Effect of Nu correlation uncertainty on safety margin for passive containment cooling system in AP1000

Thermal-hydraulic (T-H) model uncertainty is one of the important factors influencing the passive system reliability, especially for passive containment cooling system in AP1000 which is a new design concept based on natural circulation. In this paper based on the experiment data the Nusselt number (Nu) correlation is fitted firstly and then the uncertainty of such correlation is evaluated. In order to get the probabilistic density distribution, it is proposed that the ratio between experimental value and theoretical value (Nue/Nuc) can be used as the statistical variable, and the normal distribution is gained with μ = 0.99 and σ = 0.07. Finally influence of Nu correlation uncertainty on the system safety margin is evaluated under design and worst conditions for LOCA and MSLB accidents, since the cooling water vaporization at the outside surface is the important heat transfer way and the cooling water mass flow rate is lower than the evaporation rate, the Nu uncertainty at outside surface almost has little effect on the safety margin, while in the containment the steam mass flow rate is much higher than the condensation rate and the steam partial pressure is one of the main contributors to the pressure increasing, so the Nu is higher, the safety margin is higher. Moreover the difference among the results induced by the Nu correlation uncertainty can be comparable to the difference induced by input parameters and especially to the safety margin itself under worst condition for LOCA accident, so it is suitable to consider such uncertainty in the system reliability model.