Thermo-hydraulic analysis of the PBMR used fuel tank using computational fluid dynamics

Abstract

The Pebble Bed Modular Reactor (PBMR) is a 4th generation nuclear reactor based on the HTR-Modul of Siemens currently being developed by Eskom in South Africa. The major safety characteristics of the PBMR are the fuel design and physical dimensions that make it an inherently safe reactor. This means that the reactor will not melt down like a typical Light Water Reactor (LWR) when cooling of the reactor is lost. The thermo-hydraulic analysis of the Used Fuel Tank (UFT) is of great importance in the safety analysis of the PBMR. The UFT is one of two types of tanks that will be used to store fuel that has been in the reactor for a finite time. The fuel would therefore contain fission products and would generate decay heat. This decay heat should be removed to limit the temperature of the fuel. The temperature of the fuel should be limited to prevent the release of fission products to the environment. The temperature limit on the fuel during storage is required to ensure that the graphite in the fuel does not oxidize in the presence of oxygen. The fuel is normally kept in a helium environment, but it must be shown that the fuel is safe when there is air ingress into the system. The purpose of this study is therefore to determine the temperature distribution in the fuel and the components of the used fuel tank for different scenarios. This includes the forced cooling of the tanks and the possibility of cooling the tanks with natural convection. Computational Fluid Dynamics (CFD) was used to model the various heat transfer mechanisms present. This includes convection heat transfer between the gases and the solids, conduction through the solids and thermal radiation between most of the surfaces. The effect of natural convection was also included, as the pipes through the tank cause result in high mass flow through these pipes due to the buoyancy effect. The results show that the fuel temperature will not exceed the allowable limit during forced cooling if the Heating, Ventilation and Air-conditioning (HVAC) is supplied at 6 kg/s. The possibility of cooling the tanks with passive means during upset events looks promising, but it is dependant on the design of the chimneys. The chimney cross-flow area was the most significant factor influencing the air mass flow through the system. The chimney design and the rest of the system not included in this study should be analysed in detail before the passive operation of the system can be guaranteed.