Radiological consequence assessments using time-varying source terms in ONERS- decision support system for nuclear emergency response

Online Nuclear Emergency Response System (ONERS) is a Decision Support System (DSS) developed for the management of Off-Site Nuclear Emergencies for Nuclear Power Plant sites of India. In this work, ONERS is incorporated with a module ‘Time-Dependent Source term (TDS)' for real-time online plume dispersion and radiological consequence assessments using time-varying source terms which is likely during an accidental release. TDS facilitates online dispersion and dose calculations by initiating the FLEXPART particle dispersion model with WRF predicted real-time meteorological forecasts and inputs on hourly release rates of 25 significant radionuclides of both ground level and elevated releases for any reactor accident over a 72-h meteorological forecast domain. The TDS is tested against the existing uniform source term approach for a hypothetical accident for two types of frequent weather conditions over Kalpakkam nuclear site in southern India. Simulations for a complex weather condition (monsoon & local sea breeze) during 26–29 September 2019 indicate that though the simulated plume area is similar in both the cases, the area affected is different as governed by the meteorological condition during the period of releases. It is found that the area and direction of potentially affected zone is determined by most frequent wind flow in the case of uniform source term and the flow direction during the period of maximum release in the case of time-dependent source term. Simulations for winter weather condition during 10–13 January 2020 with nearly uniform flow indicate similar results in both uniform and time-varying release cases. Results indicate that while the time-dependent source terms provide more realistic projection of radiological impact under complex meteorological conditions over uniform source term, both approaches would nearly converge when the maximum release occurs under the most frequent windflow condition. The two approaches are complementary to each other for response actions given the requirements of rapid assessments with gross source terms in the initial phases and more refined assessments with detailed source terms in the later phases of emergencies.