During decommissioning of nuclear facilities, it is generally thought that the risk is relatively low after high activity inventory such as the spent fuel is removed. However, dismantlement works may be carried out with non-multiple barriers with a non-regular process depending mainly on human activities. Moreover, fire or gas incidents caused by conventional industry methods may lead to accidents with radioactivity release. This means more attention is necessary for safer dismantlement especially for nuclear reactors with high activity. Therefore, utilization of risk information based on risk assessment of the decommissioning was proposed. A method of risk assessment for decommissioning was developed and applied for the dismantlement of typical reactor facilities and nuclear fuel facilities (a uranium enrichment plant and a reprocessing plant). The results show that the consequences of such troubles are low, but their occurrences are still not negligible. This fact is supported by past trouble cases. Taking into account of the risk assessment results, a methodology to secure the safety of decommissioning was proposed. It consists of four steps i.e. 1) risk-informed approach, 2) graded approach, 3) phased approach and 4) layered approach and the results can be reflected to the management and regulation. The regulation means are for example, review of the decommissioning plan or the fitness-for-safety, the periodic safety inspections and usual monitoring. The methodology can evaluate the risk level of decommissioning more objectively and enable reasonable regulation based on the risk level. This leads to the appropriate distribution of resources with safety enhancement.

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