Monte Carlo and experimental assessment of the optimal geometry of the source and collimator for a table-top NRTA system for small nuclear material measurement

Guembou Shouop, C.; Tsuchiya, Harufumi

Nuclear Instruments and Methods in Physics Research A, 1072, p.170189_1 - 170189_14, 2025/03

https://doi.org/10.1016/j.nima.2024.170189

Evolution of radionuclide transport and retardation processes in uplifting granitic rocks, Part 2; Modelling coupled processes in uplift scenarios

Metcalfe, R.*; Benbow, S. J.*; Kawama, Daisuke*; Tachi, Yukio

Science of the Total Environment, 958, p.177690_1 - 177690_17, 2025/01

https://doi.org/10.1016/j.scitotenv.2024.177690

Uplifting fractured granitic rocks occur in substantial areas of countries such as Japan. A repository site would be selected in such an area only if it is possible to make a safety case, accounting for the changing conditions during uplift. The safety case must include robust arguments that chemical processes in the rocks around the repository will contribute sufficiently to minimise radiological doses to biosphere receptors. To provide confidence in the safety arguments, numerical models need to be sufficiently realistic, but also parameterised conservatively (pessimistically). However, model development is challenging because uplift involves many complex couplings between groundwater flow, chemical reactions between water and rock, and changing rock properties. The couplings would affect radionuclide mobilisation and retardation, by influencing diffusive radionuclide fluxes between groundwater flowing in fractures and effectively immobile porewater in the rock matrix and radionuclide partitioning between water and solid phases, via: (i) mineral precipitation/dissolution; (ii) mineral alteration; and (iii) sorption/desorption. It is difficult to represent all this complexity in numerical models while showing that they are parameterised conservatively. Here we present a modelling approach, illustrated by simulation cases for some exemplar radioelements, to identify realistically conservative process conceptualisations and model parameterisations.

Evolution of radionuclide transport and retardation processes in uplifting granitic rocks, Part 1; Key processes, conceptual models and scenario

Metcalfe, R.*; Tachi, Yukio; Sasao, Eiji; Kawama, Daisuke*

Science of the Total Environment, 957, p.177375_1 - 177375_17, 2024/12

https://doi.org/10.1016/j.scitotenv.2024.177375

A safety case for an underground radioactive waste repository must show that groundwater will not in future transport radionuclides from the repository to the near-surface environment (the biosphere) in harmful quantities. Safety cases are developed step-wise throughout a programme to site and develop a repository. At early stages, before a site is selected, safety cases are generic and based on simplified safety assessment models of the disposal system that have conservative parameter values. Later, when site-specific conditions are known, more realistic models are needed for the long-term geo-environmental evolution and their impacts on radionuclide migration/retention. Uplift is one such environmental change, which may be particularly important in countries near active tectonic plate boundaries, such as Japan. Here we review the state of knowledge about how the properties of fractured granitic rocks evolve during uplift, based on studies in Japan. Hence, we present conceptual models and a generic scenario for mass transport and retardation processes in uplifting granitic rocks as a basis for realistic numerical models to underpin safety assessment.

Deep learning-based bubble detection with Swin Transformer

Uesawa, Shinichiro; Yoshida, Hiroyuki

Journal of Nuclear Science and Technology, 61(11), p.1438 - 1452, 2024/11

https://doi.org/10.1080/00223131.2024.2348023

We developed a deep learning-based bubble detector with a Shifted window Transformer (Swin Transformer) to detect and segment individual bubbles among overlapping bubbles. To verify the performance of the detector, we calculated its average precision (AP) with different number of training images. The mask AP increased with the increase in the number of training images when there were less than 50 images but remained constant when there were more than 50 images. It was observed that the AP for the Swin Transformer and ResNet were almost the same when there were more than 50 images; however, when few training images were used, the AP of the Swin Transformer were higher than that of the ResNet. Furthermore, with regard to the increase in void fraction, the AP of the Swin Transformer showed a decrease similar to that in the case of the ResNet; however, for few training images, the AP of the Swin Transformer was higher than that of the ResNet in all void fractions. Moreover, we confirmed the detector trained with synthetic bubble images was able to segment overlapping bubbles and deformed bubbles in a bubbly flow experiment. Thus, we verified that the new bubble detector with Swin Transformer provided higher AP than the detector with ResNet for fewer training images.

Heat transfer coefficients model for SIMMER-III and SIMMER-IV

Brear, D. J.*; Kondo, Satoru; Sogabe, Joji; Tobita, Yoshiharu*; Kamiyama, Kenji

JAEA-Research 2024-009, 134 Pages, 2024/10

JAEA-Research-2024-009.pdf:2.45MB

The SIMMER-III/SIMMER-IV computer codes are being used for liquid-metal fast reactor (LMFR) core disruptive accident (CDA) analysis. The sequence of events predicted in a CDA is often influenced by the heat exchanges between LMFR materials, which are controlled by heat transfer coefficients (HTCs) in the respective materials. The mass transfer processes of melting and freezing, and vaporization and condensation are also controlled by HTCs. The complexities in determining HTCs in a multi-component and multi-phase system are the number of HTCs to be defined at binary contact areas of a fluid with other fluids and structure surfaces, and the modes of heat transfer taking into account different flow topologies representing flow regimes with and without structure. As a result, dozens of HTCs are evaluated in each mesh cell for the heat and mass transfer calculations. This report describes the role of HTCs in SIMMER-III/SIMMER-IV, the heat transfer correlations implemented and the calculation of HTCs in all topologies in multi-component, multi-phase flows. A complete description of the physical basis of HTCs and available experimental correlations is contained in Appendices to this report. The major achievement of the code assessment program conducted in parallel with code development is summarized with respect to HTC modeling to demonstrate that the coding is reliable and that the model is applicable to various multi-phase problems with and without reactor materials.

Full-f gyrokinetic simulations of hydrogen isotope mixing in tokamak plasmas

Idomura, Yasuhiro

Physics of Plasmas, 31(10), p.102504_1 - 102504_10, 2024/10

https://doi.org/10.1063/5.0229513

Hydrogen isotope mixing phenomena in tokamak plasmas are analyzed using global full-f gyrokinetic simulations. Model plasma parameters are chosen based on the hydrogen isotope pellet experiments on JET, in which hydrogen isotope mixing in the time scale of the energy confinement time occurred after injecting deuterium (D) pellets into hydrogen (H) plasmas. Two numerical experiments are conducted using plasma profiles before and after the D pellet injection. In both cases, turbulent fluctuations in the plasma core are characterized by ion temperature gradient driven turbulence, while in the latter case, trapped electron mode turbulence also exists in the outer region. In the former case, the density profile of bulk H ions is kept in a quasi-steady state, and the particle confinement time of bulk H ions is an order of magnitude longer than the energy confinement time. In the latter case, the density profiles of bulk H ions and pellet D ions show transient relaxation in the time scale of the energy confinement time, indicating the fast hydrogen isotope mixing. In the toroidal angular momentum balance, it is found that the hydrogen isotope mixing is driven by the toroidal field stress.

Beam transient studies for the JAEA-ADS LEBT

Yee-Rendon, B.; Kondo, Yasuhiro; Tamura, Jun; Maekawa, Fujio; Meigo, Shinichiro

Proceedings of 32nd Linear Accelerator Conference (LINAC 2024) (Internet), p.488 - 491, 2024/10

https://doi.org/10.18429/JACoW-LINAC2024-TUPB076

The Japan Atomic Energy Agency (JAEA) is designing a 30-MW CW proton linear accelerator (linac) for nuclear waste transmutation. Space-charge is the primary challenge in achieving low losses and high beam quality for high-power accelerators, especially at low energy levels where space-charge forces are greater. To counteract the space-charge effects, the low-energy beam transport (LEBT) uses a magnetostatic design to enable the neutralization of the beam charge, the so-called space charge compensation. The neutralization is an accumulation process that reaches a charge balance between the main beam and the opposite ionized particles. However, this equilibrium is destroyed by the chopper system used during beam ramping. During those transient regimes, the beam optics conditions are not optimal for the beam, producing considerable degradation that can end in serious damage to the accelerator. Thus, analysis of beam behavior at these periods is essential to develop a robust design and an efficient operation of the JAEA-ADS linac. This study presents the beam dynamics of neutralization build-up and chopper operation for the JAEA-ADS LEBT.

Opposing mixed convection heat transfer for turbulent single-phase flows (Contract research, Translated document)

Motegi, Kosuke; Shibamoto, Yasuteru; Hibiki, Takashi*; Tsukamoto, Naofumi*; Kaneko, Junichi*

JAEA-Review 2024-039, 45 Pages, 2024/09

JAEA-Review-2024-039.pdf:2.23MB

Several heat transfer correlations have been reported related to single-phase opposing flow; however, these correlations are based on experiments conducted in various channel geometries, working fluids, and thermal flow parameter ranges. Therefore, establishing a guideline for deciding which correlation should be selected based on its range of applicability and extrapolation performance is important. This study reviewed the existing heat transfer correlations for turbulent opposing-flow mixed convection. Furthermore, the authors evaluated the predictive performance of each correlation by comparing them with the experimental data obtained under various experimental conditions. The Jackson and Fewster, Churchill, and Swanson and Catton correlations can accurately predict all the experimental data. The authors confirmed that heat transfer correlations using the hydraulic-equivalent diameter as a characteristic length can be used for predictions regardless of channel-geometry differences. Furthermore, correlations described based on nondimensional dominant parameters can be used for predictions regardless of the differences in working fluids.

Manuals for resonance analysis code for neutron IMaging "RAIM"

Hasemi, Hiroyuki; Kai, Tetsuya

JAEA-Testing 2024-001, 39 Pages, 2024/08

JAEA-Testing-2024-001.pdf:1.4MB

RAIM is an analysis code that analyzes resonance absorption spectra measured at pulsed neutron sources such as the Materials and Life Science Experimental Facility (MLF) at the Japan Proton Accelerator Research Complex (J-PARC) to obtain information on nuclear densities and temperatures. By calculating the convolution of the pulse functions of neutron beam and the resonance capture function that is based on the nuclear cross section data, RAIM reproduces the resonance absorption spectrum measured by a pulsed neutron source. Then, RAIM determines the density and temperature of specific nuclides in a sample by performing spectral fitting on the resonance absorption spectrum data. In addition, RAIM is developed to facilitate the analysis of resonance imaging data by minimizing the number of parameters for calculation setup and by providing scripts for processing many resonance absorption spectra measured by a two-dimensional detector at once. This manual explains how to install RAIM on a computer and how to simulate resonance absorption spectra and fit them to measured data.

Thermal stability of retained austenite with heterogeneous composition and size in austempered Fe-2Mn-1.5Si-0.4C alloy

Watanabe, Miku*; Miyamoto, Goro*; Zhang, Y.*; Morooka, Satoshi; Harjo, S.; Kobayashi, Yasuhiro*; Furuhara, Tadashi*

ISIJ International, 64(9), p.1464 - 1476, 2024/07

https://doi.org/10.2355/isijinternational.ISIJINT-2024-044