AC system piping contamination caused by droplet entrainment from S/P water during containment venting at Fukushima Daiichi NPS Unit 1

The high-level contamination in the atmospheric control (AC) system piping in Fukushima Daiichi NPS Unit 1 (1F1) was believed to be related to primary containment vessel (PCV) venting; however, the mechanisms remain unclear. The Nuclear Regulation Authority’s study group examined potential causes, such as leakage from vacuum isolation valves or a decrease in the decontamination factor due to boiling under reduced pressure; however, these factors could not explain the contamination. Therefore, based on the similarity between JAEA’s ALPHA test of aerosol entrainment from a flashing pool and the 1F1 accident, we investigated whether the contamination of the AC system piping could be attributed to the deposition of droplets entrained from the pressure suppression pool. Our study showed that the radioactive entrained droplets passing through the AC system piping were nearly identical to the environmental release predicted by WSPEEDI reverse calculations during PCV venting. Furthermore, in Unit 3 (1F3), which underwent PCV venting similarly to that in 1F1, no significant contamination was observed in the AC system piping. The differences between 1F1 and 1F3 clarify why contamination increased only during PCV venting in 1F1. Therefore, from the perspective of insights gained and source term reduction, recommendations are provided regarding the factors to consider when implementing PCV venting.