Investigation on Fission Products Release Mitigated by In-Containment Relief Valve Under SGTR Accident

During a hypothesized severe accident, a containment building is designed to act as a final barrier to prevent release of fission products to the environment in Nuclear Power Plants (NPPs). However, in a bypass scenario of Steam Generator Tube Rupture (SGTR), radioactive nuclides are released to environment even if the containment is not ruptured. The radioactive nuclides are transported from primary to secondary systems through a broken steam generator tube during SGTR accident. Accordingly, the radioactive nuclides of the secondary system can be released to the environment through Main Steam Safety Valve (MSSV) or Atmospheric Dump Valve (ADV). Thus, thorough mitigation strategies are needed to prevent such unfiltered release of the radioactive nuclides during SGTR accidents. To mitigate the consequence of the SGTR accident, this study was conducted to devise In-Containment Relief Valve (ICRV) from steam generator to the free space in the containment building of the Optimized Power Reactor 1000 MWe (OPR1000). This study focuses on the conceptual development of the mitigation strategy and MELCOR code was used for the numerical simulation. The MELCOR input model of OPR1000 consists of 58 control volumes and 161 flow paths. Safety features such as Pressurizer Safety Relief Valve (PSRV), Safety Injection Tanks (SITs), and MSSV were modeled in the MELCOR model. To initiate the SGTR scenario, a flow path between secondary and primary sides of Steam Generator (SG) was modeled with a flow area of 4.49 × 10⁻⁴ m². The safety features were assumed that a few passive systems such as PSRV, MSSV, and SIT, were available. Under this condition, the ICRV connecting the SG and the free space in the containment such as dome and Reactor Drain Tank (RDT) were modeled. Specifications of the ICRV such as length, flow area, and valve opening condition were assumed to similar to those of the MSSV. Using these paths, three cases were considered; a base case, a case of steam release to the containment dome (CNMT case), and a case of release to the RDT (RDT case). Simulation results show that in the base case released radionuclides to the environment. In the other cases, the radioactive nuclides were not released to the environment although the containment pressure increased more than the base case, which is lack of the ICRV. As a result, the ICRV prevents the radionuclides release to the environment during SGTR accidents. Further studies are needed to incorporate practical valve inputs, reactor type, and safety features to gain more feasibility.