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Nagaya, Yasunobu
EPJ Nuclear Sciences & Technologies (Internet), 11, p.1_1 - 1_7, 2025/01
Japan Atomic Energy Agency (JAEA) has been developing a general-purpose continuous-energy Monte Carlo code MVP for nuclear reactor core analysis. Recently improvements to MVP have been focused on the development of an advanced neutronics/thermal-hydraulics coupling code. JAEA has also developed a new Monte Carlo solver Solomon for criticality safety analysis. Solomon aims to calculate the criticality of a damaged reactor core including fuel debris. This paper provides an overview of the capabilities and reviews recent applications of MVP and Solomon.
Ogawa, Tatsuhiko; Hirata, Yuho; Matsuya, Yusuke; Kai, Takeshi; Sato, Tatsuhiko; Iwamoto, Yosuke; Hashimoto, Shintaro; Furuta, Takuya; Abe, Shinichiro; Matsuda, Norihiro; et al.
EPJ Nuclear Sciences & Technologies (Internet), 10, p.13_1 - 13_8, 2024/11
The latest updates on PHITS, a versatile radiation transport code, focusing specifically on track-structure models are presented. Track structure calculations are methods used to simulate the movement of charged particles while explicitly considering each atomic reaction. Initially developed for radiation biology, these calculation methods aimed to analyze the radiation-induced damage to DNA and chromosomes. Several track-structure calculation models, including PHITS-ETS, PHITS-ETS for Si, PHITS-KURBUC, ETSART, and ITSART, have been developed and implemented to PHITS. These models allow users to study the behavior of various particles at the nano-scale across a wide range of materials. Furthermore, potential applications of track-structure calculations have also been proposed so far. This collection of track-structure calculation models, which encompasses diverse conditions, opens up new avenues for research in the field of radiation effects.
Sugihara, Kenta; Onodera, Naoyuki; Sitompul, Y.; Idomura, Yasuhiro; Yamashita, Susumu
EPJ Web of Conferences, 302, p.03002_1 - 03002_10, 2024/10
Times Cited Count：0 Percentile：0.00(Computer Science, Interdisciplinary Applications)In simulations of gas-liquid two-phase flows using conventional interface capture methods, we observed that when bubbles come close to each other, they tend to merge numerically, despite experimental evidence indicating that they would repel each other. Given the significant impact of sequential numerical coalescence on flow patterns, it is necessary to regulate the merging behavior of close bubbles. To address this issue, we introduced the Multi- Phase Field (MPF) method, which mitigates bubble coalescence by applying an independent fluid fraction function to each bubble. In this study, we employed the MPF based on the N-phase model to minimize numerical errors associated with surface interactions at triple junction points. Additionally, we implemented the Ordered Active Parameter Tracking (OAPT) method to efficiently store several hundreds of fluid fraction functions. To validate the MPF method, we conducted analysis of turbulent bubbly pipe flows and compared the results against experimental data from Colin et al. The validation results showed reasonable agreements with respect to the bubble distribution and the flow velocity profiles.
Sitompul, Y.; Sugihara, Kenta; Onodera, Naoyuki; Idomura, Yasuhiro
EPJ Web of Conferences, 302, p.05004_1 - 05004_10, 2024/10
Times Cited Count：0 Percentile：0.00(Computer Science, Interdisciplinary Applications)In fast reactor designs, it is of critical importance to avoid gas entrainment phenomena due to free-surface vortices. Numerical analysis is one of the key methods to understand these phenomena. However, the challenges in computational efficiency and accuracy of the previous numerical methods lead to exploring the Lattice Boltzmann method (LBM) as an alternative, known for its computational efficiency and capability in simulating complex flows. In this study, we implement free-surface LBM to accelerate gas entrainment analysis, significantly reducing computational costs while maintaining accuracy compared to traditional methods. Simulation results using LBM align well with experimental data, offering a promising avenue for faster analysis in future fast reactor designs.
Yamada, Susumu; Machida, Masahiko; Tanimura, Naoki*
EPJ Web of Conferences, 302, p.16004_1 - 16004_10, 2024/10
Times Cited Count：0 Percentile：0.00(Computer Science, Interdisciplinary Applications)When we decommission a reactor building, it is desirable to identify the radiation source distribution for safety. It has been reported that the source distribution can be estimated from the measured air dose rates at appropriate observation points by minimizing an evaluation function using the Least Absolute Shrinkage and Selection Operator (LASSO). However, it is difficult to decide on suitable points in advance. Therefore, we estimate the posterior distribution from the prior distribution of the source amounts, which are calculated by the standard LASSO, using the Bayesian LASSO, and evaluate the predictive distribution of the evaluated air dose rates at the candidate observation points from the posterior distribution. We select the additional observation point based on the variances of the predictive distributions. We confirmed that the method can estimate the source distribution with fewer additional observation points than when adding ones randomly in most cases.
Kondo, Ryoichi; Endo, Tomohiro*; Yamamoto, Akio*
EPJ Web of Conferences, 302, p.04002_1 - 04002_10, 2024/10
Times Cited Count：0 Percentile：0.00(Computer Science, Interdisciplinary Applications)In the recent study, we have developed an efficient flux distribution tallying method in the Monte Carlo calculation toward the high-fidelity, large scale multi-physics simulation. In this method, the proper orthogonal decomposition is applied to the flux distribution tallies. While the tallying method was implemented with the collision estimator in the previous study, the track length estimator is implemented in the present study to obtain the tally with lower statistical error. The implementation of the flux distribution tally with the track length estimator is compared with that of the collision estimator and the normal track length estimator in a one-dimensional problem. The numerical results reveal that the distribution tally using the POD with the track length estimator can obtain a more precise solution compared with that with the collision estimator. Therefore, in terms of the statistical error, the relationship between the distribution tally with track length and collision estimator is similar to that between the conventional track length and collision estimators.
Hasegawa, Yuta; Idomura, Yasuhiro; Onodera, Naoyuki
EPJ Web of Conferences, 302, p.03005_1 - 03005_9, 2024/10
Times Cited Count：0 Percentile：0.00(Computer Science, Interdisciplinary Applications)We implemented the ensemble data assimilation (DA) method, the local ensemble transform Kalman filter (LETKF), into the mesh-refined lattice Boltzmann method (LBM) for turbulent flows. Both the LETKF and the mesh-refined LBM were fully implemented on GPUs, so that they are efficiently computed on modern GPU-based supercomputers. We examined the DA accuracy against the flow around a cylinder. The result showed that our method enabled accurate DA with spatially- and temporarily-sparse observation data; the error of the assimilated velocity field with the observation interval of 
and the observation resolution 
(1.56% of the total computational grids) was smaller than the amplitude of the observation noise, where 
is the period of the K
rmn vortex and 
is diameter of the square cylinder.
Kawamura, Takuma; Shimomura, Kazuya; Osaki, Tsukasa*; Idomura, Yasuhiro
EPJ Web of Conferences, 302, p.11002_1 - 11002_8, 2024/10
Times Cited Count：0 Percentile：0.00(Computer Science, Interdisciplinary Applications)In the field of nuclear engineering, complex simulations on exa-scale supercomputers generate large-scale data. To facilitate efficient analysis of such simulation data, one needs to share them among scientists at remote locations. However, data I/O and data transfer for such large-scale data are quite costly. To resolve these issues, we developed a remote in-situ visualization system IS-PBVR based on the particle-based volume rendering (PBVR), which is suitable for parallel processing on modern supercomputers. In this study, we extend IS-PBVR for VR visualization on multiple client PCs, thus developing a multi-point remote VR visualization. We apply this technique to fluid simulations on GPU-based supercomputers and verify its utility by sharing in-situ VR visualization between multiple client PCs.
