Comparison of dryout power data between conflex Mk-V and canflex Mk-IV bundle strings in uncrept and crept channels

The CANFLEX Mk-V bundle is designed to improve upon the Critical Heat Flux (CHF) characteristics of the CANFLEX Mk-IV bundle. The main difference between these two bundles is an increase in bearing pad height of about 0.3 mm in the CANFLEX Mk-V bundle. This change in bearing pad height leads to an increase in gap flow at the bottom of the bundle, primarily eliminating the localized narrow-gap effect that limits the CHF of the CANFLEX Mk-IV bundle. The objective of this paper is to examine the effects of bearing pad height and pressure tube creep on the sheath-temperature distribution, dryout power, and dryout location, as observed from full-scale bundle tests, between CANFLEX Mk-IV and Mk-V bundles in uncrept and crept channels. A comparison of surface-temperature differences between the top and bottom elements of the bundles showed that increasing the bearing pad height has led to a more homogeneous enthalpy distribution in subchannels of the bundle. Initial dryout locations of the CANFLEX Mk-V bundle were mainly observed at the mid-spacer plane of either the 10th (about 80%) or 11th (20%) bundle in the 12-bundle string, as compared to the mid-spacer and downstream-bottom planes for the CANFLEX Mk-IV bundle. Dryout power and boiling-length-average (BLA) CHF values exhibit consistent trends and little scatter with varying flow conditions for both types of CANFLEX bundles in uncrept and crept channels. An increase in pressure tube creep has led to a reduction in dryout power (about 20% for the 3.3% crept channel and 27% for the 5.1% crept channel as compared to dryout powers for the uncrept channel). Increasing the bearing pad height of the CANFLEX bundle has led to an increase in the dryout power. Overall, the dryout power of the CANFLEX Mk-V bundle is 7 to 10% higher than that of the CANFLEX Mk-IV bundle at the inlet temperature range of interest (i.e., between 243 and 290 .deg. C)