Elsevier

Annals of Nuclear Energy

Volume 118, August 2018, Pages 92-98
Annals of Nuclear Energy

Application of advanced Rossi-alpha technique to reactivity measurements at Kyoto University Critical Assembly

https://doi.org/10.1016/j.anucene.2018.04.009Get rights and content

Abstract

This study presents the first application of the advanced Rossi-alpha method (theoretically introduced by Kong et al., 2014) on the reactivity measurements in a research reactor: detector count signals at the Kyoto University Critical Assembly (KUCA) facility. The detector signals in the KUCA A-type core are analyzed by three subcriticality measurement methods: (1) Feynman-alpha (F-α) method, (2) Rossi-alpha (R-α) method, and (3) advanced Rossi-alpha (advanced R-α) method. Four cases are analyzed for two different subcritical states of the core and two different neutron source locations. Two different negative reactivity ρ values are obtained by the measurements of control rod worth and regarded as the reference reactivity values, comparing the results by the four methods.

The F-α shows reactivity errors ranging between 7.1 and 7.3% due to its use of variance-to-mean ratios of detector count signals, which are not very sensitive to neutron background noise. However, the fitting uncertainties associated to the F-α results are large, ranging between 5.4 and 12.8% at one standard deviation. The R-α shows small fitting uncertainties ranging between 2.8 and 3.8%, although reactivity errors are in the range of 3.5–26.5% due to the neutron background noise. Finally, the advanced R-α that explicitly models the neutron background noise contrary to the previous methods shows the reactivity errors in the range of 1.0–11.8%, and provides the lowest uncertainties of the measured ρ in the range of 0.4–0.9%. In conclusion, among the four methods applied to the reactivity measurements at KUCA, the advanced R-α reveals the best accuracy with the lowest uncertainties.

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).Y=C2-C¯2C¯=Y1-1-e-α(t2-t1)α(t2-t1)+N.

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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