Turbulence modeling of thermal and fluid mixing in PWR's during high pressure coolant injection using COMMIX-1B

The problem of thermal and fluid mixing in a pressurized water reactor during high pressure coolant injection has been analyzed at Argonne National Laboratory using the 2-equation (k-ε) turbulence model of COMMIX-1B computer code. The numerical simulation performed is for Test No. 1 of the SAI experiment. Two constants in the ε-equation (C1 and C2) needed modification to obtain correct mixing patterns all the way from the injector to the downcomer. Results show that unstable thermal stratification (having cold fluid at the top and hot fluid at the bottom) exists in the vicinity of the injection port of HPI. Near the downcomer, cold fluid penetrates to the bottom of the cold leg resulting in a stable thermal stratification (continuously increasing temperature from bottom to top). In the core-barrel side of the downcomer, the quenching behavior is very pronounced, approximately 45 sec. after the initiation of HPI, whereas the temperature gradients in the vessel side of the downcomer are mild