A severe accident has inherently significant uncertainties due to the complex phenomena and wide range of conditions. Because of its high temperature and pressure, performing experimental validation and practical application are extremely difficult. With these difficulties, there has been few experimental researches performed and there is no plant-specific experimental data. Instead, computer codes have been developed to simulate the accident and have been used conservative assumptions and margins. This study is an effort to reduce the uncertainty in the probabilistic safety assessment and produce a realistic and physical-based failure probability. The methodology was developed and applied to the OPR1000. The creep rupture failure probabilities of reactor coolant system (RCS) components were evaluated under a station blackout severe accident with all powers lost and no recovery of steam generator auxiliary feedwater. The MELCOR 1.8.6 code was used to obtain the plant-specific pressure and temperature history of each part of the RCS and the creep rupture failure times were calculated by the rate-dependent creep rupture model with the plant-specific data.

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Recent accident at Fukushima nuclear power plant in Japan raised a great concern about the safety of nuclear power plants and the radiological consequences. About twenty percent of the operating power plants in the world are located in north-east Asia. Nuclear accident is not a domestic matter but an international matter especially among neighboring countries. The purpose of this study is to understand the general behavior of the radiological materials and the radiation dose in Korea resulted from a nuclear accident in neighboring countries of Korea. We selected a nuclear power plant unit respectively from China and Japan. We suppose a severe accident releasing radiological materials to the environment and performed a plume dispersion simulation using HYSPLIT4, considering the seasonal weather. The results show that the radiation exposure doses in the most pessimistic case could reach the nuclear emergency level in Korea, such as the indoor sheltering and the restriction on food consumption and drinking water for the public, which is based on the ICRP recommendation. The long-range plume dispersion model for the decision supportive system should be established and improved in near future.

Up to now, even though various researches related to severe accident phenomena have been going on in Korea, research results conducted in the U.S. in the 1980s are still applied to the probabilistic safety assessment as reference materials. To invigorate the risk informed regulation and application that use probabilistic safety assessment (PSA) results and information, the quality of PSA must be improved.To improve the quality and reduce uncertainties of PSA, phenomenological uncertainty analysis of the level 2 PSA has been done using the MELCOR code. For uncertainty analyses, the MELCOR 1.8.6 model for Ulchin Units 3&4 nuclear power plants were developed according to their own design data. Especially, control volumes of the reactor coolant system were divided in detail to simulate the natural circulation flow phenomenon. And then, uncertainty variables that have an effect on the early containment failure phenomenon were selected through the sensitivity analysis and the expert judgment. The MELOR uncertainty engine was used to generate samples of uncertainty variables. A number of MELCOR calculations were performed for various uncertainty variable combinations using the batch MELCOR for convenience.