ROLE OF RADIATION HEAT TRANSFER IN COOLING OF A SCALED MODEL OF A PRISMATIC GRAPHITE CORE IN A VHTR

摘要

A thermal hydraulic analysis has been carried out for a scaled model of a Very High Temperature Reactor (VHTR) consisting of a prismatic graphite core. This study presents the role of multidimensional conduction, forced and natural convection, and radiative heat transfer in overall heat dissipation from a VHTR core under various accident scenarios. The geometry considered is a prismatic graphite block surrounded on six sides by other prismatic blocks whose outer boundaries are kept at a constant temperature of 773 K. The situations considered include accident scenarios leading to pressurized conduction cooling, depressurized conduction cooling, and forced convection cooling. A multi-physics code, COMSOL Multiphysics v4.3b, was used to model this heat transfer problem and investigate the fundamental role of radiative heat transfer between the central and adjacent graphite blocks. The predicted temperature gradients in the graphite blocks were compared for stagnant gas, forced convection, or natural circulation flows in the bypass gap. From the simulation results, radiative heat transfer is seen to be essential in dissipating heat from the central graphite block when the graphite's thermal conductivity becomes substantially low; which could occur due to prolonged exposures to neutron radiation and elevated temperatures. Even under normal operating conditions, radiation heat transfer across the bypass flow gaps could account for 20% - 30% of the total heat removal rate from the central block. Under accident situations, the core temperature could rise further to above 1,373K and radiation could become the primary heat transport mode. Thus, the present work suggests that radiative heat transfer needs to be considered in cooling of a VHTR core under accident conditions, especially when the coolant flow rates through the flow channels and bypass flow gaps decrease substantially.