Analysis of Fast Pressure Transients using RELAP5 and TRACE

Abstract

Simulations of water hammer transients are important e.g. regarding structural analysis of piping systems in a nuclear power plant. This project aims to validate TRACE with respect to analysis of such transients. Validation is performed against RELAP5 and also, when available, against experimental data. RELAP5 is currently used for such analyses at Ringhals nuclear power plant, and is approved by the authorities for this purpose. Since this system code is about to be phased out in a couple of years, it is of interest to determine whether TRACE may be a suitable successor. Five main cases are studied, including e.g steam collapse in horizontal and vertical pipes, closing of inertial swing check valves and other valves, as well as some pump modelling. The parameters of interest for this project are mainly process-related ones such as pressures, void fractions and mass ows. In some cases forces on the piping systems are considered. All simulations are carried out in both codes, and the results from TRACE are compared mostly to the corresponding ones from RELAP5. Efforts are then made in TRACE to improve the results to be more similar to RELAP5, e.g. by modifying the interfacial heat transfer coefficients. It is concluded that TRACE in general works as well as RELAP5 for this type of analysis. There are some differences between the codes though, when it comes to certain modelling techniques and correlations. Further, a number of bugs and other problems exist in the software which need to be fixed. Also, both codes are rather poor at handling steam collapse. This may be due to a too small predicted interfacial area at the moments just before the last steam is about to collapse. In case the interfacial heat transfer is increased in TRACE, the modelling of steam collapse is slightly better in this code than in RELAP5. However, further improvements are required in order to fully capture cavitation water hammers. Finally, it is noted that neither RELAP5 nor TRACE is originally intended for use in analysing this kind of fast pressure transients.