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

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

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.