Analysis of Pressure Wave Phenomena in Steam Line Piping

One challenging aspect of boiling water reactor (BWR) analysis is the ability to predict the system response following the rapid closure of a steam line control or isolation valve. Of particular interest is to accurately model the effect of a pressurization wave as it transits through the reactor core. This paper describes sensitivity studies, which were performed to demonstrate the predictive capabilities of S-RELAP5 for analysis of pressure wave phenomena, and it describes the steam line models developed in support of this effort. S-RELAP5 is a RELAP5-based thermal-hydraulic system code used for realistic analyses of large break loss-of-coolant accidents (LBLOCA) in pressurized water reactors (PWRs). The code is also suitable for analyzing PWR small break LOCA and non-LOCA transients. On the extent of the analyses documented in this paper, there are not significant differences between S-RELAP5 and RELAP5. Framatome ANP is developing code models which will extend the capability of S-RELAP5 to analyze BWR transients. Within the framework of this development work, a task was established to investigate the capability of S-RELAP5 to model transients involving steam line pressure wave phenomena. An additional goal of this task was development of a steam line nodalization guideline for modeling pressurization transients. To achieve these goals, various steam line models were investigated and a series of sensitivity studies were performed using the Peach Bottom Unit 2 Turbine Trip test series as an experimental benchmark. A Turbine Trip is an anticipated operational occurrence (AOO), which, for analysis purposes, is initiated by rapid closure of the Turbine Stop Valves (TSV). The pressure oscillation generated in the main steam piping propagates with relatively little attenuation into the reactor core. The induced core pressure oscillation results in dramatic changes to the core void distribution and coolant flow. The magnitude of the resulting neutron flux transient is significantly affected by the void collapse. The performed sensitivity studies focused on steam line geometry, time step size, steam line node length, Turbine Stop Valve (TSV) model, steam Bypass Valve (BPV), and transient boundary conditions. Three models were developed for the Peach Bottom Unit 2 steam line; a single ideal steam line with no bends or elevation changes, a four steam line model of the PB2 main steam and bypass line piping, and an equivalent single steam line model of the PB2 main steam and bypass line piping. The pressure response calculated by S-RELAP5 was compared to theoretical predictions based on fundamental water-hammer equations and to experimental data from the PB2 turbine trip tests (TT1, TT2, and TT3). The results demonstrate the capability of S-RELAP5 to accurately predict pressure wave phenomena. Additional results of this work include recommendations of appropriate values for time step size and steam line node size. Models for the TSV and BPV were developed based on TT2 information, and validated through analysis of TT1 and TT3. Finally, the results of this work demonstrate the strong interaction between the steam bypass line and the main steam line, and the corresponding impact on system pressure response.