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

Abstract

Alpha-radiolysis / 238 Pu-doped UO 2 / Electrochemistry / Corrosion potential / Dissolution rate 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 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 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 deaerated 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 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.

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

“…Recent corrosion potential (E corr ) measurements obtained with 238 Pu-doped UO 2 electrodes [6] and published E corr data obtained with UO 2 electrodes in the presence of external alpha sources [2] were compared. Results showed that the E corr values for 238 Pu-doped electrodes were similar to the published values obtained with undoped UO 2 electrodes, for the same alpha source strengths [7].…”

“…Recently, MPM predictions have been compared with various independently obtained national and international experimental data (including both electrochemical E corr values and UO 2 dissolution rates) in an attempt to validate the MPM [16]. The initial E corr data measured with 238 Pu-doped UO 2 electrodes [6] were also included in the verification. Results suggested that the MPM predictions are generally in good agreement with experimental observations, especially under more oxidizing conditions.…”

Radiation Effects on Materials Used in Geological Repositories for Spent Nuclear Fuel

“…Several different types of materials have been manufactured and tested in dissolution experiments. UO 2 doped with α-emitting nuclides [51][52][53][54][55][56][57][58] has been used to mimic the radiation field from spent nuclear fuel older than 1000 years. UO 2 has been doped with Pd particles [59,60] to mimic the effect of ε-phase particles (fission products forming noble metal inclusions in the fuel matrix) and with rare earth oxides [61][62][63] to mimic the effect of fission products forming oxides in the fuel matrix.…”

Radiolytic oxidation of UO2 pellets doped with alpha-emitters (238/239Pu)

“…The reactivity of a polycrystalline UO 2 surface under alpha irradiation in contact with water has been widely investigated experimentally in recent years [1][2][3][4][5][6][7][8][9][10][11][12][13][14], motivated by the prospect of direct disposal of spent fuel in a deep geological repository. In this regard it is worth remembering that over the long term, alpha radiolysis of water, which produces oxidizing and reductive species, radicals (HO • , O 2…”

“…• external irradiation of a UO 2 /water interface by a He 2+ particle beam generated by a cyclotron [8,9] or using alpha sources [2][3][4]; • or the use of UO 2 matrices doped with alpha-emitters ( 238/239 Pu, 233 U) to reproduce the evolution of alpha activity in spent fuel in a nuclear waste repository [1,[5][6][7][10][11][12].…”