Margin Assessment of AP1000 Loss of Flow Transient

The Reactor Coolant System (RCS) of the AP1000 plant consists of two circulating loops. Each loop contains two canned motor Reactor Coolant (RC) pumps that have a rotating inertia to provide RCS flow coastdown if power to the pumps is lost. Westinghouse analysis of the complete loss of flow (CLOF) accident in support of the AP1000 design certification was based on the USNRC-approved traditional methodology applied to operating plants. The RCS response during the transient was predicted using the LOFTRAN code based on a reactivity insertion curve highly skewed to the bottom of the reactor core, but the calculation of Departure from Nucleate Boiling Ratio (DNBR) was performed assuming a top-skewed axial power profile. A more realistic margin assessment can be made by using an improved method similar to Westinghouse RAVE methodology recently approved by the USNRC. The improved method uses the three-dimensional kinetic nodal code SPNOVA coupled with the reactor core thermal-hydraulic code VIPRE-W for predicting the reactor core response during the CLOF transient. The improved method significantly improves margin predictions by generating core power distributions consistent with the trip reactivity changes for the DNBR calculation. The margin assessment showed that the improved method resulted in a 19% DNBR increase as compared to the traditional method for the AP1000 CLOF transient.