Preliminary Study for Application of the New Safety Goal related with the Limitation of Cs-137 release

In the New Safety Goal, it is clearly stated that the Probabilistic Safety Assessment (PSA) should be performed with the proper technical appropriateness, the detail, and the scope in accordance with the integrated risk assessment against the accident for the nuclear power plants. This requirement is known to be come from the provision for preventing the long term ground contamination due to the release of radioactive material. However, there were so many concerns that this goal is so severe that the current design, even in the case of the constructing nuclear power plants, cannot meet this criterion. Especially for the operating nuclear power plants, since there were no mitigation facilities against the severe accident at the design stages, the application of this new goal is known to be much severe that the constructing nuclear power plants and it is necessary to develop the alternative methods to strengthen the safety of the operating nuclear power plants. The purpose of this study is to review the new safety goal from the view point of severe accident analysis and probabilistic safety assessment, and to find the appropriate methods in order to meet that goal for the operating nuclear power plants. In order to strengthen the safety for domestic nuclear power plants, all of the domestic operating nuclear power plants are required to prepare the Accident Management Plan within 3 years. Also, this Accident Management Plan should meet the New Safety Goal including the requirement that the sum of the accident frequency that the release of the radioactive nuclide Cs- 137 to the environment exceeds the 100TBq should be less than 1.0E-6/RY. Since the operating nuclear power plants was not designed against the severe accident and they have the limited exclusive mitigation facilities, it is not easy to meet the New Safety Goal. So, it is necessary to develop the alternative methods to meet the New Safety Goal. In this study, the amount of Cs-137 released to the environment for the representative accident sequence assessed to find the method that the amount of Cs-137 released can be reduced using MAAP code. However, according to the MAAP calculation even though it is performed only for the one STC, it can be expected that the amount of Cs-137 released to the environment for each of all STCs is over 1,000TBq. Therefore, it is necessary to consider the approach from the aspect of PSA model. In the Level 2 PSA model, the improvement effects is so limited because of the uncertainties related with the severe accident phenomena and the difficulties in the installation of new facilities