Pu-doped UO2

The long-term stability of spent nuclear fuel under deep geologic repository conditions will be determined mostly by the influence of α-radiolysis, since the dose-rate for α-radiolysis will exceed that for γ/β-radiolysis beyond a fuel age of ∝ 100 years and will persist for more than 10 000 years. Dissolution rates derived from studies with currently available spent fuel include radiolysis effects from γ/β- as well as α-radiolysis. The use of external α-sources and chemically added H2O2 in UO2 studies has provided much information on the basic chemistry/electrochemistry of α-radiolysis effects on UO2 fuel dissolution. Here, alpha-radiolysis effects were measured directly by performing electrochemical experiments with α-doped UO2. The 238Pu-doped UO2 pellets, produced by milling, pressing and sintering, had activities of approximately 0.01 and 0.1 Ci/g. Discs cut from these pellets were fabricated into electrodes. Corrosion potentials (Ecorr) were measured for these electrodes, in glass cells containing deaerated 0.1 M NaClO4 (pH 9.5) for periods up to 1600 h. The effects of carbonate and Fe(II) on Ecorr values were also investigated. Preliminary results showed that Ecorr values correlated with doping levels, and were slightly higher than Ecorr values obtained with UO2 electrodes in the presence of external α-sources. The presence of carbonate reduced Ecorr values considerably. Addition of Fe(II) caused generally small reductions in Ecorr values, suggesting that Fe(II) affects the dissolution process, either by scavenging radiolytic oxidants, reducing oxidized surface states, or reducing dissolved U(VI). These effects were often transient due to depletion of the added Fe(II). Accurate dissolution rates could not be determined from measured solution and surface concentrations because of material entering the solutions during the electroreduction procedure, probably due to the dissolution of fines present on the electrode surface after polishing. Experiments are continuing in order to confirm the preliminary findings, with greater attention being paid to determining accurate dissolution rates. (orig.)