Study on Sodium Extraction and Transport at Monju

Abstract The prototype fast breeder reactor “Monju” is a loop-type, sodium-cooled nuclear reactor in Japan. Its operator, Japan Atomic Energy Agency (JAEA) started decommissioning of Monju in 2018, and the first phase of the decommissioning was completed in FY2022 when all the fuel assemblies were removed from the reactor core and Ex-vessel storage tank (EVST) and transferred to the spent fuel pool (SFP). In FY2023, we started the second phase, and are currently working to remove neutron shields and other core elements from the reactor core and EVST, to put them in SFP. We are also dismantling water and steam system such as turbine generator, condensers and feed water heaters, etc. MONJU has a primary sodium cooling system for the reactor core and its fuel, and a secondary sodium system that transferred heat. Since the sodium in the primary system is radioactive while the sodium in the secondary system is not, we had developed plans to drain/extract non-radioactive sodium first. According to these plans, we defined the sodium extraction area first, to set up International Organization for Standardization (ISO) tanks for non-radioactive sodium and developed a route for extracting sodium from the existing tanks to the ISO tanks. New piping for these operations will be installed, so that the sodium can be extracted using existing gas systems and electromagnetic pumps. We are ready to take appropriate measures to prevent sodium leakage and continue to develop methods, ensuring safety, to achieve efficient extraction and transport of the sodium.