Development of subcritical measurement technique for unknown systems

2021 Fiscal Year Final Research Report

• Project/Area Number: 19K05328
• 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: Nagoya University
• Principal Investigator: Endo Tomohiro 名古屋大学, 工学研究科, 准教授 (50377876)
• Project Period (FY): 2019-04-01 – 2022-03-31
• Keywords: 未臨界 / 原子炉雑音 / 確率母関数 / 動的モード分解 / 即発中性子減衰定数 / データ同化 / ブートストラップ法 / 一点炉動特性

Outline of Final Research Achievements

Based on the probability generating function of neutron counts, this study theoretically clarified that the ratio of (the nth-order neutron correlation) to (the (n-1)th power of 2nd-order neutron correlation) in the reactor noise measured for a subcritical system converges to the unique combination number of double factorial (2n－3)!! without depending on nuclide compositions and geometry as the system becomes to be critical. To estimate the subcriticality in dollar units from the time variation in the neutron count rates measured in an arbitrary state-change such as the reactivity and neutron source intensity, the time-domain decomposition-based integral method was newly proposed and demonstrated through actual measurements conducted at Japanese experimental facilities.
By applying the dynamic mode decomposition to the pulsed neutron source and Rossi-α methods, this study demonstrated that the prompt neutron decay constant of the fundamental mode component can be estimated robustly.

Free Research Field

原子力工学

Academic Significance and Societal Importance of the Research Achievements

測定体系の詳細情報が不明であっても、定常状態における炉雑音測定結果から「中性子相関量の比と臨界固有の値(2n-3)!!の差」を調べることで、測定体系が深い未臨界状態にあるか否かの判定や、未臨界度の深さを概算できる可能性がある。また、本研究で新たに提案した時間領域分割積分法は、反応度だけでなく中性子源強度も同時に変化するような、燃料取出時の未臨界度測定技術として活用できる。
動的モード分解を適用することで、即発中性子減衰定数αの測定結果が検出器位置に依存する問題を解決できる。本研究成果は、体系固有のα測定結果を活用したデータ同化による、評価済み核データや核特性予測結果の不確かさ低減に貢献できる。