Comparative study for modeling reactor internal geometry in CFD simulation of PWR and PHWR internal flow - Nuclear regulatory perspective

In this study, in order to examine the effect of reactor internal geometry modeling method on the prediction accuracy for PWR (Pressurized Water Reactor) and PHWR (Pressurized Heavy Water Reactor) internal flow distribution, simulations with real geometry modeling were conducted with the commercial CFD (Computational Fluid Dynamics) software, ANSYS CFX R.14 and FLUENT R.14. The predicted results were compared with those of the porous medium assumption and the measured data. Test cases were 1/5 scaled-down APR+ (Advanced Power Reactor Plus) model for PWR and moderator test vessel (Stern Laboratories Inc.) for PHWR. It was concluded that core inlet flow distribution could be predicted more accurately by considering the real geometry of the scale-down APR + internal structures, located in the upstream of core inlet. Therefore, if the sufficient computation resource is available, an exact representation of these internal structures, for examples lower support structure bottom plate and ICI (In-Core Instrumentation) nozzle support plate, is essential for the accurate simulation of PWR internal flow. For PHWR, an approach to consider the real geometry of tubes predicted the local velocity and temperature distribution better than the Porous Medium Model. However, the benefits (e.g. improvements of prediction accuracy) against the computation cost were small; compared with PWR case. The reason may be that relatively simple PHWR internal structure can make it possible to establish a generally applicable correlation for the pressure loss coefficient.