The disposal of spent nuclear fuel (SNF) in deep geological formations in combination with a resilient multi-barrier concept is the preferred option for the safe isolation of highly radioactive wastes in Germany, as well as several other countries (e.g., Belgium, Finland, Sweden, Switzerland). Nevertheless, an intrusion of ground water, intertwined with the failure of canisters and loss of SNF cladding integrity must be considered in the long-term evaluation of a deep geological repository. An assessment of SNF performance in the repository system requires a thorough process understanding of the dissolution rates, the individual radionuclide source terms as well as the alteration processes of the waste form. The dissolution process of SNF can be described in two steps: (i) a fast, initial release of radionuclides, which segregated to accessible structures of the SNF during reactor operation (ii) a slower, long-term release, originating from the dissolution of the fuel matrix itself.

In the present study, we show results obtained from ongoing leaching experiments with medium (46.9 GWd/t_HM) and high burn-up (50.4 GWd/t_HM) UO_X SNF, in highly alkaline, simulated cement water solutions under anoxic and reducing conditions induced by H₂ overpressure. Both SNF were irradiated in commercial nuclear power plants in Germany and Switzerland during the 1970s and 1980s. For the volatile radionuclides, such as the fission gases, ¹²⁹I and ¹³⁷Cs, a rapid, initial release is observed, in comparison to radionuclides assigned to the SNF matrix, e.g., ⁹⁰Sr, ²³⁸U or ²³⁹Pu. However, the initially rapid release of volatile fission products significantly slows down throughout the experiments, unaffected by neither the pH of the leachant solution nor the presence of reducing H₂, albeit a continuous release is observed. In addition, the data obtained in the current leaching experiments are compared to previous experiments conducted at KIT-INE with UO_X and (U,Pu)O_X fuels.

Spent Nuclear UOX fuel (SNF) leaching experiments were conducted in order to investigate the (fast) release of some of the most critical radionuclides with respect to long-term safety. Previously, samples with a burnup of 55 MWd.kg^-1HM have been leached in the bicarbonate solution (pH ≈ 9) used as reference leaching medium in the framework of the “FIRST-Nuclides” European program. These experiments were conducted without hydrogen, in anoxic conditions. More recently, samples of the same fuel were leached in the same type of bicarbonate solution and in a highly alkaline solution (pH 13.5) under reducing conditions imposed by hydrogen, using pressurized autoclaves at 40 bar. The latter experimental conditions are representative of a deep geological repository conditioned by the presence of cementitious materials imposing a high pH. In presence of hydrogen, the uranium concentration remained stable around 10^-7 M, whereas in anoxic conditions the concentrations increased with time. The Tc concentration was initially lower with hydrogen than in anoxic conditions in bicarbonate solution, but increased with time in reducing conditions to reach similar concentrations. A stable Tc concentration was reached with hydrogen only at high pH. The leached fraction of Sr in bicarbonate solution was higher in anoxic conditions than in reducing conditions, and (in reducing conditions) higher in bicarbonate than in the high pH solution.The released fractions of Cs and I were similar in anoxic and reducing conditions in bicarbonate solution and similar in bicarbonate and high pH solution in reducing conditions. The leached fraction of iodine was similar or slightly lower than the total fission gas release including the fission gas release during the leaching in reducing conditions, but this could not be confirmed for anoxic conditions.

This work presents experimental data of the release of Mo, Tc, Rh and Ru metallic particles from high-burnup spent nuclear fuel (63 MWd/kgU) at two different pH values, 8.4 and 13.2. The release of these elements from SF to the solution is around two orders of magnitude higher at pH=13.2 than at pH=8.4. The high Mo and Tc release at high pH would indicate that both elements would not be congruently released with uranium, as it has been pointed out in some release experiments, and would have an important contribution to the IRF, with values around 5%. On the other hand, Ru and Rh release could be explained by oxidation processes favoured at high pH.

The high release of such elements at high pH could be the consequence of the dissolution of the metallic inclusions contained in the fuel through an oxidative dissolution mechanism. Experimental data has been treated by a semi empirical model to evaluate the relative importance of the contribution of different sources on the release of Mo, Tc, Ru and Rh to deduce both the localization in the fuel and the oxidation state of the elements released to the solution as a function of time.

Understanding the leaching behavior of spent nuclear fuel is crucial for the safety assessment of deep geological repositories where spent nuclear fuel will be disposed. Consequently, numerous studies have been carried out on UO₂-based fuels aiming to determine dissolution rates as well as understanding the dissolution mechanisms. However, new types of nuclear fuels containing additives are currently being introduced in commercial reactors to improve reactor performance and reduce fuel cycle costs. Before their use on a larger scale, these fuels must be shown to be acceptable as a waste form for direct disposal in the intended repository environment. These new fuels with additives such as chromia (Cr₂O₃) have an impact on the UO₂ microstructure, e.g., enlarging fuel grain size, which might affect properties relevant to the safety assessment.

The main goal of this investigation is to gather data on the leaching behavior of fuels doped with Cr₂O₃ under relevant repository conditions. A sample consisting of spent fuel fragments is leached inside an autoclave in simplified, synthetic granitic groundwater (10 mM NaCl and 2 mM NaHCO₃) under H₂ overpressure at Studsvik’s Hot Cell Laboratory. The concentration of radionuclides of interest in the aqueous solution is monitored for 1 year as a function of time by sampling and measurement by Inductively Coupled Plasma Mass Spectrometry. In addition, the composition of the gas phase is analyzed by Gas Mass Spectrometry to detect potential air intrusion and monitor the release of fission gas from the fuel. The fuel sample was irradiated in a commercial pressurized reactor (PWR) to a local burnup of 59 MWd/kgU. The leaching data from the Cr₂O₃-doped fuel experiment is presented and compared to commercial standard UO₂ fuel.