A preliminary investigation of rapid depressurization phenomena following a sudden DLOFC in a VHTR

Abstract

Air ingress has been identified as a potential threat for Very High Temperature gas-cooled Reactors (VHTR). Reactor components constructed of graphite will, at high temperatures, produce exothermic reactions in the presence of oxygen. The danger lies in the possibility of fuel element damage and core structural failure. Previous investigations of air ingress mechanisms have focused on thermal and molecular diffusion, density-driven stratified flow due to hydrodynamic instability, and natural convection. Not yet investigated is the possibility of a rapid flow reversal of helium coolant due to a Taylor (rarefaction) wave expansion after a hypothetical sudden Depressurized Loss of Forced Cooling (DLOFC) scenario in a VHTR. Conceivably, flow reversal of the helium coolant could entrain significant quantities of air into the reactor vessel. Our goal here is to simply demonstrate this natural phenomena of compressible flow that could possibly result in rapid air ingress into a VHTR. We start with a one-dimensional shock tube simulation to simply illustrate the development of a Taylor wave. The simulation is carried out far enough in time to allow the resulting reentrant flow to occur. Then, a simulation is performed of an idealized two-dimensional axisymmetric representation of the lower plenum of General Atomics GT-MHR subjected to a hypothetical catastrophic break of the hot duct. Results show the potential for significant and rapid air ingress into the reactor vessel in the case of a large break in the cooling system.