Topical report : CFD analysis for the applicability of the natural convection shutdown heat removal test facility (NSTF) for the simulation of the VHTR RCCS.

Abstract

The Very High Temperature gas cooled reactor (VHTR) is one of the GEN IV reactor concepts that have been proposed for thermochemical hydrogen production and other process-heat applications like coal gasification. The United States Department of Energy has selected the VHTR for further research and development, aiming to demonstrate emissions-free electricity and hydrogen production at a future time. One of the major safety advantages of the VHTR is the potential for passive decay heat removal by natural circulation of air in a Reactor Cavity Cooling System (RCCS). The air-side of the RCCS is very similar to the Reactor Vessel Auxiliary Cooling System (RVACS) that has been proposed for the PRISM reactor design. The design and safety analysis of the RVACS have been based on extensive analytical and experimental work performed at ANL. The Natural Convection Shutdown Heat Removal Test Facility (NSTF) at ANL that simulates at full scale the air-side of the RVACS was built to provide experimental support for the design and analysis of the PRISM RVACS system. The objective of this work is to demonstrate that the NSTF facility can be used to generate RCCS experimental data: to validate CFD and systems codes for the analysis of themore » RCCS; and to support the design and safety analysis of the RCCS. At this time no reference design is available for the NGNP. The General Atomics (GA) gas turbine - modular helium reactor (GT-MHR) has been used in many analyses as a starting reference design. In the GT-MHR the reactor outlet temperature is 850 C, while the target outlet reactor temperature in VHTR is 1000 C. VHTR scoping studies with a reactor outlet temperature of 1000 C have been performed at GA and INEL. Although the reactor outlet temperature in the VHTR is significantly higher than in the GT-MHR, the peak temperature in the reactor vessel (which is the heat source for the RCCS) is not drastically different. In this work, analyses have been performed using reactor vessel temperatures from the GT-MHR design, and the VHTR scoping studies. To demonstrate the applicability of the NSTF facility for full scale simulation of the RCCS the following approach was used. CFD analyses were performed of the RCCS and of its simulation at NSTF to demonstrate that: all significant fluid flow and heat transfer phenomena in the RCCS can be simulated at NSTF; and RCCS simulations at NSTF can cover the whole range of variation of the parameters describing these important phenomena in the RCCS. In CFD analyses, the simulation of turbulence is one of the most significant challenges. Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES) of turbulence in large scale systems require excessive computational resources. The use of the Low-Re number k-{var_epsilon} model, which resolves the boundary layer, is computationally expensive in studies where many simulations have to be performed. In Ref. 2 it was shown that in the RCCS, heat transfer coefficient predictions of the high-Re number k-{var_epsilon} model are closer to those of the low-Re number model than those of heat transfer correlations. In this work, the standard high-Re number k-{var_epsilon} was used to simulate turbulence, and all analyses were performed with the CFD code STARCD.« less