This study was motivated by the inappropriate practices identified in the development of the design-basis ground motion at Chubu Electric Power Company. Rather than treating the case as an issue confined to a single operator, it was positioned within a broader context, and the quality management framework governing design processes in Japan’s nuclear safety regulation was examined. As a result, a potential structural pitfall in Japan’s regulatory system was identified: the institutional design of quality management for design processes requiring highly specialized expert judgment may have been relatively underdeveloped.

The radiation dose rates measured by continuous monitoring posts (MPs) are essential for understanding spatial dose rate distributions and determining appropriate radiation protection measures during a nuclear emergency. However, MPs have different characteristics, such as installation height, measurement units, and monitor types. We examined 1,692 MPs that are installed across Japan to investigate these characteristics. To quantitatively assess the effects of MP characteristics on radiation dose measurements, numerical simulations were conducted under assumed radionuclide distribution conditions following a light water reactor accident. The simulation model was validated through comparisons with observed monitoring data following the Fukushima Daiichi Nuclear Power Plant accident. The simulation results showed that the variability in measured values due to differences in installation height and measurement units generally ranged from 0.70 to 1.1 times when the ambient dose equivalent rate at a height of 1.0 m was used as reference. These findings provide important insights for the appropriate use of MP data and the improvement of emergency radiation monitoring systems in future nuclear accidents.

I reexamine the role of the Isolation Condenser (IC) at Unit 1 of the Fukushima Daiichi Nuclear Power Plant by focusing on human recognition and decision-making under extreme conditions. While the IC has been extensively examined from technical and thermal–hydraulic perspectives by post-accident analyses, research on how on-site operators perceived, questioned, and ultimately judged the system’s operational status during the accident has received limited attention. Using oral history interviews with Ikuo Izawa, who served as the shift supervisor of Units 1 and 2 at the time of the accident, I analyze operators’ actions through the analytical framework proposed by Koshizuka, which articulates three essential requirements for passive safety systems: the necessity of questioning their operation, the importance of confirmation through human senses, and the ease of recovery when malfunction is suspected. The findings indicate that operators actively doubted IC operation, relied on auditory cues such as characteristic operating noise, and attempted the visual confirmation of steam discharge despite the loss of instrumentation following the station blackout. However, structural constraints—most notably the inability to manually operate isolation valves located inside the containment—rendered recovery impossible regardless of operator skill or judgment. The results of this study demonstrate that the effectiveness of the IC was ultimately constrained not by human error but by design assumptions that underestimated the necessity of human intervention. This highlights the importance of integrating human perceptual and decision-making capacities into the design philosophy of passive safety systems.

A method to enable the simultaneous inspection of tires and the wiper area using a portable radiation portal monitor is being developed to improve the efficiency of vehicle inspection during contamination screening. To achieve this, we devised an approach to identify whether contamination is located on the tires or in the wiper area. We designed an L-shaped measurement configuration, where detectors are positioned horizontally near the ground and vertically upright. An identification approach based on this L-shaped configuration was evaluated using Monte Carlo simulations. The results showed that the magnitude relation of counts measured by the horizontal and vertical detectors varies depending on whether the point source is located on the tires or in the wiper area. We examined an identification method that utilizes this relationship to determine the contaminated location. Next, we conducted measurements using a portal monitor (Gamma Pole, manufactured by Chiyoda Technol) and attempted to identify the contaminated location by the L-shaped measurement method. As a result, in the experiment simulating tire contamination, all 30 measurements correctly identified the tires as the contaminated location. In contrast, in the experiment simulating wiper contamination, 29 out of 30 measurements correctly identified the wiper area.