(A) study on passive decay heat removal strategy and cost-benefit analysis of integrated passive safety system

Abstract

As the history of the nuclear safety passed the ages of technology and human error, nuclear power plants have been evolved by enhancing the hardware performance and the personal competence. We are in a time of safety resilience for considering unpredicted conditions or accidents. The ultimate safety goals for removing decay heat and preventing the release of radioactive materials are definite to be achieved in any conditions. For attaining the goals from various passive functions, an integrated passive safety system (IPSS), to be achieved by the use of a large water tank placed at high elevation outside containment, was proposed based on the design requirements and criteria. The functions include decay heat removal, safety injection, containment cooling, in-vessel retention through external reactor vessel cooling, and containment filtered venting. The IPSS can be applied to a nuclear power plant in many different forms with selecting functions and designing the coping time for accidents. For using the IPSS, a passive decay heat removal (PDHR) strategy was developed to cope with station blackout (SBO) and SBO-combined accidents. The PDHR strategy was developed based on the design and accident management strategy of Korean representative pressurized-water reactor, the OPR1000. The performance of the strategy was validated by the accident simulations. Under the assumption that existing engineered safety features failed to be operated, the results of the accident analyses showed that the IPSS could maintain a reactor in a safe state by removing the decay heat effectively for the designed accident coping time. The procedures of the PDHR strategy can be implemented after failures of the steps in emergency operating procedures, before the entry condition for severe accident management. Even when external conditions make a given site extremely inaccessible, the strategy can allow for cooling the core by adopting SGGI and PSIS. To estimate overall safety enhancement from the application of the IPSS, probabilistic safety analysis (PSA) was performed based on the results of the accident simulations. The results of the PSA were compared with those of other alternatives. In loss of offsite power, both emergency diesel generators and the IPSS were effective to decrease core damage frequency. In loss of feedwater, the IPSS showed the better performance to decrease a core damage frequency than the additional power sources. The changed values of core damage frequency were imported to cost-benefit analysis including the present values of implementation costs. A new model for cost-benefit analysis was developed to compare new alternatives and support a basis of risk-informed decision. In conclusion, as the proposed strategy can be applied in both current and future nuclear power plants, it is expected to enhance the overall safety of nuclear power plants by extending the resilient coping capability.