Waste Management of Small Modular Nuclear Reactors in Finland

Small modular nuclear reactors (SMRs) represent advanced technology in nuclear energy aim-ing to produce low carbon energy at smaller unit size and enhanced passive safety in compar-ison to traditional nuclear power plants (NPPs). The management of spent nuclear fuel (SNF) and low- and intermediate-level waste (LILW) from SMRs is an issue that needs to be resolved as part of any deployment of SMR technology in Finland. Currently, spent nuclear fuel from NPPs in Finland is planned to be disposed in the ONKALO® deep geological repository applying the KBS-3V disposal concept. This concept should be applicable for spent fuel from SMRs using light-water reactor (LWR) technology. However, there are some differences in the waste forms, most obviously the length of the fuel assemblies, but also in the spent fuel characteristics that need to be considered in the further development of the concept for spent fuel from SMRs. Preliminary 2D calculations were made with the continuous-energy Monte Carlo code Serpent to compare the spent fuel characteristics from two example LWR-SMRs to spent nuclear fuel from currently operating NPPs in Finland. In one example case, a NuScale Power ModuleTM was considered as it is one of the most advanced LWR-SMRs in the world. The other example case is an SMR planned in Finland for district heating purposes. The main differences between the SMR and NPP spent fuels are linked to lower burnups in the SMRs. Lower discharge burnups are to be studied further from the point of view of criticality safety at disposal. Other-wise, the lower average discharge burnup of these SMR fuel types, in principle, generally tends to make the handling of spent fuel assemblies less demanding with respect to the decay heat and ionizing radiation emitted from the assembly. However, rigorous calculation of the dose rates would require 3D calculations to determine the axial burnup distribution within a fuel as-sembly, which was outside the scope of this study. Published studies indicate that possibly larger masses (per GWe-year) of SNF and other HLW and larger volumes (per GWe-year) of LLW will be produced in a LW-SMR compared to a large NPP. However, because of the lower decay heat in the SMR SF (due to the lower burnup), less excavated volume and, consequently, less clay-based filling material (deposition tunnel back-fill) may be needed in a repository. Depending on the number of SMR units located at sites in Finland, the amounts of spent fuel and other waste streams can be relatively small so that a centralised waste management facility and repository could be the most feasible option for processing and disposal of all the nuclear waste. Alternatively, the wastes can be disposed of locally (near SMR sites in smaller facilities) or a hybrid model, where, e.g., only SNF is disposed centrally, could be considered. These alternatives will depend strongly on the ownership structure of the SMRs deployed in Finland. Local stakeholder and public opinion will be very important as well. Other issues, such as ge-ological suitability of the SMR sites for disposal, transport and interim storage will need to be assessed. In terms of final disposal of SNF from LWR-SMRs, the only currently available option is the KBS-3V concept, especially considering the state of the licencing process for this concept in Finland. Deep borehole disposal represents an intriguing, particularly in the case of local disposal for relatively small amounts of waste, but not yet fully developed alternative. The suitability of deep borehole disposal in the crystalline rock conditions prevailing in Finland will be studied in the next phase of the project. Spent fuel from non-LWR SMRs, i.e., high-temperature-gas-cooled, fast neutron-spectrum and molten salt-type SMRs, was also discussed briefly. Challenges were identified in the pre-treat-ments needed for SNF from these reactors prior to disposal including lack of suitable facilities in Finland and potential proliferation issues. In some cases, e.g., reactors with graphite mod-erators, the disposal of the LILW waste streams was considered problematic as the current methodologies in use in Finland for disposal of LILW would not be applicable. More extensive studies would be required to specifically identify the waste streams from non-LWR SMRs and how the waste characteristics would need to be taken into account for disposal