Improvements in main steam safety valve performance by root cause analysis and modifications

A summary of the root cause analysis of main steam safety valve (MSSV) vibration problems at the Davis-Besse Nuclear Power Station is presented along with modifications that have resulted in improved performance. The root cause was found to be flow-induced vibrations caused by acoustic resonance in the valve inlet nozzle and insufficient design margins in certain valve components. The highest level of flow-induced excitation was found to be at reduced power levels where acceptable performance should have been achieved, according to earlier guidelines that had been developed to avoid safety valve vibration problems. This paper presents analysis, test results, and alternative solutions for the MSSV problems