A review of uncertainties in the assessment of the radiological impact of liquid releases to the Ignalina NPP cooling pond lake Drūkšiai

Abstract

The methodology used for assessment of the public exposure from the Ignalina nuclear power plant (INPP) liquid radioactive releases to the plant cooling pond, Lake Drūkšiai is based on the application of so called “release-to-dose calculation factors” (RDFs). The application of RDFs is straightforward and was used to assess impacts of actual and planned releases. The RDFs were derived a few decades ago, and thus are based on that time understanding of the plant operation and the environment characteristics. The early shutdown of the INPP power reactors and the startup of the plant decommissioning changed the initially planned conditions for radionuclide release and dispersion. The appropriateness of RDF values became questionable in their past application for prospective and in their current application for retrospective impact assessments.This publication addresses uncertainties in RDF values taking into consideration the actual operation and the planned decommissioning of the INPP. The model for calculation of RDFs for liquid releases was updated to the current status of knowledge and regulatory requirements without changing the complexity of the model itself. The analysis includes a maximum possible or reasonably large variation of the model parameters, defining radionuclides dispersion in the environment and exposure to the population.The assessment results show that RDF values for majority of the considered radionuclides 3H, 54Mn, 55Fe, 60Co, 90Sr, 134Cs and 137Cs are supported by the probabilistic analysis of the model, scenario and parameter uncertainties. The RDF values for radionuclides 93mNb and 94Nb are overestimated, while a higher RDF value for radionuclide 14C is recommended.