Comparison between RELAP5 and TRACE for modelling different loads on pipe systems during transient conditions

Abstract: This is a M. Eng. degree project at Uppsala University carried out at the Forsmark nuclear power plant in Sweden. The purpose of it is to compare the two codes RELAP5 and TRACE during transient changes in mass flow against experiment. The change in mass flow will create a pressure wave and generate pipe loads. RELAP5 is a transient analysis code used to model thermal hydraulic systems. TRACE is an effort to combine the previous codes TRAC-B, TRAC-P, RAMONA and RELAP5. Both RELAP5 and TRACE has been compared to experiments. These comprise two abrupt valve closures, the closure of an inertial swing check valve (a flapper disc which closes when the flow is reversed) and a pump start and stop. Both RELAP5 and TRACE conforms well to the experiments with the abrupt valve closures. The check valve closes faster in the calculations compared to the experiment, both for RELAP5 as with TRACE. The amplitude of the pressure wave from the closure of the inertial swing check valve is lower compared to the experiment in both RELAP5 and TRACE. Numerical disturbances become visual as very high amplitudes in the time history diagram of the force in TRACE. The check valve oscillates between its open and closed position in RELAP5, but not in TRACE. Both RELAP5 and TRACE conforms well to the pump start. The mass flow decreases faster in both RELAP5 and TRACE compared to the pump stop.