Three-dimensional analysis of thermal and fluid mixing in cold leg and downcomer of PWR geometries

This report describes the three-dimensional transient and steady-state computations using the COMMIX-1A computer code for the analysis of six (6) 1/5-scale thermal and fluid mixing experiments conducted at Creare, Inc. under EPRI sponsorship. The tests chosen for analyses emphasized the effects of vent valve flow, cold leg and high pressure injection (HPI) coolant flow rates, and HPI location and geometry. The COMMIX-1A computations will provide fluid temperatures and velocities in the belt-line region of the downcomer for assessment of boundary conditions for thermal stress analysis in the vessel walls. A realistic prediction for thermal and fluid mixing significantly helps establish what overcooling transients can lead to in pressurized thermal shock (PTS) events. Sample three-dimensional steady-state computations are presented for three (3) generic full-scale pressurized water reactors (PWR's) typical of Westinghouse (W), Combustion Engineering (CE), and Babcock and Wilcox (B and W) configurations as part of the code assessment