An Experimental Investigation of Critical Heat Flux on a Vertical Surface Abutting a Coarse Porous Medium

Abstract

A postulated severe accident in a CANDU reactor may involve the collapse of core materials to the bottom of the calandria vessel. Avoidance of further degradation of the reactor state would require retention of these materials within the vessel. Successful retention depends upon transport of radioactive decay heat produced inside the vessel to its surroundings, without undue escalation of the vessel shell’s temperature. Near the bottom of the vessel, this is only possible if nucleate boiling of water is maintained on the vessel’s outer surfaces. The maximum heat flux removable by nucleate boiling is referred to as the Critical Heat Flux (CHF), and depends strongly on the geometry in question. At each end of a CANDU reactor, the vertical calandria tubesheets abut an end shield cavity filled with steel shielding balls and water, constituting a coarse porous medium. Predictions of CHF in this geometry prior to the work presented in this thesis were subject to large uncertainties; an undesirable situation given the potentially significant impact of CHF on accident outcomes. The primary goal of the thesis is the quantification of CHF in this geometry, permitting improved assessments of severe accidents. Progress toward this goal constitutes a contribution to the more broadly-defined subject of externally-heated coarse porous media. Experiments were performed, quantifying CHF and its dependence on key variables (location on the calandria tubesheet and shielding ball diameter). Investigation of potential mechanisms led to insights into the nature of the CHF phenomenon in this geometry, and its dependence on relevant variables (shielding ball thermal conductivity, and calandria tubesheet surface conditions). Similarities to and differences from existing literature pertaining to externally-heated porous media are noted. Finally, future work is proposed, that is expected to yield a semi-mechanistic model of CHF accounting for the additional parameter of spatial heat flux variation. The contents of this thesis represent significant progress in the understanding of CHF on the surface of a calandria tubesheet facing the end shield cavity, and more generally, externally-heated coarse porous media.