Simulation of natural convective flow in an experimental reactor cavity cooling system facility

Abstract

The very high temperature reactor (VHTR) has many safety features. One of these features is the reactor cavity cooling system (RCCS). This system is intended to remove decay heat from the reactor cavity during upset conditions. The Korea Atomic Energy Research Institute (KAERI) constructed a facility that represents a ¼ scale model of the RCCS of a VHTR. The preliminary testing on the facility has been completed and a simulation model has been set up for the facility, using the system code GAMMA+. GAMMA+ was intended to be used to simulate the phenomena in gas-cooled reactors, particularly the PMR200 (under development by KAERI). This study aims to simulate the facility using the 1D CFD program Flownex SE and compare the results with the results obtained with GAMMA+. The Flownex simulation was set up as close as possible to the GAMMA+ model by using the same initial- and boundary conditions. The fluid and surface temperatures, as well as the mass flow rates in the riser tubes, were compared to determine the agreement of the results. The results show very good agreement. There are differences in the philosophies of the programs, as well as some differences in the calculation of the fluid properties. The small differences in the results are attributed to these factors. The mixed convection regime was found to be present and therefore the relevant correlations were used to calculate the heat transfer. The convection heat transfer coefficient had to be calculated based on a Nusselt number which is a combination of the forced and free convection Nusselt numbers. The mixed convection regime can either increase or decrease the amount of heat that is transferred. In this particular study, the heat transfer was impeded, since the forced convection and free convection was orientated in the same direction while in the flow was in the turbulent regime. This was due to a laminarizational effect that the mixed convection regime can have on the boundary layer.