General Outline of the Decommissioning of the Prototype Fast Breeder Reactor MONJU

Abstract The prototype fast breeder reactor “MONJU” is a loop-type sodium-cooled nuclear reactor under decommissioning in Japan. Japan Atomic Energy Agency (JAEA) started its decommissioning in 2018, and will complete the four phases of decommissioning in about 30 years. The phase I is already over. It was the “Fuel unloading” stage during which secondary sodium was drained, the fuel assemblies in the reactor core were removed and put in storage in the spent fuel pool, while the distribution of radioactive contamination in buildings, components, equipment, etc. was evaluated. The phase II is ongoing. It is focused on the “Preparation for dismantling the plant”. We have been removing the neutron shields in the reactor core and putting them in storage in the spent fuel pool in preparation for the dismantling of the sodium-adhered equipment. We will also transport sodium, dismantle and remove the power-generating facility such as water/steam systems. In addition, we continue to assess the distribution of radioactive contamination in buildings, components, equipment, etc. The phase III will be started in 2032. This phase, the “Decommissioning Period I” is focused on the dismantling and removing of sodium-adhered equipment, the transport of spent fuel, and the removal of the water and steam power generation system. The final Phase is the “Decommissioning Period II” during which the radiation-controlled area will be lifted and all the buildings will be dismantled and removed. This paper presents an overview of MONJU decommissioning, the results of the phase I, and details of the phase II currently ongoing.