Realistic evaluation of source terms for steam generator tube rupture accidents

Abstract

In the existing Probabilistic Safety Assessment (PSA) model, all the Steam Generator Tube Rupture (SGTR) sequences have been assumed to have identical fission product release characteristics without considering the sequences of no source term release resulting in a conservative evaluation. Since the Korean Nuclear Safety Act was revised in 2016, a realistic method that can reasonably evaluate the risk became needed to satisfy the PSA targets. Therefore, a new method is proposed to improve the Level 2 PSA model (decomposition event tree, containment event tree, source term category logic diagram) that can identify the SGTR sequences without fission product release to the environment. The sequences are classified by dividing the Source Term Category (STC) of SGTRs into SGTR_CON (containment) and SGTR_ENV (environment). If fission product is trapped inside the containment building and not released to the environment, it is classified as SGTR_CON. Otherwise it is classified as SGTR_ENV. About 72% of the existing STC for SGTR is treated as SGTR_CON, which does not release fission product to the environment. If SGTR STCs are divided into SGTR-CON and SGTR-ENV, then a more reasonable source term evaluation of SGTR sequences is expected.