Estimation of Source Term and Large Early Release Frequency for Level-2 PSA of 700 MWe Indian PHWR

Indian Nuclear Power Plants are designed to be safe and have high standards of safety with reliable safety equipment’s and design measures to make the occurrence of severe accident highly improbable. Further, Probabilistic Safety Assessments (PSAs) for NPPs are conducted to gain insights of safety, design and performance of the plants and their potential environmental effects. The Level-2 PSA provides insights with progression of severe accident and containment performance and deals with frequency and magnitude of radioactivity releases to environment. Various damage states of plant which include accident within design and design extension conditions have been taken into account in a well-planned manner with the response of containment for a typical 700 MWe Pressurized Heavy Water Reactor (PHWR). Computation of source term for postulated accident scenario was performed with state of art methodology. The radionuclide release from the core is estimated and its behaviour in primary heat transport & containment system is taken into account. The key factors impacting potential radionuclide leakage into atmosphere are described. Engineered safety features and accident management guidelines perform a crucial function to control and mitigate the accident progression and hence helps in reduction of radioactivity release to the environment. If containment becomes overly pressurized, containment integrity can be ensured by using Containment Filtered Venting System (CFVS). The CFVS also plays an important part to retain particulates in its scrubber tank by maintaining appropriate pH, which reduce the radionuclide leakage to atmosphere. This study defines Large Early Release Frequency (LERF) based on dose criteria as well as release quantity of I-131. LERF is estimated for typical 700 MWe PHWR that is well within regulatory recommended limits. Based on the release of radio-nuclides to atmosphere, release categorization is done, which aids to determine accident’s severity and hence emergency preparations may be planned appropriately.