Probabilistic risk assessment on maritime transportation and port cargo handling of spent nuclear fuel

Spent nuclear fuels (SNF) of Japan have been transported by sea under an exclusive vessel with high seaworthiness. A high level of safety has been ensured by domestic regulatory actions according to the IAEA regulations. That is proved by a long history of safe transportation achieved to date. Meanwhile, by the start of operation of nuclear fuel cycle facilities including an interim storage, safety concerns are raised regarding near-future increase of the traffic of radioactive materials. For that reason, the relevant data has recently been prepared to assess the transport risk quantitatively, aiming at the proactive safety planning based on the perception of potentially hazardous situation and the risk information. Principal feature of Japan's SNF transportation is both a coastal shipping operation and a port cargo handling by crane. We have developed the risk evaluation process and method suitable for the domestic environment. This study provides a few of the methods for identifying potentially significant accident using the accident-scenario management system, and evaluating an occurrence frequency of the accident for vessel and crane. Note however that it is conservatively estimated on the basis of general accident statistics due to lack of actual data. Then a case study of shipping SNF along the Pacific coast of Japan was carried out to demonstrate the applicability of estimated data. It was found that most part of the accident risk could be contributed from a vessel foundering, then followed by package drop from crane. The general survey also revealed that quite a few cases of foundering are caused by severe weather conditions, without any collision or fire. A radiological impact by foundering depends heavily on two factors, water depth distribution along the route and ingestion population of contaminated marine-products. The latter is one of the major sources of uncertainty. Even under conservative assumptions, the future-planned health risk will stay very low at a comparative level with the train-accidental risk of the Yucca Mountain EIS.