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ABSTRACTThere are two units with RBMK-1500 type reactors at the Ignalina Nuclear Power Plant (Ignalina NPP) in Lithuania where graphite was used as a neutron moderator and reflector. These reactors are now being decommissioned, and Lithuania has to find a solution for safe irradiated graphite disposal. It cannot be disposed of in a near surface repository due to large amounts of 14C (radiocarbon, carbon-14); thus, a deep geological repository (DGR) is analyzed as an option. This study had the aim to evaluate 14C migration from the RBMK-1500 irradiated graphite disposed of in a potential DGR in crystalline rocks taking into account the outcomes of the research performed under the collaborative European project CAST (CArbon-14 Source Term) and to identify the potential to reduce the conservatism in the assumptions that was introduced in the lack of data and led in the overestimated 14C migration. The information gathered during the CAST project was used to model 14C transport in the near field by the water pathway and to perform uncertainty analysis. The study demonstrated that more realistic assumptions could reduce the estimated 14C flux from the near field by approximately one order of magnitude in comparison with the previous estimations based on very conservative assumptions.

Approximately 22 600 spent nuclear fuel (SNF) assemblies originating from the RBMK-1500 reactor of the Ignalina nuclear power plant in Lithuania need to be managed and disposed of safely. Generic investigations of RBMK-1500 SNF disposal options in Lithuania were initiated. This paper presents insights on RBMK-1500 SNF disposal in crystalline rocks gained during participation in the International Atomic Energy Agency Coordinated Research Project “The Use of Numerical Models in Support of Site Characterization and Performance Assessment Studies for Geological Repositories,” as well as in the Lithuanian Science Development Program. The research was focused on the analysis of disposal behavior of different SNF types under generic geological conditions and for a one-canister defect scenario with two different corrosion rates. A comparison of peak fluxes from the near field for Lithuanian RBMK-1500 and Swedish boiling water reactor SNF revealed differences that are not directly proportional to the differences in SNF inventory.