Critical Heat Flux and Pressure Drop for a CANFLEX Bundle String Inside an Axially Non-Uniform Flow Channel

Abstract

Experimental data of dryout power and pressure drop have been obtained with a simulated string of twelve aligned, full-scale, CANFLEX fuel bundles. The bundle string consisted of 43 elements and was equipped with junction and appendages simulations. It was installed inside three flow tubes simulating three different creep profiles: one had a uniform inside diameter of 103.86 mm and the other two had axially varying inside diameters, with a peak of either 107.29 mm or 109.16 mm (3.3% and 5.1 % larger than the uniform tube). Pressure variations along the fuel string were obtained with differential-pressure cells connected to a number of pressure taps. Sliding thermocouples were used to obtain surface-temperature measurements and detect dryout. A wide range of steam-water flow conditions was covered in the current tests: an outlet-pressure range from 6 to 11 MPa, a mass-flow-rate range from 7 to 25 kg/s, and an inlet fluid- temperature range from 200 to 290 degrees C. This paper focuses primarily on data obtained at normal operating pressures with the axially non-uniform channel that had a maximum diameter 5.1 % larger than the reference pressure tube. Local and boiling-length-average (BLA) critical-heat-flux values were derived from the dryout-power data for various flow conditions. Unlike the traditional BLA approach, the averaging process was initiated from the onset of significant void (OSV), instead of from the saturation point. This allowed the extension of the BLA approach to subcooled dryout conditions. The OSV values were evaluated from the pressure distribution along the bundle string. Comparisons of various parameters were made between the 37-element and CANFLEX bundles. Overall, the dryout-power values were consistently higher for the CANFLEX bundle than the 37-element bundle. At inlet-flow conditions of interest, the dryout-power measurements were, on average, 17% higher for the CANFLEX bundle than the 37-element bundle. The fuelstring pressure drop was similar between the CANFLEX and 37-element bundle strings.