Development of quick neutron spectrum reconstruction module based on POD for burnup calculation of fast reactor

Aizawa, Naoto*; Watanabe, Tomoaki; Chiba, Go*; Tada, Kenichi; Fujita, Tatsuya*; Yamamoto, Akio*

Nuclear Engineering and Technology, 58(5), p.104176_1 - 104176_16, 2026/05

https://doi.org/10.1016/j.net.2026.104176

Projections of coastal landscape evolution on a 10 years considering uncertainty of sea level change

Takai, Shizuka; Uchikoshi, Emiko*; Shimada, Taro; Takeda, Seiji

MRS Advances (Internet), 6 Pages, 2026/00

https://doi.org/10.1557/s43580-026-01551-5

In Japan, regulations require the disposal of intermediate-level radioactive waste at depths of at least 70 m below ground surface for 10 years. Therefore, it is essential to assess the long-term safety of landscape evolution by uplift, denudation, and sea-level change, which will decrease the depth of disposal and change groundwater flow. Numerical simulations are invaluable for analyzing landscape evolution while taking into account future uncertainties. However, the projection over the next 10 years has not been conducted for coastal areas, which are preferable to disposal in terms of waste transportation. In this study, we developed a framework for projection of coastal landscape evolution on a 10 years based on simulations over the past 125,000 years. Using the LEMs developed by JAEA, the constructed framework following two steps was verified in the Kamikita coastal plain. Step 1) Simulation from past to present: Based on simulations of the past 125,000 years (the last glacial-interglacial cycle), we first identified the best parameter set that can reproduce the actual topography. The deviation between reproduced and recent topography is within 15 m for 80% of the area. Step 2) Projection over the next 10 years: Using the best parameters, we make projections over the next 125,000 years. On the basis of sea level change in the late Quaternary and previous research on the glacial isostatic adjustment modelling, three scenarios of future global sea level were considered; base case (the next glacial period starts 5,000 years after present), glacial case (the next glacial period starts from the present), and global warming case (the current interglacial lasts over the next 125,000 years). In the case of a hypothetical disposal depth of 100 m, erosion depth should be less than 30 m to maintain the waste at a depth of 70 m below ground. For the next 125,000 years, erosion exceeding 30 m is projected to occur in the river (mid and downstream) and coastal areas (~below highest projected sea level of 15 m). Vertical erosion by fluvial incision is the most affected by sea level decline. In the base and global warming cases, sea level rises up to 15 m; erosion occurs several hundred meters landward from the current 15-m contour. We also confirmed that the average basin relief increases significantly in accordance with sea level fall.

Development of 3D-air dose rate evaluation system

Kim, M.

Kankyo Sozo Senta Chosa Kenkyu Seika Hokokusho, Heisei 27-Nendo (2015) Reiwa 6-Nendo (2024) (Internet), P. 31, 2025/10

3D-Air Dose Rate Evaluation System has been developed that can accurately estimate complex spatial dose rate distributions due to radiation emitted into the environment. The applicability of the system was examined, and the results of the analysis and measurements were in general agreement.

Development of a probabilistic risk assessment method for combined hazards; An Application example of multihazard events

Choi, B.; Nishida, Akemi; Tsutsumi, Hideaki*; Takada, Tsuyoshi

Transactions of the 28th International Conference on Structural Mechanics in Reactor Technology (SMiRT28) (Internet), 10 Pages, 2025/08

One of the key lessons learned from the Fukushima Daiichi Nuclear Power Plant accident is the necessity of developing a probabilistic risk assessment (PRA) method that considers multiple external hazards (multi-hazards), such as earthquakes and tsunamis. In this study, we organized classification concepts for multi-hazards that could lead to significant disasters. A classification method for multi-hazard scenarios was developed based on causal relationships (dependent or independent) and temporal relationships (simultaneous or time-lagged occurrence). Furthermore, we examined modeling approaches for multi-hazard scenarios using the developed classification method. The modeling considered elements such as hazard curves, hazard intensity, hazard frequency, time, and event duration, and these were defined in terms of conditional probability density functions. This paper reports application examples of multi-hazard evaluation under hypothetical site and analysis conditions, focusing on representative scenarios such as earthquake-tornado and earthquake-tsunami combinations, based on the multi-hazard classification method developed in previous studies.

Fast burnup calculation method based on neutron spectrum reconstruction with proper orthogonal decomposition and regression model

Watanabe, Tomoaki; Aizawa, Naoto*; Chiba, Go*; Tada, Kenichi; Yamamoto, Akio*

Proceedings of International Conference on Mathematics and Computational Methods Applied to Nuclear Science and Engineering (M&C 2025) (Internet), p.288 - 297, 2025/04

Currently, a major burnup calculation method for the nuclide composition of nuclear fuel conducts neutron transport calculations at each burnup step to account for changes in the neutron spectrum. While this method is highly accurate, the large computational cost of neutron transport calculations can be problematic. Therefore, a fast burnup calculation method based on neutron spectrum reconstruction with the proper orthogonal decomposition (POD) and regression model is investigated. In this method, dimensionality reduction by POD is applied to many neutron fluxes obtained from detailed burnup calculations for various input parameter sets, and regression models are constructed to connect the dimensionality-reduced neutron fluxes and parameters. By substituting arbitrary input parameters to the regression models, the neutron flux is reconstructed and the burnup calculation is performed. This method performs burnup calculations that consider changes in the neutron spectrum based on input conditions without neutron transport calculations. The present method was applied to a PWR UO fuel pin cell model. The results show the nuclide inventory can be calculated with a prediction accuracy within a few percent. In addition, it is found that the calculation error is dominated by the regression models, which implies the further improvement of the regression models leads to improving the accuracy.

Improvement of uranium identification based on electron microscope image analysis using deep learning models for nuclear forensic analysis

Kimura, Yoshiki; Matsumoto, Tetsuya*; Yamaguchi, Tomoki

Dai-45-Kai Nihon Kaku Busshitsu Kanri Gakkai Nenji Taikai Kaigi Rombunshu (Internet), 4 Pages, 2024/11

no abstracts in English

Development of a probabilistic risk assessment method for combined hazards; A Classification and modeling framework for multihazard events

Choi, B.; Nishida, Akemi; Tsutsumi, Hideaki*; Takada, Tsuyoshi

Proceedings of Probabilistic Safety Assessment and Management & Asian Symposium on Risk Assessment and Management (PSAM17 & ASRAM2024) (Internet), 8 Pages, 2024/10

The Fukushima Daiichi Nuclear Power Plant accident highlighted the importance of developing safety assessment methods that consider multihazard events involving numerous simultaneously occurring events such as earthquakes (shaking) and tsunamis (submersion). When addressing such multihazard events, traditional methods often focus on assessing the load combinations of general structures in their structural designs and adopt simple selection criteria. However, these methods fall short when evaluating, countering, and screening external events, such as earthquakes, tsunamis, strong winds, and concentrated heavy rainfall, that occur simultaneously or in a chain. To address this, we reviewed existing literature on multihazard assessment methods, focusing particularly on scenarios involving earthquake and tsunami events. Based on concepts and basic theories, we examined various methods for addressing multihazard scenarios and classified their characteristics. Specifically, several multihazard scenarios were surveyed, and the relationships between multiple hazards were organized. In addition, common causes leading to combined events, their mutual influences, and potential cascading effects were analyzed.

A Study of evaluation method for buffer material erosion in vertical in-hole emplacement

Kikuchi, Hirohito*; Uda, Toshiaki*; Hayashi, Daisuke*; Emori, Minoru*; Kimura, Shun

Genshiryoku Bakkuendo Kenkyu (CD-ROM), 31(1), p.11 - 20, 2024/06

https://doi.org/10.3327/jnuce.31.1_11

no abstracts in English

Development of radiation measurement and digital technologies for the decommissioning of the Fukushima Daiichi Nuclear Power Plant

Takasaki, Koji

Hokeikyo Nyusu, (73), p.2 - 5, 2024/04

The development of radiation measurement and digital technology for the decommissioning of the Fukushima Daiichi Nuclear Power Plant, which is being conducted by the Remote System and Sensing Technology Division of CLADS, will be presented.

Evaluation of temporal changes in fracture transmissivity in an excavation damaged zone after backfilling a gallery excavated in mudstone

Aoyagi, Kazuhei; Ishii, Eiichi

Environmental Earth Sciences, 83(3), p.98_1 - 98_15, 2024/02

https://doi.org/10.1007/s12665-023-11416-x

The long-term geological disposal of high-level radioactive waste relies on predictions of future changes in a disposal facility's hydro-mechanical characteristics to assess potential leakage through fractures in the excavation damaged zone (EDZ) after backfilling the facility. This study evaluated the transmissivity of EDZ fractures using in situ hydraulic tests around the area of a full-scale, experimental, engineered barrier system in the Horonobe Underground Research Laboratory, Hokkaido, Japan. After their installation, the buffer blocks swelled, altering the stresses within the EDZ fractures. The effects of these changing stresses on the fractures' transmissivity were assessed over a period of 4 years. The transmissivity continuously decreased in this period to about 41% of its value measured prior to the swelling. Using the Barton-Bandis normal-stress-dependent fracture-closure model, the decrease in transmissivity is quantitatively attributed to closure of the EDZ fractures, which was caused by the swelling pressure increasing up to 0.88 MPa. Evidence of fracture closure came from seismic tomography surveying, which revealed a slight increase in seismic velocity in the study area with increasing swelling pressure. The results show that EDZ fractures were closed by swelling of the full-scale buffer material. They also demonstrate the applicability of the Barton-Bandis model to preliminary estimation of the long-term transmissivity of EDZ fractures in facilities for the geological disposal of radioactive waste.