Simulation of decay heat removal in high temperature gas-cooled reactors

As one of the next generation nuclear power systems, the Gas Turbine Modular Helium Reactor (GT-MHR) achieves improved safety by relying upon inherent thermal storage characteristics and passive cooling features to maintain fuel and reactor vessel integrity if all its active cooling systems fail to operate in the unlikely event referred to as conduction cooldown. In designing the passive natural draft cooling system for the GT-MHR, computational fluid dynamic (CFD) models have been developed to model the passive heat transfer processes and predict the resultant reactor fuel and component temperatures. To initiate validation of the analytical methods used for this critical safety application, a benchmark heat transfer experiment performed at the Japanese Atomic Energy Research Institute (JAERI), which partially models the in-vessel and ex-vessel decay heat removal heat transfer of the GT-MHR, was simulated using the finite element-based FIDAP CFD analysis package. The steady state model incorporated conjugate heat transfer, diffuse grey-body surface radiation, and a Boussinesq model for buoyancy driven convection. Since Rayleigh numbers for the experiment ranged from 106 to 107 inside the vessel and from 108 to 109 outside the vessel, both laminar and turbulent flow models were evaluated. The analysis results show good agreement with the experiment test data