RELAP5 MOD3 MODELING OF WATER COLUMN REJOINING AND A WATER SLUG PROPELLED BY NONCONDENSABLE GAS/

The capability of the RELAP5/MOD3 computer code to analyze water hamme r transients due to wafer column rejoining and a water slug propelled by noncondensable gas is investigated. The code-calculated results hav e been compared with those obtained from simple ideal analytical model s. Good agreement is obtained between the calculation and analytical r esults in the initial period of the transient during which the water c olumn or slug retains its sharp interface and suffers from little brea kup or dissipation. As the transient proceeds, the code-calculated hyd rodynamic loads are generally less than those implied by the analytica l models. This is most likely due to the breakup of the water phase, w hich is not taken into account in the analytical models. Effects of ri me step and mesh sizes have also been studied. The results show that t he usual Courant time limit applies. Finally, a sample calculation, co rresponding to a wafer hammer transient in a typical Westinghouse four -loop reactor head vent system piping, is presented. The transient is induced by the opening of a relief valve and accelerating a trapped wa ter slug through the pipeline. Hydrodynamic loads (i.e., force-time cu rves) on various pipe segments have been evaluated by appropriate post processing of the transient results. The calculated peak forces at sel ected pipe segments compare favorably with those estimated from the an alytical models.