R&D works on the spent fuel criticality safety, including burnup credit, are ongoing in Switzerland in relation to the preparation of a general license application for a deep geological repository, which is led by NAGRA (Swiss National Technical Competence Centre in the field of deep geological disposal of radioactive waste) in cooperation with the Paul Scherrer Institute (PSI). The task of PSI in this joint project is deriving loading curves for a reference canister design provided by NAGRA, intended to cover, ultimately, the entire park of the fuel assemblies operated and currently stored in Switzerland as well as the component of spent fuel foreseen for the future NPP's operation. Comparing to existing methodologies, one can characterize the approach currently chosen by PSI as a combination of the best estimate plus uncertainty (BEPU) concept with certain bounding conservative assumptions and conditions. Regarding the uncertainty assessments, the presently identified major sources of the uncertainties include the nuclear data and the spent nuclear fuel composition affected as well by the nuclear data uncertainties and also by operating conditions for every particular fuel assembly. A critical feature of the modern burnup credit methodologies is that the benchmarks available for validation of the criticality calculations were performed with fresh fuel. Therefore, it is important to consider whether the available benchmarks are representative for the final methodology application for systems with spent nuclear fuel and also accurately assess the uncertainties related to the spent fuel nuclear data. For that purpose, the benchmark-to-application system similarity assessment is under introduction into the PSI criticality safety evaluation methodology for the spent fuel geological repository, called BUCSS-R. The present paper describes the current status of the BUCSS-R methodology with the focus on its BEPU specific characteristics.