A probabilistics safety assessment of ultimate response guidelines of Maanshan Nuclear Power Plant

Ultimate Response Guidelines (URGs) was proposed to mitigate the so called 'Fukushima type accident' of nuclear power plants (NPPs). As specified in URGs, if it is necessary, operators will depressurize the secondary side of steam generators (SGs) to bring in the low pressure water. The injection damages the economic value of the plant. However, the injection prevents the cladding temperature from rising above 1500 F (1089 K), which is the temperature that release of volatile fission products from gap is initiated. In the present study, the impact of URGs on the Core Damage Frequency (CDF) in a SBO (Station Blackout) accident of a Pressurized Water Reactor (PWR) designed by Westinghouse and adopts large dry containment is quantified. In the Base Case analysis of the study, the actions in the URGs are not considered. The CDF of SBO in the base case calculation is 2.49x10^-7 year. The actions specified in URGs can reduce CDF by 62%. The strategies in URGs are very effective in enhancing the reliability of decay heat removal via SG secondary side. Therefore, the implementation of these strategies can reduce the CDF of sequences involving late failure of AFWS. After the incorporation URGs, the dominant sequences of SBO are the sequences with seal failure. (author)