Analysis of Radioactive Material in Consideration of Containment Filtered Venting System for Wolsong NPP Unit 1

The containment filtered venting system (CFVS) was first installed at Wolsong unit 1 in 2012. CFVS is used to prevent the build-up of excessive pressure in a reactor containment during a severe accident. When the containment pressure exceeds the design pressure of the containment building, the operator opens CFVS isolation valves. Then steam and air in the containment flows out to the environment through the CFVS and the containment pressure decreases below 150 kPa(a). In addition, the key function of CFVS is to reduce the radioactive material releasing from the containment to the environment. The aim of this study is to analyze the radioactive material in consideration of the CFVS operation for Wolsong unit1 using the MELCOR 1.8.6 code developed at Sandia National Laboratories (SNL) for the U.S. Nuclear Regulatory Commission (NRC). In addition, in order to evaluate the effects of filtration efficiencies for the filters of the CFVS, a sensitivity study according to decontamination factor (DF) of the filters was carried out. The SBO accident is chosen to analyze the release fraction of radioactive material of Wolsong unit 1 in the consideration of the CFVS operation using MELCOR 1.8.6 code. Also, a sensitivity study was carried out to evaluate the filtration performance according to decontamination factor of the filters for CFVS. The results show that the small amounts of radioactive material are slightly released to the environment except for noble gases. That is, the CFVS filtered most of aerosol form and released noble gases to the environment directly. Therefore, the CFVS has a good filtration performance to reduce radioactive material such as aerosol form. In addition, the filtration efficiencies of the filters for CFVS considerably depend on DFs according to the calculation results of CsI. However, there are no different results of Xe according to DFs because the CFVS cannot retain noble gases. In the future, a proper requirement for a criteria of decontamination factor for CFVS according to accident scenarios such as LBLOCA, SBLOCA and SGTR, etc. should be evaluated in order to review the licensing for CFVS