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Technical paper submitted for peer-review, accepted, and presented at the 2021 Topfuel conference (October 24-28, 2021). The conference was hosted as a hybrid event with on-site attendance (Santander, Spain) and remote attendance via a web application. ---Abstract--- In the context of interim or long-term storage of spent nuclear fuel, as well as in final disposal scenarios it is crucial to understand and accurately predict the materials��� behavior, specifically towards the release of radionuclides into the environment. Within the Spent Fuel Autoclave Leaching Experiments (SF-ALE) project at SCK CEN an improved experimental setup for performing leaching studies was developed, and six experiments (three on UOX fuel, three on MOX fuel) are being conducted in parallel over a timeframe of 36 months. To allow an accurate interpretation of the leaching data, depletion calculations were performed to establish the nuclide inventory of the MOX fuel samples at the time of the experiments. Preliminary results of the fission gas release measured during the leaching experiments are reported and discussed.