SF-ALE report: Report – Phase I

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Initiated in 2015, a research program has been defined in collaboration with KIT-INE and ONDRAF/NIRAS in order to assess the dissolution of the spent fuel matrix and the fast release of a selection of relevant radionuclides from UOX spent nuclear fuel in a deep geological environment, more specifically under conditions representative for the Supercontainer design, as part of the current long-term management strategy for spent nuclear fuel in Belgium. Two critical parameters are investigated: a highly alkaline environment (Young Cement Water, pH 13.5) caused by the presence of concrete in the Supercontainer design, and reducing conditions imposed by the presence of hydrogen from the corrosion of iron-based materials present in the engineered barriers. Because the control of the hydrogen partial pressure during the experiment and the measurement of the fission gas release are fundamental, the experiments are conducted using autoclaves. The influence of hydrogen is evaluated by imposing different partial hydrogen pressures. Apart from the Young Cement Water (YCWCa), tests are also performed with the bicarbonate solution that was used as leaching medium in the European program FIRST-Nuclides (2012-2015) for samples of the same spent nuclear fuel rod under anoxic conditions [1]. The same type of leaching solution is used here under reducing conditions in order to link the two series of experiments and to help in understanding the leaching behavior. Initially planned for a total duration of three years, three leaching experiments are thus conducted under 40 bar, using (1) a mixture of Ar/4%H2 with YCWCa as the leaching medium, (2) Ar/0.75%H2 with YCWCa, and (3) Ar/0.75%H2 with the bicarbonate solution. The experimental program is divided in two phases with a transfer of the SNF samples into new autoclaves at mid-term. Some delay occurred in Phase I, started in September 2018 and only completed in October 2020, because of the pandemic situation and the associated work restrictions. This report provides details on the SNF samples used in the leaching experiments (radionuclide inventory, radiation history...), a description of the experimental setup, the data treatment and a summary of the results obtained in Phase I. Over a total duration of 733 days, 6 samplings of the gas phase and leaching solution were performed and about 40 RN’s were measured. Only a selection of RN’s is reported in this report, especially those that are common with KIT-INE [2] for the comparison of the results, but the full dataset will be made available once the program is fully accomplished. This report includes only the data of the first phase and is thus incomplete. Therefore, the discussion is focused on the release of the accessible fraction of the inventory. A more general discussion, including the matrix dissolution, will be performed once all the data is available and will be part of the final report of Phase I and Phase II in 2023.
Original languageEnglish
PublisherSCK CEN
Number of pages49
StatePublished - 15 Sep 2022

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NameSCK CEN Reports
PublisherSCK CEN

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