Assessment of CHF Characteristics at Subcooled Conditions for the CANDU CANFLEX Bundle

Abstract

An analysis has been performed to assess the Critical Heat Flux (CHF) characteristics for the CANFLEX bundle at subcooled conditions. CHF characteristics for CANDU bundles have been established from experiments using full‑scale bundle simulators. These experiments covered flow conditions of interest to normal operation and postulated loss‑of‑flow and small break loss‑of‑coolant accidents. Experimental CHF values obtained from these experiments were applied to develop correlations for analyses of regional overpower protection and safety trips. These correlations are applicable to the saturated region in the reference uncrept channel and the slightly subcooled region in postulated high‑creep channels. Expanding the CHF data to subcooled conditions facilitates the evaluation of the margin to dryout at upstream bundle locations, even though dryout occurrences are not anticipated there. In view of the lack of experimental data, the ASSERT‑PV subchannel code has been applied to establish CHF values at low qualities and high subcoolings (thermodynamic qualities corresponding to ‑25%). These CHF values have been applied to extend the CHF correlation to the highly subcooled conditions.