Qinshan-II Extension Project Severe Accident Countermeasure Analysis

The frequency of the severe accident happened in NPP is rarely low, once it occurred, it will brings on very severe result, the severe accident had been occurred in historic alarmed us, it is non-abundant that nuclear safety only considers design basic accident, we must consider the preventing and mitigating of the severe accident to meet with the increasing requirement of energy and safety. In the"nuclear power plant design safety regulation"(HAF102) issued by Nation Nuclear Safety Administration (NNSA), April 18, 2004, aim at the preventing and mitigating of severe accident, the regulation bring forward new requirement, which required that during design phase, NPP should consider setting the preventing and mitigation measurement of severe accident as actually as possible.Qinshan-II extension project is based upon Qinshan-II (unit 1&2) and abides by"replicate and improvement"principle. Aim at the severe accident, since the PSA of Qinshan-II extension project is not completed, can't use the PSA result to analyze the severe accident, so we refer to the relating safety study and the practice of the similar nuclear power plant, through analysis and comparing, basis reasonable and feasible to set the preventing and mitigation measurement of severe accident as actually as possible.According the international universal severe accident choosing principle, this paper confirmed the eleven accident sequences which could result Qinshan-II EP in severe accident, and calculating and analyzing the each severe accident sequence. According the result of the severe accident calculating & analyzing, referencing the relating experience of the domestic and oversea plant and combine the object of the accident mitigation, Qinshan-II EP adopted three items severe accident mitigation measurement:1. Adopting pressurizer depressurization function extension to prevent the high-pressure core melt resulting in direct heating containment;2. Installing EUH system to preventing hydrogen burning;3. Installing containment filtrate exhaust system (EUF) to prevent the containment failed on over-pressure.This paper apply the severe accident analyzing program to comparing analyze the sequence which could make high-pressure core melt, hydrogen burning and containment failed in over-pressure, and which is the biggest core damage frequency, and the following is the main conclusion:1. In LOFW accident, SBO accident and SBLOCA accident, adopting the measure that the operators opened the pressurizer safety valve to depressurization. When the pressure vessel failed, the measure could reduce the pressure of the first loop availably, and then mitigate the risk of the high-pressure core melted resulted in containment direct heating.2. Qinshan-II EP installed the EUH system in containment, which could composite the plentiful hydrogen producted in SBLOCA, LBLOCA and SBO accident. The concentration of hydrogen in containment maintains the low level, which availably restrict the risk of hydrogen burning.3. Qinshan-II EP containment filtrate exhaust system (EUF) could mitigate the increasing of containment pressure after the severe accident occurred, that eliminated the risk of containment slowly over-pressured effectively.Through analyzing the severe accident sequence of Qinshan-II EP 650MWe nuclear power plant,this paper put forward the severe accident mitigating measure for plant, and demonstrating the validity of the severe accident mitigating measure by severe accident analyses. It is very important for the Qinshan-II EP to advance the safety performance, and it will put forward the specific way for the next phases'investigating work, and it also has magnitude significant reference merit for the new nuclear power plant to do the similar work in future.