Development of 1D-CFD coupling method for natural circulation analyses through benchmark analyses of shutdown heat removal tests in EBR-II

Yoshimura, Kazuo; Doda, Norihiro; Tanaka, Masaaki; Fujisaki, Tatsuya*; Murakami, Satoshi*

Annals of Nuclear Energy, 226, p.111896_1 - 111896_11, 2026/02

https://doi.org/10.1016/j.anucene.2025.111896

At the Japan Atomic Energy Agency, a multilevel simulation (MLS) methodology which enables consistent evaluation from whole plant behavior to local phenomena in the plant components is being developed to attempt plant design and enhance the safety of sodium-cooled fast reactors. To validate the coupling method in the MLS system, the 1D-CFD coupling method using Super-COPD for 1D plant dynamics analysis and Fluent for multi-dimensional CFD analysis was applied to the analyses of loss of flow tests in EBR-II. It was confirmed that it could predict multi-dimensional thermal-hydraulic phenomena such as thermal stratification in the upper plenum, Z-shaped pipe, and cold pool, holding the whole plant behavior simultaneously. Moreover, the applicability of the 1D-CFD coupling method to the evaluation of the phenomena in natural circulation conditions was confirmed by comparing the results of the 1D-CFD couple analyses and the measured data.

Validation of underwater radiation monitoring detector

Ji, W.*; Lee, E.*; Ji, Y.-Y.*; Ochi, Kotaro; Yoshimura, Kazuya; Funaki, Hironori; Sanada, Yukihisa

Nuclear Engineering and Technology, 58(2), p.103933_1 - 103933_6, 2026/02

https://doi.org/10.1016/j.net.2025.103933

We aimed to validate the performance of an in situ underwater radiation detector, MARK-U1 (Monitoring of Ambient Radiation of KAERI - Underwater), was used to estimate Cs activity concentration in river and reservoir sediment at predicted sites of contamination. Additionally, underwater core samples were collected to measure the radioactivity using a high-purity germanium (HPGe) detector. To estimate radioactivity, a conversion factor was derived by comparing the measured spectrum and Cs activity in the sample. A Monte Carlo N-Particle (MCNP) simulation was conducted to determine the effective source geometry for in situ measurement. The simulation results correlated well with the on-site MARK-U1 monitoring results, with a deviation of 31.62%. These findings validate the performance of the in situ detector. This device can therefore be used to estimate Cs activity concentration in the underwater sediment via on-site monitoring, without requiring sample collection.

Development of a new THINC/WLIC method based on a separate evaluation of the geometrical fidelity and the interface sharpness

Fukuda, Takanari; Yamashita, Susumu; Yoshida, Hiroyuki

Journal of Computational Physics, 545, p.114485_1 - 114485_32, 2026/01

https://doi.org/10.1016/j.jcp.2025.114485

This paper puts forward a novel approach for the evaluation of the geometrical fidelity and the interface sharpness of the VOF advection schemes separately and quantitatively. This new evaluation has elucidated the trade-off relationship of the geometrical fidelity and the interface sharpness between the existing schemes of the original THINC and the THINC/WLIC. By investigating and resolving this trade-off relationship, we have developed a novel THINC-based scheme that exhibits high performance with regard to both geometrical fidelity and interface sharpness, despite employing an algorithm as concise as those of the original THINC and the THINC/WLIC. The novel scheme, designated "THINC/Advanced WLIC (THINC/AWLIC)," has been developed by redefining the weight function of the preceding THINC/WLIC so that the contribution of the first-order upwind flux can be variably blended with the usage of the control parameter. The results of the multiple benchmark tests in two and three dimensions demonstrate that both the geometrical fidelity and the interface sharpness are significantly enhanced if the control parameter is appropriately determined. Furthermore, the associated error of THINC/AWLIC is comparable to that of the geometrical scheme, although the implementation complexity is unchanged from that of the simple THICN/WLIC.

Development of LASSO based optimized scheme for reconstructing radioactive source distributions using monitoring air dose rates

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

Measurement, 258(Part D), p.119444_1 - 119444_15, 2026/01

https://doi.org/10.1016/j.measurement.2025.119444

Demonstration of the inherent safety feature of HTGRs through the loss-of-forced-cooling test in the HTTR

Nagasumi, Satoru; Hasegawa, Toshinari; Iigaki, Kazuhiko; Nakagawa, Shigeaki; Kubo, Shinji; Shimazaki, Yosuke; Nakajima, Kunihiro; Sakurai, Yosuke; Shinohara, Masanori; Saito, Kenji; et al.

Nuclear Engineering and Design, 446, p.114542_1 - 114542_14, 2026/01

https://doi.org/10.1016/j.nucengdes.2025.114542

To demonstrate HTGR's safety features, a loss-of-forced-cooling (LOFC) test was conducted using the HTTR. In this test, the forced cooling in the reactor core was intentionally lost by shutting down all helium gas circulators (HGCs) without reactor scram. During steady-state operation at 100% reactor power (30 MW), after the LOFC, the reactor power spontaneously decreased. This power reduction occurred due to the negative reactivity feedback effect triggered by an increase in core temperature. The power stabilized at a lower value of 1.2% after re-criticality. Additionally, the measured radioactivity concentration in the primary coolant remained nearly unchanged during this LOFC operation and during an immediately subsequent HTTR operation. This indicates no failure of the coated particle fuel, even after the increase in core temperature associated with the LOFC event. These results provide experimental evidence of the safety features of HTGRs.

Random media criticality analysis using randomized Fourier series and incomplete randomized Weierstrass function

Ueki, Taro

Progress in Nuclear Energy, 191, p.106007_1 - 106007_11, 2026/01

https://doi.org/10.1016/j.pnucene.2025.106007

Criticality analysis of continuously mixed random media is crucial for safely retrieving fuel debris. Initially, a Monte Carlo method was established using the Incomplete Randomized Weierstrass Function (IRWF) to model a single-mode inverse power law power spectrum. However, image analysis showed that oxide debris mock-ups require a more complex model. To address this, a new function called the Randomized Fourier Series (RFS) was developed to represent arbitrary power spectra. RFS is versatile, incorporating Brownian motion models and aiding reactor physicists in analyzing various scenarios. Numerical results compare the fluctuation of neutron multiplication factor in various media generated by RFS and IRWF, identifying the spectral range most affecting k.

Mechanistic origin of oxygen-induced twin suppression in titanium

Chong, Y.*; Tsuru, Tomohito; Gholizadeh, R.*; Minor, A. M.*; Tsuji, Nobuhiro*

Acta Materialia, 301, p.121523_1 - 121523_12, 2025/12

https://doi.org/10.1016/j.actamat.2025.121523

Twinning is essential for achieving large ductility in hexagonal close-packed (HCP) titanium alloys that inherently lack independent slip systems due to an asymmetrical HCP crystal structure. Unfortunately, twinning is in principle suppressed by a trace amount of interstitial oxygen, resulting in a substantially deteriorated ductility in titanium. However, the underlying mechanism remains in dispute so far. Here, we report a systematic multiscale study on the twinning/detwinning behaviors of Ti-O alloys that provides a clear mechanistic view of how interstitial oxygen inhibits twinning. We reveal for the first time that oxygen atoms segregate to both {102}2} compression twin boundaries using atom probe tomography. Combined with theoretical simulations that unravel a strong pinning effect of oxygen atoms on twin boundary due to an oxygen shuffling mechanism, we explain the distinctive migration abilities of twin boundaries in Ti-O alloys at different temperatures. The insights from our experimental and computational work provide a rationale for the design of titanium alloys with increased tolerance to variations in interstitial impurity content, with significant implications for more widespread use of this high strength, light weight material.

Evaluation of Aluminum Hexacyanoferrate utilization for PGM and Mo removal from simulated high-level-raffinates in reprocessing for repository area minimization

Nakase, Masahiko*; Mishima, Ria; Abe, Takumi; Okamura, Tomohiro*; Asano, Hidekazu*

Annals of Nuclear Energy, 224, p.111569_1 - 111569_14, 2025/12

https://doi.org/10.1016/j.anucene.2025.111569

Reversible and irreversible changes in protein secondary structure in the heat- and shear-induced texturization of native pea protein isolate

Nakagawa, Hiroshi; Ubbink, J.*

Food Hydrocolloids, 168, p.111453_1 - 111453_9, 2025/12

https://doi.org/10.1016/j.foodhyd.2025.111453

The molecular mechanism of plant protein texturization under extrusion conditions was unraveled at the secondary structure level by decoupling the effects of heating, cooling and shearing on protein secondary structure. Upon heating without shearing, native alpha-helices and intramolecular-beta-sheets unfold to random domains, followed by the formation of intermolecular beta-sheets, inducing aggregation. During cooling, the intermolecular beta-sheets become increasingly ordered, and random domains partially fold into non-native beta-structures. Combined heating and shearing results in more extensive beta-sheets than heating alone. The resulting beta-rich structures provide for an entangled network of protein chains and a cohesive protein matrix.

Performance of UAV-based airborne gamma-ray spectrometry for wide-area radiation monitoring of contaminated sites

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

Journal of Radiological Protection, 45(4), p.042501_1 - 042501_11, 2025/12

https://doi.org/10.1088/1361-6498/ae0cd4

This study reports the development and field validation of KAERI's UAV-based gamma-ray spectrometry system equipped with LaBr(Ce) detectors. Joint surveys with JAEA near Fukushima Daiichi Nuclear Power Plant (FDNPP) showed reliable dose rate estimation after applying altitude based attenuation correction, through discrepancies occurred in sloped terrain. Incorporating terrain data is recommended to enhance accuracy for emergency response applications.