Integrated thermal power measurement in the modified STACY for the performance inspections

Araki, Shohei; Aizawa, Eiju; Murakami, Takahiko; Arakaki, Yu; Tada, Yuta; Kamikawa, Yutaka; Hasegawa, Kenta; Yoshikawa, Tomoki; Sumiya, Masato; Seki, Masakazu; et al.

Annals of Nuclear Energy, 217, p.111323_1 - 111323_8, 2025/07

https://doi.org/10.1016/j.anucene.2025.111323

JAEA has modified the STACY from a homogeneous system using solution fuel to a heterogeneous system using fuel rods in order to obtain criticality characteristics of fuel debris. The modification of the STACY was completed in December 2023. A series of performance inspections were conducted for the start of experimental operations. A new thermal power calibration is required for the performance inspections in order to operate at less than 200 W, which is the permitted thermal power. However, the thermal power measurement method and calibration data used in the former STACY is no longer available due to the modification of the modified STACY. We measured the thermal power of the STACY using the activation method that was improved to adapt to the measurement condition and calibrated the power meter system. Since the positions where activation foils could be installed were very limited, the thermal power was evaluated using numerical calculations supplemented by experimental data. Neutron flux data at the positions of the activation foil was measured by the activation method. Neutron distribution in the core was calculated by the Monte Carlo code MVP. A response function of the activation foil was calculated using the PHITS. The uncertainty of the thermal power measurement was conservatively estimated to be about 15%. Four operations were conducted for the thermal power measurement. The power meter was calibrated by using three operational data and tested with the one operational data. It was found that the indicated value of the meter adjusted by the STACY before the modification work would tend to overestimate the actual output by about 40%. In addition, the current calibration was able to calibrate the meter to within 3% accuracy.

Development of extremely small amount analysis technology for fuel debris analysis (Contract Research); FY2023 Nuclear Energy Science & Technology and Human Resource Development Project

Collaborative Laboratories for Advanced Decommissioning Science; Tohoku University*

JAEA-Review 2024-064, 118 Pages, 2025/06

JAEA-Review-2024-064.pdf:6.73MB

The Collaborative Laboratories for Advanced Decommissioning Science (CLADS), Japan Atomic Energy Agency (JAEA), had been conducting the Nuclear Energy Science & Technology and Human Resource Development Project (hereafter referred to "the Project") in FY2023. The Project aims to contribute to solving problems in the nuclear energy field represented by the decommissioning of the Fukushima Daiichi Nuclear Power Station (1F), Tokyo Electric Power Company Holdings, Inc. (TEPCO). For this purpose, intelligence was collected from all over the world, and basic research and human resource development were promoted by closely integrating/collaborating knowledge and experiences in various fields beyond the barrier of conventional organizations and research fields. The sponsor of the Project was moved from the Ministry of Education, Culture, Sports, Science and Technology to JAEA since the newly adopted proposals in FY2018. On this occasion, JAEA constructed a new research system where JAEA-academia collaboration is reinforced and medium-to-long term research/development and human resource development contributing to the decommissioning are stably and consecutively implemented. Among the adopted proposals in FY2019, this report summarizes the research results of the "Development of extremely small amount analysis technology for fuel debris analysis" conducted from FY2019 to FY2023. Understanding the properties of fuel debris is necessary for handling, criticality control, storage control, etc. A key technique is the chemical analysis of actinide nuclides. We developed sample pretreatment technology and separation / analysis process required for chemical analysis. The purpose of this study is to streamline future planned fuel debris analysis. To promote 1F decommissioning, we will train human resources through on-the-job training. In particular, we applied the extremely small amount analysis (ICP-MS/MS), which has recently been successful in the fields of analytical chemistry and radiochemistry, to the nuclear field. This method allows high-accuracy analysis without pretreatment to isolate the nuclide to be measured. The separation pretreatment can be skipped and a rapid analysis process can be established.

Uncertainty analysis of the inverse LASSO estimation scheme on radioactive source distributions inside reactor building rooms from air does rate measurements

Shi, W.*; Machida, Masahiko; Yamada, Susumu; Okamoto, Koji

Progress in Nuclear Energy, 184, p.105710_1 - 105710_10, 2025/06

https://doi.org/10.1016/j.pnucene.2025.105710

Very recently, Least Absolute Shrinkage and Selection Operator (LASSO) has been proposed as a scheme capable to inversely estimate radioactive source distributions inside reactor building rooms from air dose rate measurements together with the predicted lower bound of the measurement numbers for successful reconstructions. However, no one has ever analyzed how the uncertainty of input data including the measurement errors influences the accuracy of the inverse estimation results. In this paper, we therefore perform uncertainty analysis of the LASSO scheme and suggest an uncertainty estimation function derived based on the theory of Candes. We actually demonstrate in two types of numerical tests with different input uncertainties obtained by using Monte Carlo code, Particle and Heavy Ion Transport code System (PHITS) that the calculated errors obey the proposed uncertainty estimation function. Thus, the LASSO scheme allows to successfully estimate radioactive distributions within the predicted uncertainty.

Analysis of dissolved radionuclides trapped into corrosion products formed on carbon steel and the corresponding increase in radioactivity

Aoyama, Takahito; Ueno, Fumiyoshi; Sato, Tomonori; Kato, Chiaki; Sano, Naruto; Yamashita, Naoki; Otani, Kyohei; Igarashi, Takahiro

Annals of Nuclear Energy, 214, p.111229_1 - 111229_6, 2025/05

https://doi.org/10.1016/j.anucene.2025.111229

Human resource development project for decommissioning of Fukushima Daiichi NPS; Focusing on engineering and management skills in severe environment

Usami, Hiroshi; Yoshinaga, Kyohei*; Fujikawa, Keigo*

Nihon Genshiryoku Gakkai-Shi ATOMO, 67(5), p.295 - 299, 2025/05

https://doi.org/10.3327/jaesjb.67.5_295

no abstracts in English

Three-dimensional localization and radioactivity quantification of radiation sources through inverse estimation based on Compton camera measurements

Sato, Yuki

Radiation Protection Dosimetry, p.ncaf046_1 - ncaf046_11, 2025/05

https://doi.org/10.1093/rpd/ncaf046

Comparison of analysis results based on flight methods using a CZT detector system on an unmanned aerial vehicle near the Fukushima nuclear power plant

Joung, S.*; Ji, Y.-Y.*; Choi, Y.*; Lee, E.*; Ji, W.*; Sasaki, Miyuki; Ochi, Kotaro; Sanada, Yukihisa

Journal of Instrumentation (Internet), 20(4), p.P04027_1 - P04027_10, 2025/04

https://doi.org/10.1088/1748-0221/20/04/P04027

Development of a compact detector for measurement of alpha contamination in piping

Morishita, Yuki; Peschet, L.; Yamada, Tsutomu*; Nakasone, Takamasa*; Kanno, Marina*; Sasaki, Miyuki; Sanada, Yukihisa; Torii, Tatsuo*

Radiation Measurements, 183, p.107414_1 - 107414_6, 2025/04

https://doi.org/10.1016/j.radmeas.2025.107414

In the decommissioning nuclear facilities, it is crucial to inspect piping for contamination to prevent worker exposure to alpha-emitting nuclides. Traditional methods using gamma rays and neutrons are inadequate for detecting small amounts of alpha nuclides due to the short range (approximately 4 cm) of alpha particles in air. To address this, we developed a compact detector capable of distinguishing between alpha particles for direct measurement within pipes. This detector, comprising a ZnS(Ag) scintillator for alpha particles and a plastic scintillator for beta particles (gamma rays), was coupled to a small photomultiplier tube. The system demonstrated high accuracy in differentiating between alpha and beta radiation through pulse shape discrimination (PSD). Monte Carlo simulations and empirical measurements confirmed the detector's effectiveness, achieving a 51.3% detection efficiency for alpha particles with negligible sensitivity to beta and gamma radiation. This innovation presents a significant advancement for direct alpha contamination measurement in environments with high beta and gamma backgrounds, such as the Fukushima Daiichi Nuclear Power Plant decommissioning site.

Aerial monitoring around TEPCO's Fukushima Daiichi Nuclear Power Station and development of radiation monitoring technology for unmanned airplanes in fiscal year 2023 (Contract research)

Futemma, Akira; Sanada, Yukihisa; Nakama, Shigeo; Sasaki, Miyuki; Ochi, Kotaro; Nagakubo, Azusa; Sawahata, Yoshiro*; Kawasaki, Yoshiharu*; Iwai, Takeyuki*; Hiraga, Shogo*; et al.

JAEA-Technology 2024-021, 232 Pages, 2025/03

JAEA-Technology-2024-021.pdf:25.79MB

The 2011 off the Pacific coast of Tohoku Earthquake on March 11, 2011, caused a tsunami that led to the TEPCO's Fukushima Daiichi Nuclear Power Station (FDNPS) accident, releasing a large amount of radioactive material into the surrounding environment. Since the accident, Aerial Radiation Monitoring (ARM) has been used to quickly and widely measure radiation distribution. As a commissioned project from the Nuclear Regulation Authority, the Japan Atomic Energy Agency (JAEA) has continuously conducted ARM around FDNPS using manned and unmanned helicopters. This report summarizes the monitoring results for fiscal year 2023, evaluates changes in dose rate from past results, and discusses the factors contributing to these changes. Additionally, an analysis considering terrain undulation was conducted to improve accuracy for converting ARM data into dose rate. Furthermore, a method to discriminate airborne radon progeny was applied for ARM results to evaluate its impact. Moreover, to perform wide-area monitoring more efficiently, we advanced the development of unmanned airplane monitoring technology.

Investigation of chemical substances affect the fluidity of paste on cement solidification

Taniguchi, Takumi; Matsumoto, Saori; Hiraki, Yoshihisa; Sato, Junya; Fujita, Hideki*; Kaneda, Yoshihisa*; Kuroki, Ryoichiro; Osugi, Takeshi

JAEA-Review 2024-059, 20 Pages, 2025/03

JAEA-Review-2024-059.pdf:1.0MB

The basic performance required for solidifying waste into cement, such as fluidity before curing and strength after curing, is expected to be affected by the chemical effects of substances and components contained in the waste. The fluidity before curing and the strength properties after curing are greatly influenced by the curing speed of the cement. We investigated existing knowledge with a focus on chemical substances that affect the curing speed of cement. In this report, chemical substances that affect fluidity are broadly classified into inorganic substances such as (1) anion species, (2) metal elements such as heavy metals, (3) inorganic compounds as cement admixtures, and (4) organic compounds as cement admixtures. Based on the investigation, we actually added chemicals and measured the setting time. As a result, it was found that there are multiple mechanisms contributing to accelerated hardening. We investigated chemical substances that inhibit the curing reaction of cement, and were able to compile information to consider ingredients that are contraindicated in cement curing.