The Effect of an Outer-Element Bow on Dryout Power and Post-Dryout Transfer of a 37-Element Bundle String

Abstract

Dryout and post-dryout tests were performed with a modified 37-element simulated CANDU® fuel string, with one outer element of the last bundle gradually bowed toward the flow-tube wall. The element had 8% higher heat flux than the remaining outer-ring elements and had the narrowest gap between it and the flow-tube wall. The initial dryout occurred on the bowed element for all element-to-wall gap sizes. The dryout power decreased moderately (4% average) as the gap size was reduced to 13.5% of the nominal (unbowed) gap. For smaller than 13.5% gaps, however, the dryout power increased slightly (1.2%) at the low (10.5 kg/s) flow rate and decreased by 5% at the high (16.0 kg/s) flow rate, compared to the nominal gap dryout power. Surface temperatures of the bowed element were recorded for different gap sizes and up to 20% overpower (maximum). The temperature increased by 26% at the maximum overpower as the element was moved from nominal to zero gap position.