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.
Highlights
In this study, the theoretical formula for the third-order neutron-correlation technique is derived to take the dead-time effect in a neutron-detection process into account
Using detection probabilities shown in Eqs. (3)–(5), the expected numbers of single-neutron, pairs and trios detected during the counting gate width T can be derived to obtain the theoretical formulae for the neutron correlation factors Y and Y3
In actual reactor noise analysis, the dead-time effect of a neutron detector could affect on the measurement results of the second- and third-order neutron-correlation factors Y and Y3
Summary
The theoretical formula for the third-order neutron-correlation technique is derived to take the dead-time effect in a neutron-detection process into account. In the previous study [1], it was theoretically clarified that the neutron-correlation ratio of the third-order factor Y3 to the squared value of the second-order factor Y converges to the unique combination number “3” at a near-critical state in an arbitrary system, as the neutron counting gate width increases sufficiently. One of the issues is the detector dead time effect on the ratio Y3⁄Y2, which might occurs under a situation where the neutron count rate is high compared with the dead-time of the neutron counter Another issue is an actual experimental validation for a non-multiplication system and a deeper subcritical system.
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