Experimental study of the effect of interfacial shear stress on jet

Masaki, Naoto*; Kaneko, Akiko*; Horiguchi, Naoki; Yoshida, Hiroyuki

Extended Abstracts for the 11th JSME-KSME Thermal and Fluid Engineering Conference (J-K TFEC11) (Internet), 6 Pages, 2025/10

In the event of a core meltdown in a nuclear reactor, molten fuel has a possibility to enter a coolant pool in a jet-like manner. Jet atomization promoting heat transfer between molten fuel and coolant is crucial for severe accident management. Previous studies of jet atomization have focused on interfacial shear stress acting on a jet because it contributes to the growth and deformation of interfacial waves and ultimately promote droplet formation. However, it is extremely difficult to observe interfacial wave evolution on a cylindrical jet and quantify interfacial shear stress acting on an interface. Then, the effect of interfacial shear stress acting on a jet is not fully clarified. This study aims to experimentally clarify the effect of interfacial shear stress on jet atomization. We conducted experiments in a liquid-liquid system using the 3D-LIF method and PIV. Herein, a glycerol-water solution was used as a jet phase and silicone oil as a continuous phase to match their refractive indices. A jet entered vertically downward through a 3 mm nozzle into a pool. The injection velocity was 2.6 m/s, and the pool depth was 30 mm. To use the 3D-LIF method and PIV with refractive index matching, Rhodamine 6G and tracer particles were used. As a result, waves on a jet interface were observed to have two modes. The larger waves appeared in a two-dimensional (axisymmetric) form at the interface more than 15 mm away from the nozzle, and three-dimensional smaller waves (wavelength = 1 mm) were found to form on the surface of these larger waves. Furthermore, the interfacial shear stress acting on the smaller waves was found to be greater than that acting on the larger waves. Droplets were also observed to form easily from the smaller waves. From these results, we experimentally concluded that high interfacial shear stress acting on small waves promotes droplet formation.

Research on radioactive aerosol control and decontamination at Fukushima Daiichi Nuclear Power Station Decommissioning (Contract research); FY2023 Nuclear Energy Science & Technology and Human Resource Development Project

Collaborative Laboratories for Advanced Decommissioning Science; The University of Tokyo*

JAEA-Review 2025-015, 73 Pages, 2025/09

JAEA-Review-2025-015.pdf:5.9MB

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, 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 FY2021, this report summarizes the research results of the "Research on radioactive aerosol control and decontamination at Fukushima Daiichi Nuclear Power Station decommissioning" conducted from FY2021 to FY2023. The present study aims to develop a safe laser decontamination system that integrates advanced particle detection and characterization systems with aerosol dispersion control, in collaboration with UK researchers. For aerosol dispersion control, new containment methods ranging from simple mechanical containment hoods to optical laser shields are jointly investigated. The Japanese team will develop a radioactive dispersion control method based on the use of water mist and water spray to reduce radiation risks during laser cutting and decontamination applications. Based on data on aerosol particles provided by the UK team, the potential to enhance aerosol scrubbing efficiency will be explored. Additionally, the effect of charge enhancement will be evaluated. The UK team will develop laser-based aerosol containment methods and conducting experiments to improve the condensation of aerosol particles and mist in spray scrubbing. The development of aerosol removal technologies and strategies will be carried out through comprehensive experimental and computational studies. Experiments will be conducted at UTARTS facility to verify simultaneous operations such as laser decontamination, cutting, and spray operations. Aerosol measurements will also be performed to obtain high spatial resolution data that can better validate CFD models. In the final year, mock-up tests will be conducted to verify the system's operation and decontamination performance at full scale, evaluating the decontamination system developed through this research.

Development of importance measures reflecting the risk triplet in dynamic probabilistic risk assessment; A Case study using MELCOR and RAPID

Zheng, X.; Tamaki, Hitoshi; Shibamoto, Yasuteru; Maruyama, Yu; Takada, Tsuyoshi; Narukawa, Takafumi*; Takata, Takashi*

Journal of Nuclear Engineering (Internet), 6(3), p.21_1 - 21_18, 2025/06

https://doi.org/10.3390/jne6030021

Selective resonance ionization of odd calcium isotopes using linearly polarized lasers

Iwata, Yoshihiro; Miyabe, Masabumi; Wells, S. R.*; Yamamoto, Yuta*; Hasegawa, Shuichi*

Proceedings of International Topical Workshop on Fukushima Decommissioning Research 2024 (FDR2024) (Internet), 4 Pages, 2024/10

In this study, triple and double resonance ionization schemes of atomic Ca were developed, aiming for the separation of odd isotopes by (i) laser polarization-dependent selection rules, and (ii) large isotope shifts of odd Ca isotopes in the Rydberg levels. Separation of odd isotopes was confirmed under the orthogonal condition. Suppression of non-resonant ionization of Ca by the electric field in the ionization region could further improve the optical isotope selectivity of Ca.

Research on radioactive aerosol control and decontamination at Fukushima Daiichi Nuclear Power Station Decommissioning (Contract research); FY2022 Nuclear Energy Science & Technology and Human Resource Development Project

Collaborative Laboratories for Advanced Decommissioning Science; The University of Tokyo*

JAEA-Review 2024-031, 75 Pages, 2024/08

JAEA-Review-2024-031.pdf:3.74MB

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 FY2022. The Project aims to contribute to solving problems in the nuclear energy field represented by the decommissioning of the Fukushima Daiichi Nuclear Power Station, 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 FY2021, this report summarizes the research results of the "Research on radioactive aerosol control and decontamination at Fukushima Daiichi Nuclear Power Station decommissioning" conducted in FY2022. The present study aims to develop a safe laser decontamination system that can control aerosol diffusion during laser decontamination while simultaneously allowing advanced particle measurement and evaluation, in cooperation with a research group in the United Kingdom.

Resonance ionization spectroscopy of high-lying 4sns and 4snd Rydberg levels of odd calcium isotopes

Iwata, Yoshihiro; Miyabe, Masabumi; Akaoka, Katsuaki; Wakaida, Ikuo; Hasegawa, Shuichi*

Journal of the Optical Society of America B, 41(1), p.119 - 126, 2024/01

https://doi.org/10.1364/JOSAB.507539

Calcium Rydberg levels are of significant interest for efficient and isotope-selective resonance ionization of trace radionuclides such as calcium-41 (Ca). In this study, we report novel measurement data on the energy level shifts of calcium-43 (Ca) for 4sns S and 4s(n-1)d D (n=40, 45, 50, 55, 60) Rydberg levels due to hyperfine-induced singlet-triplet mixing specific to isotopes with an odd mass number.

Data assimilation of two-dimensional isotropic turbulence by lattice Boltzmann method and local ensemble transform Kalman filter (LBM-LETKF)

Hasegawa, Yuta; Onodera, Naoyuki; Asahi, Yuichi; Idomura, Yasuhiro

Keisan Kogaku Koenkai Rombunshu (CD-ROM), 28, 5 Pages, 2023/05

We implemented and investigated the data assimilation (DA) of two-dimensional isotropic turbulence using the lattice Boltzmann method and the local ensemble transform Kalman filter (LBM-LETKF). We carried out the numerical experiment with 256 grids, 256 or less observation points, 10% root mean square (RMS) observation noise in the velocity observation, and 4, 16, or 64 ensemble members. The numerical experiment showed that the accuracy of the LETKF was better than the nudging DA with both dense and sparse observation. The lack of observation points caused the numerical instability in the LETKF, but such a numerical instability can be suppressed by increasing the number of ensemble members. In the sparse observation case (88 observation points) with 64 ensemble members, the root means squared error (RMSE) of the velocity in the LBM-LETKF was smaller than the RMS of the observation noise, while the nudging DA required 3232 observation points to obtain the same accuracy. Overall, the LETKF with sufficiently large number of ensemble members was highly accurate and robust, thus, the LETKF was a good choice for the DA of turbulent flows using the LBM.

Aerosol characterization during heating and mechanical cutting of simulated uranium containing debris; The URASOL project in the framework of Fukushima Daiichi fuel debris removal

Porcheron, E.*; Leblois, Y.*; Journeau, C.*; Delacroix, J.*; Molina, D.*; Suteau, C.*; Berlemont, R.*; Bouland, A.*; Lallot, Y.*; Roulet, D.*; et al.

Proceedings of International Topical Workshop on Fukushima Decommissioning Research (FDR2022) (Internet), 5 Pages, 2022/10

One of the important challenges for the decommissioning of the damaged reactors of the Fukushima Daiichi Nuclear Power Station (1F) is the fuel debris retrieval. The URASOL project, which is undertaken by a French consortium consisting of ONET Technologies, CEA, and IRSN for JAEA/CLADS, is dedicated to acquiring basic scientific data on the generation and characteristics of radioactive aerosols from the thermal or mechanical processing of fuel debris simulant. Heating process undertaken in the VITAE facility simulates some representative conditions of thermal cutting by LASER. For mechanical cutting, the core boring technique is implemented in the FUJISAN facility. Fuel debris simulants have been developed for inactive and active trials. The aerosols are characterized in terms of mass concentration, real time number concentration, mass size distribution, morphology, and chemical properties. The chemical characterization aims at identifying potential radioactive particles released and the associated size distribution, both of which are important information for assessing possible safety and radioprotection measures during the fuel debris retrieval operations at 1F.

GPU implementation of local ensemble transform Kalman filter (LETKF) with two-dimensional lattice Boltzmann method

Hasegawa, Yuta; Onodera, Naoyuki; Asahi, Yuichi; Idomura, Yasuhiro

Keisan Kogaku Koenkai Rombunshu (CD-ROM), 27, 4 Pages, 2022/06

We developed GPU implementation of ensemble data assimilation (DA) using the local ensemble transform Kalman filter (LETKF) with the lattice Boltzmann method (LBM). The performance test was carried out upto 32 ensembles of two-dimensional isotropic turbulence simulations using the D2Q9 LBM. The computational cost of the LETKF was less than or nearly equal to that of the LBM upto eight ensembles, while the former exceeded the latter at larger ensembles. At 32 ensembles, their computational costs per cycle were respectively 28.3 msec and 5.39 msec. These results suggested that further speedup of the LETKF is needed for practical 3D LBM simulations.

A Proposal of optimum calculation settings of continuous wavelet transform in magnetotelluric data processing

Ogawa, Hiroki; Hama, Yuki*; Asamori, Koichi; Ueda, Takumi*

Butsuri Tansa, 75, p.38 - 55, 2022/00

https://doi.org/10.3124/segj.75.38

In the magnetotelluric (MT) method, so as to identify the subsurface resistivity structure, the apparent resistivity and phase profiles are calculated by transforming time-series data into spectral data. The continuous wavelet transform (CWT) is well known as a new method of time-frequency analysis instead of the short-time Fourier transform. The CWT is superior in processing non-stationary wideband signals like the MT signal by adjusting the size of the wavelet according to the value of frequency. However, the calculation settings of the CWT, such as the type of basis function and the wavelet parameter, are often determined empirically because of the arbitrariness of the shape of the wavelet. Although there might be differences between the calculated MT responses and the true responses due to improper settings of the CWT, there are no detailed studies considering the effect of numerical errors derived from spectral transforms on MT data. In this study, focusing on the frequency band between 0.001 Hz and 1 Hz, we examined the optimum calculation settings of the CWT in processing MT data in terms of suppressing the numerical errors caused by the spectral transform of time-series data. We also show the validity of the proposed calculation settings by applying the CWT to MT survey data of different types. Superiority of the CWT with proposed settings is suggested especially when the signal-to-noise ratio of observed data is low. Consequently, the proposed calculation settings were confirmed to strike a balance between the resolutions of the time and frequency domains well and will therefore be effective in obtaining reliable MT responses.