Prediction of the offsite exposure dose trends and preliminary derivation of DCGLs for a hypothetical contaminated NPP site in Vietnam using RESRAD-OFFSITE code

Abstract

The potential radiation exposure for an offsite receptor living near the proposed Ninh Thuan 1 Nuclear Power Plant (NPP) site in Vietnam is assessed using the RESRAD-OFFSITE v4.0 code. Radioactive contamination is assumed to occur on 23 ha of surface soil in the Ninh Thuan 1 NPP's protected core site due to residual radioactive inventory resulting from operation or decommissioning activities. Nine radionuclides of concern in this work are selected based on the previous decommissioning experience of typical VVER, the same Russian reactor type intended for the Ninh Thuan 1 NPP project, and those of typical PWRs in the United States. The radionuclide migration through the aqueous and air medium is modeled based on the geological and physical characteristics of the specific site gathered through the feasibility safety report for the Ninh Thuan 1 NPP project and the related literature. Identifying the dose contribution fraction of each radionuclide to the annual dose limit through individual DCGL values can assist in weighting the influence faction of each radionuclide to the final total exposure dose. The results show that the total offsite exposure dose from nine radionuclides considered in the present work under a given contamination condition (1 pCi/g) is relatively lower than the annual dose limit (0.1 mSv/yr) for the public in Vietnam. In addition, the dominant exposure pathway for an offsite receptor is determined to be fish ingestion due to the remarkably high transfer factors, followed by plant ingestion. Consequently, observation of the sensitivity analysis for the solubilities and distribution coefficients of the radionuclides shows that while the change of distribution coefficients significantly influences the output dose, the impact of the solubility variation is relatively minor according to the small concentration inventory of each radionuclide in the survey area.