RELAP5 Capability to Predict Pressure Wave Propagation Phenomena in Single- and Two-Phase Flow Conditions

Correct evaluation of the hydrodynamic loads induced by large and rapid pressure waves propagating with the speed of sound along the reactor piping systems and Reactor Pressure Vessel (RPV) is an important and difficult issue of nuclear power plant safety. The pressure shock transients and resulting hydrodynamic loads on the pipes and RPV structures are commonly calculated with one-dimensional thermo-hydraulic system codes such as RELAP5, TRACE, DRAKO and ROLAST. In Sweden, the most widely used computer code for this purpose is RELAP5. This code needs, therefore, to be assessed for its capability to predict pressure wave behavior. The conducted assessment involves simulations of single- and two-phase shock-tube problems and two-phase blowdown as well as water hammer experiments. The performed numerical experiments clearly show that RELAP5, with the proper time step and spatial mesh size, is capable of predicting the complex dynamics of single- and two-phase pressure wave phenomena with good to reasonable accuracy.