Influence of Radiolysis on UO 2 Fuel Matrix Dissolution Under Disposal Conditions

The objective of this study was to examine the recent published literature on the influence of water radiolysis on UO2 fuel matrix dissolution under the disposal conditions. The -radiation is considered to be dominating over the other types of radiations at times longer than 1000 years. The presence of the anaerobic corrosion products of iron, especially of hydrogen, has been observed to play an important role under radiolysis conditions. It is not possible to exclude gamma/beta radiolysis effects in the experiments with spent fuel, since there is not available a fuel over 100 years old. More direct measurements of -radiolysis effects have been conducted with -doped UO2 materials. On the basis of the results of these experiments, a specific activity threshold to observe -radiolysis effects has been presented. The threshold is 1.8 · 10 to 3.3 · 10 Bq/g in anoxic 10 M carbonate solution. It is dependent on the environmental conditions, such as the reducing buffer capacity of the conditions. The results of dissolution rate measurements at VTT with U-doped UO2 samples in 0.01 to 0.1 M NaCl solutions under anoxic conditions did not show any effect of -radiolysis with doping levels of 5 and 10% U (3.2 · 10 and 6.3 · 10 Bq/g). Both Fe and hydrogen can act as reducing species and could react with oxidizing radiolytic species. Fe concentrations of the order of 10 M can decrease the rate of H2O2 production. Low dissolution rates, 2 · 10 to 2 · 10 /yr, have been measured in the presence of metallic Fe with 5 and 10% U-doped UO2 in 0.01 to 1 M NaCl solutions. The tests with isotope dilution method showed precipitation phenomena of U to occur during dissolution process. The concentrations of dissolved U were extremely low ( 8.4 · 10 M). No effects of -radiolysis could be seen. It is difficult to distinguish the effects of metallic Fe, Fe or hydrogen in these tests. Hydrogen could also act as a reducing agent. Interaction tests between U(VI) and Fe have shown the reduction of U(VI) hydroxide complexes to take place by Fe in aqueous solution. A decrease in corrosion and release rates has been observed in the experiments with spent fuel and alpha-doped UO2 in the presence of hydrogen. Hydrogen seems to suppress the oxidation and dissolution of UO2. The H2 concentration of around 10 M seemed to be a threshold for oxidation of UO2 to occur in the tests with 10% U-doped UO2 in 10 M NaCl (+2 · 10 M NaHCO3) solution. Dissolved hydrogen concentrations from the anaerobic corrosion of iron are estimated to be in the 10 to 10 M range. High burnup fuel did not seem to display any enhanced dissolution in the presence of 3.3 · 10 M H2. The level of U concentrations in solution has been very low in the presence of H2, ~ 10 M, lower than the solubility of amorphous UO2. The dissolved U concentrations at the same level have been measured in the presence of metallic Fe. A hypothesis of -radiation induced hydrogen annealing, or a crystallization process in the amorphous phase has been presented in literature to explain low U concentrations. A hypothetical mechanism to explain the effects of hydrogen under radiolysis conditions has been presented. The observations suggest that the consumption of oxidants is a surface-catalyzed process.