BEST ESTIMATE PLUS UNCERTAINTY ANALYSIS TO EVALUATE SAFETY MARGIN IN CASE OF LARGE BREAK LOSS OF COOLANT ACCIDENT

摘要

Traditionally, conservative deterministic safety analysis has been used to calculate available safety margin that indicated the nuclear power plant (NPP) safety. Subsequently, best estimate calculations plus with uncertainty have been developed to be able to provide realistic information about physical behavior.It is of interest to evolve methods to integrate the insights of deterministic analysis into probabilistic domain to address the issue of adequate safety margin. The paper deals with the concept of an integrated methodology for safety margin assessment which was carried out for large break coolant accident (LBLOCA). In the present study, five input uncertain parameters namely, nominal power level, fission yield, fuel clad gap conductivity, fuel thermal conductivity and discharge coefficient, were selected. These parameters and their nominal, minimum and maximum values were selected based on literature and engineering judgment. A uniform probability density function was assigned to the uncertain parameters and these uncertainties are propagated using Latin hypercube sampling (LHS) technique. An event tree for the initiating event (LBLOCA) has been used from an NPP of WER-1000 type. In the accident analysis the success/ failure of the safety systems were modeled for each non-core damage (NCD) accident sequence from the event tree. The peak clad temperature (PCT) was obtained for each of 25 code runs for each NCD accident sequence. A Kolmogorov - Smirnov goodness-of-fit test was carried out for each accident sequence PCTs. The PCT were found to follow normal distribution. The safety margin in an NCD accident sequence was calculated as the ratio of the difference between the mean and the acceptance criteria for PCT (= 1473°K) and the standard deviation of the respective sequence. The relative values of the safety margins in accident sequences indicated the importance of the safety systems. Since the safety margin for each sequence was positive, the overall safety margin for the case of LBLOCA was obtained as the weighted sum of the safety margins of the four sequences. The weights are the ratio of the NCD accident sequence frequency and the LBLOCA frequency.