Performance evaluation for rapid-dose estimation of radioactive plume dispersion based on pre-simulation database of wind conditions by large-eddy simulation

Sato, Takuto; Nakayama, Hiromasa; Satoh, Daiki

Journal of Nuclear Science and Technology, 17 Pages, 2025/09

https://doi.org/10.1080/00223131.2025.2555784

We developed a framework for rapid monitoring of radioactive plumes in the vicinity of nuclear facilities based on a quick and practical high-resolution atmospheric dispersion simulation method that combines a large-eddy simulation (LES) model pre-simulation database (pre-sim DB) of wind conditions and onsite meteorological observation results, as proposed by the previous study. However, this framework was not quantitatively demonstrated using measurement data. In this study, we evaluated the performance of the wind condition reproduction and plume dispersion analysis methods. Air dose rates observed at monitoring posts around the stack were compared with the values reproduced by the method using the pre-sim DB, and the reproducibility of both air dose rate and flow field was discussed. The pre-sim DB-based method successfully captured the temporal variation of air dose rates at the monitoring posts, though it tended to overestimate the peak values. Particularly when the vertical wind shear was pronounced, the method using the pre-sim DB could cause significant errors. This is likely because the method relies on wind conditions from a single observation point, which inherently limits its ability to represent vertical wind shear within the pre-sim DB. Despite these limitations, particularly in reproducing complex wind fields, the method utilizing the pre-sim DB offers a valuable and practical tool for rapid dose rate simulation due to its lower computational cost compared to unsteady simulations using an LES model.

Sub-pin level distribution tallies and statistical error estimation with POD tallies in two-dimensional C5G7 benchmark

Kondo, Ryoichi; Yamamoto, Akio*; Endo, Tomohiro*

Journal of Nuclear Science and Technology, 12 Pages, 2025/09

https://doi.org/10.1080/00223131.2025.2554969

The flux distribution tallies using proper orthogonal decomposition, called POD tallies, have been developed to address issues of high-fidelity Monte Carlo simulations. The POD tallies can reduce both dimensionality and statistical error. The present study verifies the applicability of the POD tallies to sub-pin level flux distribution in the two-dimensional C5G7 benchmark. Statistical error estimation is also proposed by applying the circular block bootstrap method to the POD tallies to estimate the statistical error of the flux distribution in a single Monte Carlo calculation. In the verification, the dimensionality of the finely discretized distribution is reduced by more than 90% compared with conventional cell tallies. The statistical error is reduced by more than half as the average value of all tally regions. The proposed approach is confirmed to properly estimate the statistical error of flux distribution considering both the inter-cycle correlation and the correlation between the expansion coefficients of different POD orders.

Evaluation of Co inventory in the core of the Fukushima Daiichi Nuclear Power Plant; Contribution of fuel deposits to the reactor core inventory

Uchida, Shunsuke*; Kino, Chiaki*; Karasawa, Hidetoshi; Takahatake, Yoko; Koma, Yoshikazu

Journal of Nuclear Science and Technology, 17 Pages, 2025/07

https://doi.org/10.1080/00223131.2025.2499187

Evaluation of radioactive nuclide behavior on and after the accident is important for the estimating radioactive nuclide composition in the wastes. The reactor core inventories have been obtained from the ORIGEN2 analysis, but the inventory of activation products is determined by the amount of their parent nuclides which are impurities contained in the structural materials. The ORIGEN2 does not treat fuel deposits including the impurities. Estimation of the initial Co-60 inventory in accurate is needed on the evaluation of some kinds of radioactive nuclide amount, since it is possible Co-60 is standard in the scaling factor. In this study, contribution of fuel deposits to the reactor core inventory was estimated by comparing the amount of Co-60 and Ni-63 calculated by the amounts of deposition by the microlayer-evaporation and drying-out model (MEDO model) and the result of the ORIGEN2 analysis, and then the method of estimating the reactor core inventory was proposed.

Estimation of the beam trip frequency of a proton linear accelerator for an accelerator-driven nuclear transmutation system and comparison with the allowable beam trip frequency

Takei, Hayanori

Journal of Nuclear Science and Technology, 45 Pages, 2025/06

https://doi.org/10.1080/00223131.2025.2523023

The Japan Atomic Energy Agency is working on the research and development of an accelerator-driven nuclear transmutation system (ADS) for transmuting minor actinides. This system combines a subcritical nuclear reactor with a high-power superconducting proton linear accelerator (JADS-linac). One of the factors limiting the advancement of the JADS-linac is beam trips, which often induce thermal cycle fatigue, thereby damaging the components in the subcritical core. The average beam current of the JADS-linac is 32 times higher than that of the linear accelerator (linac) of the Japan Proton Accelerator Research Complex (J-PARC). Therefore, according to the development stage, comparing the beam trip frequency of the JADS-linac with the allowable beam trip frequency (ABTF) is necessary. Herein the beam trip frequency of the JADS-linac was estimated through a Monte Carlo program using the reliability functions based on the operational data of the J-PARC linac. The Monte Carlo program afforded the distribution of the beam trip duration, which cannot be obtained using traditional analytical methods. Results show that the frequency of the beam trips with a duration exceeding 5 min must be reduced to 27% of the current J-PARC linac level to be below the ABTF.

Evaluations on the thermal properties of Cs-B-O compounds using density functional theory and phonon vibration calculations

Suzuki, Chikashi

Journal of Nuclear Science and Technology, 62(6), p.542 - 551, 2025/06

https://doi.org/10.1080/00223131.2025.2451824

Boron (B) can chemically react with cesium (Cs) to form Cs-B-O compounds and affect the chemical behavior of Cs during severe accidents. The author evaluated the thermal properties of solid Cs-B-O compounds using density functional theory and phonon vibration calculations with CsO and BO as solid reference materials. These evaluations indicate that the calculated thermal properties are in good agreement with the reported ones. The author calculated the reaction enthalpy and Gibbs free energy values of solid CsBO from CsO and BO to obtain fundamental data in solid systems. The deviations between the calculation and the reported data in this study are comparable with those in the previous study. The author estimated the Gibbs free energy values of the CsBO reaction from CsBO and boric acid. The differences between these estimations and those in the recent investigation are within the error of the reaction energies in the experiments.

Numerical investigation of the accuracy of a conductance-type wire-mesh sensor for a single spherical bubble and bubbly flow

Uesawa, Shinichiro; Ono, Ayako; Nagatake, Taku; Yamashita, Susumu; Yoshida, Hiroyuki

Journal of Nuclear Science and Technology, 62(5), p.432 - 456, 2025/05

https://doi.org/10.1080/00223131.2024.2447366

We performed electrostatic simulations of a wire-mesh sensor (WMS) for a single spherical bubble and bubbly flow to clarify the accuracy of the WMS. The electrostatic simulation for the single bubble showed the electric current density distribution and the electric current path from the excited transmitter to receivers for various bubble locations. It indicated systematic errors based on the nonuniform current density distribution around the WMS. The electrostatic simulation for the bubbly flow calculated by the computational fluid dynamics code, JAEA Utility Program for Interdisciplinary Thermal-hydraulics Engineering and Research (JUPITER), indicated that the WMS had difficulty in quantitatively measuring the intermediate values of the instantaneous void fraction between 0 and 1 because they cannot be estimated by previous transformation methods from the WMS signal to the void fraction, such as linear approximation or Maxwell's equation, and have a significant deviation of the void fraction of 0.2 for the WMS signal. However, the electrostatic simulation indicated that the time-averaged void fractions around the center of the flow channel can be estimated using linear approximation, and the time-averaged void fraction near the wall of the flow channel can be estimated using Maxwell's equation.

Impact of nuclear data updates from JENDL-4.0 to JENDL-5 on burnup calculations of light-water reactor fuels

Watanabe, Tomoaki; Tada, Kenichi; Endo, Tomohiro*; Yamamoto, Akio*

Journal of Nuclear Science and Technology, 16 Pages, 2025/04

https://doi.org/10.1080/00223131.2025.2486798

This study investigated the impact of nuclear data updates from JENDL-4.0 (J4) to JENDL-5 (J5) on the light-water reactor fuel burnup calculations. Burnup calculations were conducted with J4 and J5 for PWR pin-cell and BWR fuel assembly geometries. The calculation results revealed significant burnup-dependent differences in the neutron multiplication factor (k). Across the burnup range of 0-50 GWd/t, k values of J5 were consistently smaller than those of J4 and the difference gradually increased as burnup progressed. Direct sensitivity calculations, in which each nuclide data was replaced from J4 to J5, indicated that updates to the cross-sections of U, U, and Pu and the thermal scattering law data of H in HO notably impacted the k differences. For the BWR assembly geometry containing Gd fuels, large k differences were observed in the burnup range of 10-15 GWd/t. This difference was primarily attributed to updates in the U, Gd, and Gd cross-sections, and thermal scattering law data of H in HO. Furthermore, we investigated how the nuclear data updates affected the k differences by examining nuclide number densities, the energy-dependent sensitivities, and the neutron spectra.

Development of neutron self-indication thermometry at J-PARC

Segawa, Mariko; Toh, Yosuke; Maeda, Makoto; Kai, Tetsuya

Journal of Nuclear Science and Technology, 62(3), p.268 - 277, 2025/03

https://doi.org/10.1080/00223131.2024.2424519

Feasibility study of reactor radiation photon spectroscopy in Fugen for nuclear decommissioning

Kaburagi, Masaaki; Miyamoto, Yuta; Mori, Norimasa; Iwai, Hiroki; Tezuka, Masashi; Kurosawa, Shunsuke*; Tagawa, Akihiro; Takasaki, Koji

Journal of Nuclear Science and Technology, 62(3), p.308 - 316, 2025/03

https://doi.org/10.1080/00223131.2024.2417786

Extraction behaviors of minor actinides and rare earth elements with NTA amide extractants

Suzuki, Hideya*; Ban, Yasutoshi

Journal of Nuclear Science and Technology, 62(2), p.157 - 166, 2025/02

https://doi.org/10.1080/00223131.2024.2402366