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

Abstract

Summary 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 10000 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 H 2 O 2 in UO 2 studies has provided much information on the basic chemistry/electrochemistry of α-radiolysis effects on UO 2 fuel dissolution. Here, alpha-radiolysis effects were measured directly by performing electrochemical experiments with α-doped UO 2 . The 238 Pu-doped UO 2 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 (E corr ) were measured for these electrodes, in glass cells containing de-aerated 0.1 M NaClO 4 (pH 9.5) for periods up to 1600 h. The effects of carbonate and Fe(II) on E corr values were also investigated. Preliminary results showed that E corr values correlated with doping levels, and were slightly higher than E corr values obtained with UO 2 electrodes in the presence of external α-sources. The presence of carbonate reduced E corr values considerably. Addition of Fe(II) caused generally small reductions in E corr 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 electro-reduction 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.