Preliminary study of pre-closure safety assessment in the NUMO safety case

The methodology of assessment for pre-closure safety has been developed as a part of NUMO safety case. The ongoing update of the safety case for co-disposal of HLW and TRU waste in Japan will include a more extensive assessment of pre-closure safety than has been carried out in the past. The pre-closure safety case aims to assure both radiological and conventional safety. Radiological safety for public and workers requires radiation shielding and radionuclide containment within the disposal facilities in case of operational perturbations. Radiation control and facility design are based on guidelines applied for other nuclear facilities. Within the radiation-controlled zones, most operations will be remote-handled or will involve appropriate shielding, avoiding any significant dose to workers. For conventional safety for workers, the working environment will be maintained to ensure worker comfort and safety during normal operation. After identifying the evaluation scenarios that take into account the progress of abnormal situation, an impact analysis is conducted to evaluate the damage to waste packages and/or workers. In the evaluation of the abnormal state, the relationship between the abnormal state that may occur at the geological disposal facility and the potential countermeasures are schematised by taking the same approach as the event analysis method. (author)