Atomic Energy Society of Japan

Dismantling in the radiation-controlled area is planned in phase 3 of the “Monju” decommissioning program, but an assessment of contamination of the facilities is essential before starting the dismantling.The unique Ex-Vessel Storage Tank (EVST) unique to "Monju" stores fuel in liquid sodium at 200℃. It requires an evaluation of radioactivity due to neutron-induced structural material activation. The neutron flux distribution and radioactivity of the EVST were assessed by calculation using a 2D RZ system, validating the results through comparison with a 3D system. The assessment confirms that the maximum concentration of activity of activation product in the EVST structural material is at L3 level and generally below clearance level in most areas.