Influence of Debris Bed Shape on Buoyancy Driven Flow of Low Prandtl Number Fluid

ABSTRACT

Fast Breeder Reactors (FBR) are provided with redundant and highly reliable reactor protective systems with adequate safety margins. Hence, an accident like whole core meltdown is extremely unlikely and is considered as a Beyond Design Basis Event (BDBE). To protect the environment and public, some enveloping BDBEs are still analyzed for their consequences and provisions are made in the reactor to tackle such events. Whole core meltdown following the power failure event coupled with the failure of both the redundant and diverse shut down systems is a BDBE. Under such an event, the molten fuel interacts thermally with sodium (solidification of fuel and fragmentation) in the pool and forms a debris bed on a horizontal tray within the primary vessel below the core (known as core catcher), within minutes after the occurrence of the event. The bed so formed may not be the uniformly distributed on the collection tray of the core catcher and depending upon the location and quantity of the debris, the bed can form as a conical shape with different repose angles initially. However, after some time the heap may become shallow and can re-distribute the debris uniformly. The debris continues to generate decay heat and adequate cooling of the debris is to be ensured to protect the structures of core catcher. It is important to assess the natural convective cooling capability of sodium in the cold plenum and the ensuing temperature distribution in the debris. Towards this, natural convection of sodium in a cylindrical enclosure surrounding the debris bed trays has been investigated by computational fluid dynamics studies. Three different shapes of debris heat source (2 MW) viz. (a) uniformly distributed (b) conical with 7° repose angle and (c) conical with 20° repose angle are considered in the present study. The decay heat load varies exponentially with time. It also depends on the amount of debris in the core catcher. As the focus of the present study is a comparison of the various shapes of debris bed, a constant decay heat load of 2 MW_t is considered. A strong re-circulating flow above the heat source is observed with almost stagnant regions below heat source plate. The maximum temperatures in the heat source plate and in the debris bed are higher for higher repose angle. For 20° repose angle these values are found to be 865 K and 1430 K respectively.

Keywords: Natural convection, Liquid metal CFD, Internal heat generation

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