Passive cooling of the PBMR spent and used fuel tanks

Abstract

The pebble bed modular reactor (PBMR) is a new generation high temperature gas-cooled reactor, making use of spherical fuel elements. The spent fuel and partially burnt fuel (called used fuel) is stored in large storage tanks. This paper presents the cooling design of the storage tanks, with special emphasis on its passive cooling ability. For corrosion protection, the tanks are cooled with a closed loop active system, however, passive cooling is seen as the ultimate cooling mode for the storage tanks. If the active cooling fails, the flow automatically bypasses the active system and passive cooling takes over. The active cooling is thus not safety-related; rather its purpose is for investment protection. The storage tank design with its longitudinal internal cooling pipes has a good passive cooling ability. The layout of the tank concrete cubicle ensures that cooling air can flow only in the desired direction. Computational fluid dynamics (CFD) analyses have been done for various heat load scenarios inside the tank. Passive cooling exists for tanks with a low spent fuel fill level with heat load below 25 kW up to a tank containing a full PBMR core (used fuel) with heat load of 640 kW. For all scenarios, the maximum fuel temperature is below 400 °C. A method was developed to calculate the passive cooling characteristics of the tank at a fraction of the time it takes CFD by using the pipe network simulation software Flownex. The method was also used to analyze transient passive cooling events and showed flow phenomenon similar to what CFD analyses have predicted. A small-scale two-dimensional representation of the storage tank and cubicle layout has been built. This experiment demonstrates the passive cooling ability of the tank. It also proved the flow characteristics that were predicted by the CFD and Flownex analyses. It has been shown through diverse techniques that the fuel inside the tanks can be cooled passively. There are still a few aspects which need to be explored in more detail, but overall it can be said that passive cooling of the PBMR spent and used fuel in bulk storage tanks is viable.