廃止措置段階の被ばくリスク評価コードDecAssess-Rの開発

Development of exposure risk assessment code at decommissioning stage of nuclear power station, DecAssess-R

島田 太郎 ; 三輪 一爾 ; 笹川 剛  ; 高井 静霞 ; 武田 聖司 

Shimada, Taro; Miwa, Kazuji; Sasagawa, Tsuyoshi; Takai, Shizuka; Takeda, Seiji

原子力発電所の廃止措置段階における原子力規制検査のリスク情報に基づいた実施のために、解体対象機器ごとに事故シーケンス別被ばく線量と発生確率から廃止措置工程に応じた被ばくリスクの経時変化を評価するコードDecAssess-Rの開発を進めている。廃止措置段階での起因事象からイベントツリーを構築するとともに、過去のトラブル事例から起因事象の発生頻度及び事故進展確率の整備を進めた。開発をしているリスク評価の手法により、米国の110万kW級のBWR解体作業を例として、火災を起因事象とした被ばくリスクを評価した。その結果、放射能保有量の大きい炉内構造物の水中解体作業時に発生する火災よりも、放射能保有量の小さい原子炉建屋内の周辺機器の気中解体時に発生する火災の方が被ばくリスクが大きくなるなど、単なる残存放射能インベントリの大小だけではない、廃止措置工程に応じたリスク情報を提供できることが確認できた。

For risk-informed implementation of nuclear regulatory inspections during the decommissioning phase of nuclear power plants, we are developing DecAssess-R, a code that evaluates changes in exposure risk over time according to the decommissioning process based on exposure dose and occurrence probability by accident sequence for each component to be dismantled. Event trees have been constructed from the initiating events that occurred during the decommissioning phase, and the occurrence frequency and accident progress probability of the events have been developed from past trouble cases. Using the developed risk assessment method, we evaluated the exposure risks due to fire as an example of dismantling of a 1.1 MW BWR in the United States. As a result, it was confirmed that the method can provide risk information according to the decommissioning process, not only the quantity of the residual radioactivity inventory, such as the result that the exposure risk is larger for a fire that occurs during air dismantling of peripheral equipment in the reactor building, which has a smaller radioactivity inventory, than for a fire that occurs during underwater dismantling of in-core structures, which has a larger radioactivity inventory. The results confirmed that the information on the risk according to the decommissioning process can be provided.