Application of advanced Rossi-alpha technique to reactivity measurements at Kyoto University Critical Assembly
Introduction
Experiments conducted in research reactors are crucial to increase our knowledge of nuclear physics and validate reactor analysis codes and methods. In early 2017, the Kyoto University Critical Assembly (KUCA) facility has reopened after its safety equipment was reinforced to satisfy the stricter nuclear regulations in Japan consecutively to the Fukushima accident. Using the KUCA facility, we carried out subcriticality measurement experiments and analyzed the experimental results with different methods to investigate and compare the reliability of each method. The three subcriticality measurement methods used in this study are (1) the Feynman-alpha (F-α) method, (2) the Rossi-alpha (R-α) method, and (3) the advanced Rossi-alpha (advanced R-α) method.
After reopening of the KUCA facility, experiments on reactivity measurements have been carried out at the polyethylene-moderated core (A-core) in Kyoto University Research Reactor Institute (Pyeon et al., 2017a, Pyeon et al., 2017b). The KUCA A-core has been mainly engaged in a feasibility study on the accelerator-driven system (ADS) (Van et al., 2017, Zheng et al., 2017), and this study is focusing on improvement of subcriticality measurement in a core system. This research differs from the previous experiments in the way that (1) a new core configuration is investigated, different from previous KUCA core configurations and (2) a state-of-the-art technique (advanced R-α method; proposed by Kong et al., 2014) is applied for the first time to neutron count signals obtained from the detectors. The advanced R-α method was applied for only virtual signals generated by Monte Carlo real-time simulation. The objective of this study is to evaluate performance of the advanced R-α method on the real neutron signals in comparison with those of the traditional R-α and F-α methods.
The structure of this paper is as follows: Section 2 describes the underlying principles behind the three measurement methods applied to the reactivity measurements at KUCA. Section 3 introduces the configuration of the core and the cases analyzed by the three methods. Section 4 describes the results of experimental analyses by three methods using the measured data. Section 5 concludes this paper.
Section snippets
Feynman-alpha method
The F-α method uses the principle that the variance-to-mean ratio of detector count signals is theoretically equal to a unit when delayed neutrons are neglected. When the effect of delayed neutrons is taken into account, the variance-to-mean ratio of detector count signals follows the Poisson’s distribution (Taninaka et al., 2011).
Eq. (1) expresses the variance-to-mean ratio of the detector signals (Tonoike et al., 2004).
In Eq. (1), Y is the
Description of KUCA facility and experiment
The KUCA facility was established in 1974 for nuclear reactor physics experiments. It is composed of three types of cores: two of them are solid-moderated cores (A-core and B-core), and the other is light-water-moderated core (C-core) (Misawa et al., 2010). The three cores are operated at very low power and therefore the nuclear fuel can always be considered as fresh fuel. The subcriticality of the cores is determined by the thickness and the arrangement of fuel and moderator plates.
Feynman-alpha results
Fig. 5 shows the F-α fitting results using the detector FC#1. The information about the type of detector as well as the associated acquisition electronics are well presented in Dr. Lee’s paper (2010). The F-α method uses the variance-to-mean ratios of the detector counts along time bins. In Fig. 5, the x-axis is the time bin size, and the y-axis is the variance-to-mean ratios of the detector counts.
The analyses were carried out for two different neutron sources (Am-Be and Cf-252). Fig. 5 shows
Conclusions
The state-of-the-art subcriticality measurement technique, advanced Rossi-alpha, was tested using practical measurement data from the KUCA core.
In the KUCA facility, a new core configuration made of normal fuel assemblies, special fuel assemblies including lead, and polyethylene moderator assemblies were used for experiments. Three fission chambers gave detector count signals, which were analyzed by the following three methods: (1) Feynman-alpha (F-α) method, (2) Rossi-alpha (R-α) method, and
Acknowledgments
This work was partially supported by KETEP, which is funded by the Korea government Ministry of Trade, Industry and Energy. (No. 20131610101850).
This work was partially supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT). (NRF-2017M2B2B1072806).
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