3. Reactor Physics and Reactor Engineering

概要

INTRODUCTION: In the fuel debris removal process of Fukushima Daiichi nuclear power plant, subcriticality monitoring system should be equipped to prevent crit- cality accident. For this purpose, International Research Institute for nuclear Decommissioning (IRID) is devel- oping criticality control techniques for fuel debris remov- al based on neutron noise analysis using Feynman-alpha method or Rossi-alpha method. Prototype of the sub-criticality monitoring system was tested to verify applicability on various sub-criticality measurement con- ditions.
For this measurement, a small neutron detector based on a SiC with boron coated film is one of the candidates at Fukushima because of its toughness against gamma-ray and neutron radiation exposure and low detection effi- ciency for gamma-ray. We are also developing a data transfer system from this SiC neutron detector to data acquisition system which is placed at outside of a reactor vessel by a specially designed optical fiber with high re- sistance against radiation. In this research, we used this new data transfer system to measure subcriticality.

INTRODUCTION: In the last study, a neutron detector located about 55 cm away of fuel assembly measured the auto power spectral density. However, the prompt neutron decay constants obtained by this detector was different from that of other detectors. The objective of this study is experimental study of reactor noise analysis by the power spectrum method using neutron detector placed outside

INTRODUCTION: The Japan Atomic Energy Agency (JAEA) started the Research and Development (R&D) to improve nuclear prediction techniques for High Temper- ature Gas-cooled Reactors (HTGRs) in 2018. The objec- tives are to introduce the generalized bias factor method to avoid full mock-up experiment for the first commercial HTGR and to improve neutron instrumentation system by virtue of the particular characteristics due to a graphite moderation system. For this end, we composed B7/4”G2/8”p8EU(3)+3/8”p38EU in the B-rack of Kyoto University Critical Assembly (KUCA) in 2021.

INTRODUCTION: The reactor noise methods can measure the subcriticality without additional instruments. International Research Institute for Nuclear Decommis- sioning (IRID) is developing the monitoring subcriticality system based on the reactor noise methods using the Feynman-alpha method. A prototype system was devel- oped using B-10 neutron detectors [1]. The system is required with radiation hardness. We, IRID, picked up the corona detectors and multi-cell detector as the candidate detectors with radiation hardness.
For this measurement, the applicability of the monitoring subcriticality systems with corona counters with B-10 or He-3 and a multi-cell He-3 detector are verified.

INTRODUCTION: The estimation of reactivity of an amount of unknown fissile material is one of important is- sues in the field of criticality safety.
JAEA has been theoretically developing a method to esti- mate the reactivity from neutron count rate alone[1-2]. The method is based on a newly developed equation of power in quasi-steady state after prompt jump/drop of power due to reactivity and/or neutron source change.
The purpose of the experiment is to obtain the experi- mental data for the verification and validation of the de- veloped method. This time, the data were obtained for deep subcritical states around keff = 0.95, which is a threshold value for a subcritical condition in a numerical analysis.

INTRODUCTION: We are developing an in-core power distribution estimation method (PHOEBE) using ex-core neutron detectors to reduce the cost and improve the maintainability of nuclear instrumentation in small reac- tors as a distributed power source [1]．Theoretical and numerical examinations were initially studied with the experimental demonstration conducted at UTR-KINKI [2]. The demonstration was conducted with a simple core Hence, more complex experiment geometry is required to evaluate the power distribution of the core inner region for the experimental demonstration of PHOEBE. In this study, a new core was constructed at KUCA and tested to confirm the effectiveness of PHOEBE.

INTRODUCTION:
In engineering discussions of the feasibility of new reac- tor systems, it is necessary to evaluate the impact of the fuels and materials for the neutronics characteristics such as criticality, conversion rate, and fuel balance. In order to develop of Thorium (Th) nuclear system, critical ex- periments on Th loaded cores using the KUCA with solid moderator core have been systematically carried out to perform neutronics characteristics measurements of Th loaded thermal neutron systems and integral evaluation of Th cross sections [1]. In order to perform nuclear design for Th loaded reactors, it is important to validate U-233 nuclear data. In order to perform an integral validation of U-233 fission cross section, measurements of sample reactivity worth in KUCA solid moderated core were carried out.

INTRODUCTION: Subcriticality monitoring system has to be used to detect criticality approach for each step of debris removal in Fukushima Daiichi nuclear power plant. For this purpose, International Research Institute for nuclear Decommissioning (IRID) is developing criti- cality control techniques for fuel debris removal based on neutron noise analysis using Feynman-alpha method. A prototype of the sub-criticality monitoring system was tested to verify applicability on various sub-criticality measurement conditions.
For this measurement, a small neutron detector based on a SiC with boron coated film is one of the candidates at Fukushima because of its toughness against gamma-ray and neutron radiation exposure and low detection effi- ciency for gamma-ray. We are also developing a data transfer system from this SiC neutron detector to data acquisition system which is placed at outside of a reactor vessel by a specially designed optical fiber with high re- sistance against radiation. In this research, we used this new data transfer system to confirm the availability for subcriticality measurement.

INTRODUCTION: Precise estimation of the reactor kinetics is essential for the nuclear safety. In CRIEPI, continuous energy Monte Carlo (MC) method has been studied to estimate the point kinetics parameters in a crit- ical condition [1, 2] and reactor periods [3]. Besides, time dependent neutron transport calculation techniques are now under development [4, 5]. In order to validate those calculations, comprehensive data sets of reactor kinetics were measured in A3/8”p36EU(3) core.

INTRODUCTION: Reactor noise for high-power re- actors were actively measured in the 1960’s and 1970’s. The major focuses of those researches were for the ab- normality diagnosis or the output stabilization diagnosis, and almost researchers were in the field of system control engineering or instrumentation engineering. High-power reactor noise measurements for dynamics’ analysis of reactivity change, reactivity feedback or reactor charac- teristics itself were few in the time (1960’s and 1970’s), because of the powerless measurement system. In this research, we plan to measure KUR’s output with pre- sent-day measurement system and plan to analyze with several analysis methods. The results of this work will supply some knowledges and technics in the aspect of sophistication of reactor noise analysis or simulation methods.
In this year, we tried to measure the reactor nuclide noise of the critical state KUR core via a 1-inch 3He counters at CN-1 port focused on epi-thermal neutrons. The experi- mental work was done on 25th November 2021. As the result of the experiment, a result looks like the nuclear reactor noise was observed in 1MW critical state.

INTRODUCTION: To safely control the reactor, moni- toring neutron flux inside the reactor is necessary. There- fore, a reliable technology is needed to measure neutrons of high intensity, as in the reactor. Self-powered neutron detectors (SPNDs) are available as detectors to perform the measurement. The SPND is possible to obtain infor- mation on neutron flux by measuring the current signal generated with the use of activation of emitter material (Rh, V, Co etc.). In the detection principle, the response time of the detector is determined by the time constant of activation decay, which makes it difficult to respond quickly. In this study, we will explore a method to speed up the response of SPND by digital data processing.

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