Benchmark of CFD Simulations Using Temperatures Measured within an Enclosed Array of Heater Rods Oriented Vertically and Horizontally

Abstract

Experiments and computational fluid dynamics/radiation heat transfer simulations of an 8 8 array of heated rods within an aluminum enclosure are performed. This configuration represents a region inside the channel of a spent boiling water reactor (BWR) fuel assembly between two consecutive spacer plates. The heater rods can be oriented horizontally or vertically to represent transport or storage conditions, respectively. The measured and simulated rod-to-wall temperature differences are compared for various heater rod power levels (100, 200, 300, 400 and 500W), gases (Helium and Nitrogen), enclosure wall temperatures, pressures (1, 2 and 3 atm) and orientations (Horizontal and Vertical) to assess the accuracy of the computational fluid dynamics (CFD) code. For analysis of spent nuclear fuel casks, it is crucial to predict the temperature of the hottest rods in an assembly to ensure that none of the fuel cladding exceeds its temperature limit. The measured temperatures are compared to those determined using CFD code to assess the adequacy of the computer code. Simulations show that temperature gradients are much steeper near the enclosure walls than they are near the center of the heater rod array. The measured maximum heater rod temperatures are above the center of heater rod array for nitrogen experiments in both horizontal and vertical orientations, whereas for helium the maximum temperatures are at the center of heater rod array irrespective of the orientation due to the high thermal conductivity of the helium gas. The measured temperatures of rods at symmetric locations are not identical, and the difference is larger for rods close to the enclosure wall than for those far from it. Small but uncontrolled ii deviations of the rod positions away from the design locations may cause these differences. For 2-inch insulated nitrogen experiment in vertical orientation with 1 atm pressure and a total heater rod power of 500 W, the maximum measured heater rod and enclosure wall temperatures are 375oC and 285oC respectively with the measured rod-towall temperature difference of 90oC. The simulated rod-to-wall temperature difference for this case is 91.2oC. The simulations reproduce the measured temperature profiles. The ∆TSIM vs. ∆TMEA for all experiments (i.e. N = 3384 measured/simulated temperatures), the linear regression line " ∆TSIM,LR = 0.97∆TMEA + 0.8°C" shows that the simulations slightly but systematically under predict the heater rod temperatures with 95% of the simulated temperatures are within 11°C. The ∆TSIM vs. ∆TMEA for the hottest heater rod temperatures yields a linear regression line "∆TSIM = 1.01∆TMEA - 1.1oC" with 95% of the simulated temperatures are within 7.3°C which is 34% smaller than it was for all the temperatures. These results can be used to assess the accuracy of using simulations to design spent nuclear fuel transport and storage systems.