Estimation of the radiation dose equivalent for the hypothetical submergence of a sea-transport package of radioactive materials by the state-of-the-arts oceanic dispersion model

The requirement for a 200 m water submersion test for irradiated fuel packages was originally added to the 1985 edition of the Regulations to reduce the radiological impact by the submergence accident based on the impact assessment study on the hypothetical submergence of spent fuel package. After that, CRIEPI has carried out assessments of the dose to the public from a hypothetical release of radioactive materials from a submerged irradiated nuclear fuel packages, such as spent fuel, high level wastes and fresh mixed-oxide fuel to gain the public acceptance for sea transport even if the probability of the accident is estimated to be extremely low.The method of impact assessment for both near shore and deep sea areas consists of estimation of the release rate, simulation of the radionuclide concentration by the oceanic dispersion model and estimation of the dose by ingestion of sea foods to the public. We have employed the state-of-the-arts oceanic dispersion model which has been making rapid progress due to the rapid progress of supercomputing.We applied oceanic dispersion models to simulate the concentration of radioactive materials due to the Fukushima Daiichi Nuclear Power Plant accident. Release amounts of radioactive materials were estimated by comparison with measured data and simulated result by oceanic dispersion model. Validations of the model was carried out in comparison with measured data after the accident to improve the model. Improved model reasonably represented measured data and helped understanding ocean-ic contamination due to the accident.We carried out impact assessment of submergence accident of a radioisotope package near shore by the improved oceanic dispersion model. The estimated dose equivalents were extremely lower than the ICRP recommendation (1 mSv year–1). The results of tracer studies by the improved model indicated that the results of dose assessment by previous model are also reasonable. Improved model can predict the affected area reasonably because the model can represent the realistic oceanic condition, such as current and stratification. This is an advantage to employ to make the emergency plan for the submergence accident.