Characterization study of four candidate technologies for nuclear material quantification in fuel debris at Fukushima Daiichi Nuclear Power Station, 2; Numerical simulations for passive neutron technique

能見 貴佳 ; 長谷 竹晃 ; 小菅 義広*; 白茂 英雄 ; 浅野 隆 

Nomi, Takayoshi; Nagatani, Taketeru; Kosuge, Yoshihiro*; Shiromo, Hideo; Asano, Takashi

This report describes evaluation results of the passive neutron technique (PN) which is conducted as a part of the characterization study of candidate technologies for nuclear material quantification in fuel debris at Fukushima Daiichi Nuclear Power Station. The PN consists of Differential Die-away Self-Interrogation (DDSI) and neutron coincidence counting. The DDSI evaluates the neutron multiplication and absorption effect and corrected coincidence count rate determines Cm-244 effective mass. Nuclear material in a canister is quantified by using isotopic data obtained from -ray measurement and/or burn up code. The uncertainty derived from the variation of composition of fuel debris and from the variation of position of fuel debris in the canister were evaluated. For both evaluations, wet storage models have larger uncertainties than dry storage models. That comes from the presence of water in the canister. Neutron moderation by water leads to increase neutron multiplication and absorption effect and to vary neutron detection efficiency. These results indicate that the presence of water and variation of water content in canister should be carefully managed to apply the passive neutron technique.