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.
Yasunobu TAKINAMI (Thu,) studied this question.