Coupling RELAP5-3D and Fluent to analyze a Very High Temperature Reactor (VHTR) outlet plenum

The Very High Temperature Reactor (VHTR) system behavior should be predicted during normal operating conditions and during transient conditions. To predict the VHTR system behavior there is an urgent need for development, testing and validation of design tools to demonstrate the feasibility of the design concepts and guide the improvement of the plant components. One of the identified design issues for the gas-cooled reactor is the thermal mixing of the coolant exiting the core into the outlet plenum. Incomplete thermal mixing may give rise to thermal stresses in the downstream components. This analysis was performed by coupling a RELAP5-3D???? VHTR model to a Fluent outlet plenum model. The RELAP5 VHTR model outlet conditions provide the inlet boundary conditions to the Fluent outlet plenum model. By coupling the two codes in this manner, the important three-dimensional flow effects in the outlet plenum are well modeled without having to model the entire reactor with a computationally expensive code such as Fluent. The two codes were successfully coupled. The values of pressure, mass flow rate and temperature across the coupled boundary showed only slight differences. The coupling tool used in this analysis can be applied to many different cases requiring detailed three-dimensional modeling in a small portion of the domain.