A Study on the Off-site Consequence Analysis for a VHTR

Abstract

Risk-Informed Design included in a VHTR design to ensure the level safety has been considered as an effective methodology. The use of risk information is essential to develop the next generation of NPPs. Therefore, a PSA for a VHTR was performed to assess the health effects on the public. However, there is no plant state of a VHTR comparable to the ‘core damage frequency’ and ‘large early release frequency’. Thus, it is necessary to implement a new PRA procedure for a VHTR. This paper deals with the sequence and consequence levels of a PSA. In the sequence level, several scenarios where radioactive materials could be release are selected. In addition, a traditional event tree method is used to find a probability distribution of the release frequency. To calculate the system unavailability used for an event tree head, a couple of analysis methods are used. One of them is the survival reliability, which is used as a traditional method. For the other method, a structural reliability was used for a passive safety system such as a Reactor Cavity Cooling System (RCCS) because it has 100% reliability when calculated based on the survival reliability. After calculating the release frequency, the end states of an accident were defined according to the release categories of the radioactive materials. The consequence level of a PSA for off-site radioactive materials released during a severe accident was performed using the MACCS2 code. In addition, a risk profile was performed with a compensated cumulative distribution function (CCDF), which provides overall insights regarding the risk of VHTR accidents. In addition, the CCDF were compared to the CCDF of other plants in order to verify the safety of the VHTR. For further study, an assessment of the characteristics of a VHTR safety system and a precise quantification of its accident scenarios are expected to be conducted for a more certain consequence analysis. The methodology shown in this study might contribute to enhancing the safety of the VHTR design by utilizing the results having a far lower effect on the environment than the LWRs.