MEASUREMENT OF CRITICAL HEAT FLUX IN A CANDU END SHIELD CONSISTING OF A VERTICAL SURFACE ABUTTING A PACKED BED OF STEEL SHIELDING BALLS

Abstract

The critical heat flux (CHF) was measured in an experimental apparatus representative of a Canada Deuterium Uranium (CANDU) reactor end-shield porous medium heated by a vertical surface. The understanding of peak heat transfer rates in this geometry is crucial to the demonstration of in-vessel retention of corium during a postulated severe accident in a CANDU reactor. In this paper, a large-scale experimental apparatus is described, which is capable of measuring CHF from a vertical surface to a packed bed of steel balls representative of a CANDU end shield. Data are presented from a large matrix of tests investigating the impact of heated surface height on CHF. Preliminary analysis of a subset of these data is also presented as well as the derivation of an empirical correlation for CHF as a function of elevation along the vertical wall. Qualitative comparisons are made between the results of the tests and predictions of peripherally related work carried out in the past, with reasonable agreement being observed in vapour-phase transport in the porous medium and in trends of CHF. It is expected that future work including further analysis of the current data will allow for the development of an improved mechanistic understanding of the phenomenon, leading to a more broadly applicable correlation for CHF that is subject to smaller uncertainties and could be incorporated into severe accident codes such as MAAP-CANDU.