Radiological impact and emergency zones for small iPWR with different approaches for source term calculation

Abstract

Assessment of the potential radiological impact and emergency zones for small Integral Pressurized Water Reactors (iPWR) is very important for their employment in the near term. In this work, an attempt is made to provide an analysis and estimation of the envelope of the potential impact from a severe accident in an iPWR. The source term is calculated from the solution of the lumped aerosol concentration equation in the containment, by using different approaches for estimating the various parameters. The first approach comprises a straightforward use of the methodology and parameters used for large reactors differing only in the lower power level of an iPWR. To account for the anticipated enhanced aerosol retention in the containment and the slower core damage progression in case of iPWRs, two additional approaches are then used. The values of the deposition velocities required are specified using relevant data from available detailed computational and experimental studies. Conservative, yet representative, accidental conditions in the containment are specified, based on insights offered by available analyses with severe accident codes available in the open literature and disclosed regulatory safety assessments. The radiological impact is assessed using a Gaussian plume model, whereas the exposure is assessed against the latest reference levels and dose criteria. Fetus exposure was found to determine the level of the radiological impact during the plume phase. The radioactive releases and the associated radiological impact may differ decisively depending on the approach used to estimate the source term. Consideration of the enhanced safety attributes of iPWRs (namely, enhanced radioactivity retention in the containment and slower accident progression) leads to a reduction of more than one order of magnitude, from about 75TBq to 4TBq and 6TBq to 0.3TBq, in terms of I131 and Cs137 releases to the atmosphere, respectively. Correspondingly, emergency distances are estimated to span from about 4 km - a level commensurate with the generic distances suggested in international standards for low power nuclear reactors - to a distance as low as 600 m if the favorable characteristics of iPWRs are considered, thus implying that no significant measures are required off-site.