Detection of criticality approaching in irregular systems based on node search of spatial higher-order modes using reactor noise analysis method

2023 Fiscal Year Final Research Report

• Project/Area Number: 20K05381
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Multi-year Fund
• Section: 一般
• Review Section: Basic Section 31010:Nuclear engineering-related
• Research Institution: Kyoto University
• Principal Investigator: Kitamura Yasunori 京都大学, 複合原子力科学研究所, 准教授 (60332706)
• Co-Investigator(Kenkyū-buntansha): 三澤 毅 京都大学, 複合原子力科学研究所, 教授 (70219616)
• Project Period (FY): 2020-04-01 – 2024-03-31
• Keywords: 福島第一原子力発電所 / 燃料デブリ回収 / 再臨界阻止 / 炉雑音解析法 / 不規則体系

Outline of Final Research Achievements

In the planned fuel debris retrieval process at the Fukushima-Daiichi plant, it is necessary to prevent the criticality due to changes in shape and mixing state with water. Therefore, we conducted a research on a method for monitoring the neutron decay constant and detect the signs of criticality approacing by reactor noise analysis. First, to verify the applicability of the method in deep subcriticality, systems with the effective multiplication factors of 0.48 - 0.71 were constructed and reactor noise analysis was performed. As a result, it was demonstrated that the signs of criticality approaching can be detected by monitoring the neutron decay constant even in deep subcriticality. On the other hand, further research is required on the removal of higher mode effects, which are problematic in subcriticality systems distributed over a wide area such as fuel debris.

Free Research Field

原子炉物理

Academic Significance and Societal Importance of the Research Achievements

東京電力福島第一原子力発電所において、近い将来に実施される燃料デブリ回収作業が予定されている。現状の燃料デブリ体系は未臨界であると推定できるが、今後の燃料デブリ取り出し作業等に伴い、燃料デブリ形状や水との混合状態が変化することによる臨界を防止することは、一般公衆及び作業員に過度の被ばくが生じることのないようにするために必要である。本研究の成果は、これに貢献しようとするものである。