Improvement and Assessment of the CATHARE 2 Three-Dimensional Module Compared with the UPTF Downcomer Test 7

Abstract

The CATHARE 2 three-dimensional module is assessed in comparison with the Upper Plenum Test Facility downcomer test 7, which was performed to obtain full-scale data on downcomer and lower plenum refill behavior during the refill phase of a loss-of-coolant accident. New discretizations for the equation of motion, named Mods. D and R, are suggested and implemented in the three-dimensional module. Mod. A is also investigated, which defines a new junction void fraction used to calculate interfacial friction. Using the standard and the modified three-dimensional modules, the four experiments, test 7 runs 200 through 203, are simulated with the downcomer nodalized as an 8 × 1 × 8 mesh. Sensitivity calculations associated with interfacial friction, condensation, and nodalization are also performed. The calculation results show that the discretization of the momentum convection is very important in strongly heterogeneous flow conditions. Mod. D + A gives the best results so far, and Mod. R + A yields the smallest scatter in the predicted water deliveries to the lower plenum. The results of the sensitivity calculations show that the interfacial friction coefficient of CATHARE 2 is somewhat overestimated and the 8 × 1 × 8 mesh downcomer is fine enough for test 7.