Simulation of Critical-Heat-Flux Enhancement in Fuel Bundles by Tubes Equipped with Inserts

Abstract

CHF-enhancement data obtained in tubes equipped with inserts and tested in water and Freon-134a, were compared to those obtained in 37-element bundles equipped with extra planes of spacer and bearing pads. The objective of the studies was to identify the following effects on CHF enhancement in both geometries: (i) spacing between insert planes, (ii) the number of similar/dissimilar insert planes, (iii) fluid type, and (iv) flow conditions. The spacing was found to be the major geometric factor affecting the CHF-enhancement ratio K (defined as the ratio in CHF enhancement-equipped tube to the CHF in a bare tube for the same local flow conditions). By decreasing the relative spacing, L/Dhy, for example from 30 to 10, K can reach the value from 1.0 to 1.7, depending on flow conditions. Among the flow parameters, the critical quality, xc, can very strongly affect K (K increases with xc in both geometries, for mass flux GW > 2 Mg/m2s); pressure has a much weaker effect on K. CHF enhancement does not depend on fluid type, provided that the conditions in the fluids meet the CHF fluid-to-fluid modelling requirements. The above-mentioned K characteristics found in tubes with inserts are qualitatively similar to those in rod bundles. This observation has permitted the derivation of a CHF-enhancement prediction method for a subchannel code based on the tube data, and has subsequently been used to perform many separate-effect studies in support of optimization of CHF-enhancing devices applied to CANFLEX: a new 43-element fuel bundle for CANDU reactors.