Coolability of a Corium Pool in a Debris Bed: Impact of Debris Size, Steam, and Liquid Flow Rate; Tilting Angle; and Pressure on Critical Heat Flux—First Numerical Evaluations

In case of a severe accident in a light water reactor, a debris bed may form in the core and possibly melt, as happened in Three Mile Island Unit 2. Knowledge about the coolability of such a molten pool surrounded by debris is crucial to investigate the possibility of stabilizing a part of the fuel inside the vessel. In particular, it is of primary interest to determine the maximum size of a molten pool surrounded by debris that may be stabilized under water. A key parameter is the maximum heat flux [critical heat flux (CHF)] that may be extracted from the pool boundary by water flowing within the debris bed. A facility was built at IRSN to determine the CHF under various conditions. A heated copper surface simulates the boundary of the pool and is placed in contact with a debris bed (monodisperse steel balls), under water. In this article, we first complete previous experimental work with steam and liquid flow rates, ball diameter, tilting angle of the heated surface, and pressure up to 2.5 bar absolute. A general CHF correlation depending on the tilting angle and the steam flow rate is derived, and an example of the use of this correlation to evaluate the maximum mass of the corium pool that can be stabilized under water is given, for some typical reactor conditions. In the last part of the article, the accuracy of the ASTEC code is evaluated based on first calculations intending to reproduce the experimental impact of liquid and steam flow rates.