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The denominated instant release fraction (IRF) is considered in performance assessment (PA) exercises to govern the dose that could arise from the repository. A conservative definition of IRF comprises the total inventory of radionuclides located in the gap, fractures, and the grain boundaries and, if present, in the high burn-up structure (HBS). The values calculated from this theoretical approach correspond to an upper limit that likely does not correspond to what it will be expected to be instantaneously released in the real system. Trying to ascertain this IRF from an experimental point of view, static leaching experiments have been carried out with two commercial UO2 spent nuclear fuels (SNF): one from a pressurized water reactor (PWR), labelled PWR, with an average burn-up (BU) of 52 MWd·kgU

One of the mechanisms that may decrease the mobility of cesium released from spent fuel in a high level nuclear waste repository (HLNW) is its sorption onto uranyl-containing alteration phases formed on the spent fuel surface such as studtite (UO2O2·4H2O). The results obtained in this work show that sorption is a very fast process; cesium in solution is sorbed in less than one hour at pH 5. Sorption as a function of initial concentration in solution was also studied between initial cesium concentrations ranging from 7.6×10−9 mol dm−3 to 1.0×10−3 mol dm−3. The data have been modelled considering a Freundlich isotherm, with K F and n values of 10±1, and 1.4±0.1, respectively (r 2=0.998). Sorption is very dependent on ionic strength, suggesting that cesium sorbs onto studtite by forming an outer-sphere complex involving electrostatic interactions. Sorption is observed to be very low at acidic pH, while relatively high at alkaline pH ( i.e. , almost 60% of the total cesium concentration in solution is sorbed at pH>9). The results point to the importance of sorption processes on uranyl alteration phases on the retention of radionuclides.

The instant release fraction of a spent nuclear fuel is a matter of concern in the performance assessment of a deep geological repository since it increases the radiological risk. Corrosion studies of two different spent nuclear fuels were performed using bicarbonate water under oxidizing conditions to study their instant release fraction. From each fuel, cladded segments and powder samples obtained at different radial positions were used. The results were normalised using the specific surface area to permit a comparison between fuels and samples. Different radionuclide dissolution patterns were studied in terms of water contact availability and radial distribution in the spent nuclear fuel.The relationship between the results of this work and morphological parameters like the grain size or irradiation parameters such as the burn-up or the linear power density was studied in order to increase the understanding of the instant release fraction formation.

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