https://doi.org/10.1051/epjconf/202124709004
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
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.
Key words: reactor noise / subcriticality / third-order neutron correlation / dead-time
© The Authors, published by EDP Sciences, 2021
This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
