EXPERIMENT OF UNIQUE COMBINATION NUMBER DUE TO THE THIRD-ORDER NEUTRON-CORRELATION

¹ Nagoya University Furo-cho, Chikusa-ku, Nagoya-shi, 464-8603, Japan
² Kindai University 3-4-1, Kowakae, Higashiosaka-shi, Osaka, 557-8502, Japan
³ Kyoto University Asashiro-nishi, Kumatori-cho, Sennan-gun, Osaka, 590-0494, Japan

t-endo@energy.nagoya-u.ac.jp
imai.sho@a.mbox.nagoya-u.ac.jp
k-watanabe@energy.nagoya-u.ac.jp
a-yamamoto@energy.nagoya-u.ac.jp
sakon@kindai.ac.jp, kengoh@pp.iij4u.or.jp
m-yamanaka@rri.kyoto-u.ac.jp
pyeon@rri.kyoto-u.ac.jp

Published online: 22 February 2021

Abstract

From zero-power reactor noise measurement, the second- and third-order neutron correlation factors Y and y₃ can be evaluated by analyzing mean, variance, the third-order central moment of neutron count data. Theoretically, it is expected that the neutron-correlation ratio y₃/Y² converges to the unique combination number “3” at a near-critical state in an arbitrary system without depending on the fissile material and the neutron-energy spectrum of core, as the neutron counting gate width T increases sufficiently. Thus, the information about the difference between y₃/Y² and “3” has the potential to judge whether a target unknown system is critical or not and to roughly guess the absolute value of subcriticality. In this study, the detector dead-time effect on y₃/Y² is theoretically investigated based on the heuristic method using the single-, pair-, and trio-detection probabilities with the fundamental mode approximation. As a result, it is clarified that the saturation value of y₃/Y² converges to “3” independent of the dead time, when a target system is a critical state. For validation, actual experimental results are presented for a non-multiplication system driven by ²⁵²Cf spontaneous source, and shallow and deep subcritical systems at Japanese experimental facilities (UTR-KINKI and KUCA) under the shutdown state. Consequently, it is demonstrated that y₃/Y² shows a significant difference from “3” in the non-multiplication system. In the case of subcritical systems driven by inherent neutron sources, it is confirmed that the ratios y₃/Y² are close to the unique combination number “3,” and the slight difference from “3” is measurable by the long-time reactor noise measurement for the deep subcritical system.