Nuclear regulation inspection should be carried out based on the risk-informed during decommissioning stage at nuclear power station. However, it is difficult because the methodology for evaluating the radiation exposure risk quantitatively during dismantling activities has not been established. Therefore, a decommissioning risk assessment code DecAssess-R has been developed based on the decommissioning safety assessment code, DecAssess, creating event trees from initiating events and evaluating the radiation risk resulting from public exposure dose for each accident sequence. In the evaluation, that movable radioactive inventory which may be easily dispersed into workspace such as radioactive dust accumulated in HEPA filters attached at a contamination control enclosure is fluctuated according to the progress of dismantling works was taken into account. Initiating events were selected based on the investigation for accidents and troubles during activities of dismantling, disassembly and replace of components around the world, and event trees from the initiating events indicating progress scenario were created. Frequencies of occurrence were set with reference to data of general industry in addition to data regarding above accidents and troubles, event progression probabilities were set with reference to failure data during the operating stage. It was also taken into account that safety functions to be maintained during decommissioning stage which the operator applied at decommissioning plan were reduced according to the progress of dismantling activities. Public exposure dose after accidental release of radioactive dusts is evaluated at exposure pathway such as cloud shine, respiration and surface deposition by atmospheric dispersion. The exposure risks during dismantling of components in reference BWR were evaluated. As a result, public exposure dose was maximum in the case of fire during dismantling of reactor internals and spread of fire to combustibles and filters including radioactivity temporally stored at the workspace. Exposure risk also showed the maximum because the occurrence possibility of this accident sequence was larger than other scenario.

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