Assessment of Mitigating Strategies Using CFVS and CVWM at CANDU Plant under Severe Accident Conditions

As a mitigating action to control the containment pressure, venting using a Containment Filtered Venting System (CFVS) is considered. On the other hand, the containment of a CANDU plant can fail by rapid steam generation caused by the quenching of the corium as it relocates to the calandria vault after the failure of the Calandria Vessel (CV). In Korea, the CFVS and CVWM were first introduced in Wolsung Nuclear Power Plant (NPP) unit 1, which is a CANDU plant. If the mitigating strategies to use the CFVS and CVWM are well established, it will minimize the releases of the fission products into the environment. In this study, sensitivity analyses were performed by using MAAP (Modular Accident Analysis Program) - ISAAC (Integrated Severe Accident Analysis Code for CANDU plants) 4.03 to establish appropriate mitigating strategies to use CFVS and CVWM for reducing fission product releases to the environment. The CFVS and CVWM are effective measures to maintain the containment integrity against the gradual and rapid increase of the containment pressure. Since the operation of these measures can cause releases of a significant amount of radioactive materials, the mitigating strategies should be well established. Thus, in this study, various sensitivity analyses are carried out to evaluate the appropriateness of the current strategies. According to the analysis results, CVWM operation in itself and the operation time does not contribute significantly in reducing the release of fission products. In addition, the vented radioactive aerosol mass is not affected by the vent line size strongly. Therefore, it can be concluded that the current strategy to use the CVWM before the CV failure and the current vent line size are appropriate to mitigate severe accidents.