CFD Calculations of Natural Circulation in a High Temperature Gas Reactor Following Pressurized Circulator Shutdown

Abstract

It is anticipated that in the event of the failure of the gas circulator in a prismatic gas-cooled very high temperature gas reactor (VHTR), there will develop natural convection currents in the core with the helium coolant. It is of interest to know the amount of energy transported by the helium plumes impinging on material surfaces in the upper plenum. Additionally, in the event of a rupture in an intermediate heat exchanger which contains water, it will be of great interest to understand the potential for free convection as it will convect water vapor, which will have detrimental effects on the core graphite. It is well known that heating a gas causes it to rise because the buoyant forces overcome gravitational forces. In the reactor, there will be hot walls that can provide heating to the helium, though the temperature of the coolant channel walls will be a function of the core depth, which makes the presence of free convection dependent on the particular conditions. In addition to the uncertainty of whether there will be sufficient buoyant forces to drive free convection, there is uncertainty as to what paths the helium will take in forming natural circulation loops. Computational fluid dynamic (CFD) calculations are reported herein that demonstrate the potential for the occurrence of natural circulation considering the core itself along with upper and lower plena and including flow paths in the gaps between the graphite blocks that allow bypass flow to occur. It is shown that multiple paths are possible for circulating flow.