Hydrogen Inhibition Effect on Spent Nuclear Fuel Matrix Dissolution Under Anoxic/Reducing Atmosphere and Alkaline pH

In many countries (United States, Germany, China, etc.), final disposal of spent nuclear fuel (SNF) in deep geological repositories is the preferential option to permanently and safely isolate the radionuclide inventory from the biosphere. In post-closure safety assessments of such facilities, access of water to the emplaced SNF and the consecutive release of radionuclides from the waste is considered. The UO₂ matrix of the SNF retains many radionuclides, which can be released upon its dissolution. Therefore, an improved understanding on the interaction of SNF with pore water solutions under repository conditions is required in safety analyses of deep geological repositories.<br/>In this study, the release of radionuclides from spent UO₂ fuel under relevant geochemical conditions, representative of the Belgian “Supercontainer concept” were investigated. Three leaching experiments with SNF cladded pellets were performed in highly alkaline (cementitious) solution and anoxic/reducing atmosphere using autoclaves to analyse the impact of the presence of hydrogen and how influences the hydrogen partial pressure on the release of relevant fission products and actinides.<br/>The SNF specimens, comprising one full and two half pellets in each experiment, were sampled from a fuel rod with an average burn-up of 50.4 MWd/kg_HM. The fuel rod was irradiated in the Gösgen pressurised water reactor in Switzerland. Each experiment was divided in two stages, replacing the previous titanium liners and using fresh solution when starting the new stage. The experiments were conducted under different gas atmospheres and total pressures. All other experimental conditions were the same.<br/>- 40 bar of an Ar/H₂ gas mixture with a H₂ partial pressure of 3.2 bar in experiment 40/3.2H.<br/>- 3.75 bar of an Ar/H₂ gas mixture with a H₂ partial pressure of 0.3 bar in experiment 3.75/0.3H.<br/>- 1.0 bar Ar atmosphere (in absence of H₂) during the first stage of experiment 1-40/0; in this experiment the Ar pressure was raised to 8 bars after 497 days until termination of the first stage, 288 days later. Then, the experiment was conducted for another 603 days during the second stage at 40 bar Ar atmosphere.<br/>The findings demonstrate a hydrogen inhibition effect on the dissolution of the fuel matrix (as displayed by an inhibited release of U-238) and on the release of mainly matrix bound radionuclides e.g., Sr-90. Measured uranium and strontium concentrations are lower in the two experiments with hydrogen overpressure compared to respective results of the experiment under solely anoxic conditions. A comparison of the measured U-238 concentrations in the current experiments with calculated solubility limits of respective U(IV) and U(VI) solids indicate that U-238 concentrations approach towards the solubility limit of UO₂(am, hyd) in presence of hydrogen. The observed concentration levels and kinetic data show that the presence of hydrogen inhibits the dissolution kinetics of SNF and favours the formation of a U(IV) solid phase. At the end of the experiments, the Cumulative Fraction of Inventory in Aqueous Phase (CumFIAP) values of Sr-90 are similar to the CumFIAP of U-238. It is concluded that the strontium release behaviour is coupled to the dissolution of the UO₂ matrix.<br/>In the three experiments the CumFIAP values of Cs-137 are several orders of magnitude higher than the respective values of U-238. During the experiments, there was no pronounced effect of the redox conditions on the Cs-137 concentrations or release rates observed.<br/>The fission gases Kr and Xe are continuously released throughout the experiments, too. An effect of the redox conditions on the fission gas release was not detected.