Abstract: The finite element software is used to calculate the equivalent static force of a main steam isolation valve to predict its seismic weak point, and to calculate seismic margin of the main steam isolation valve by using probability vulnerability method. The results show that a seismic weak point of the main steam isolation valve may shift with amplification of seismic vibrations. By doing calculations, the seismic weak point of the main steam isolation valve is determined to be supporting bracket, and the seismic capacity of the supporting bracket under 95% confidence along with 5% failure probability is calculated to be 11.14g. On this basis, fragility curves of the main steam isolation valve with different confidence levels under elastic failure mode were drawn. The calculation method of seismic margin of nuclear power valves was combed out, and the importance of seismic margin for seismic safety was demonstrated when the earthquake occurs beyond the design benchmark.

Key words: over design benchmark, main steam isolation valve, seismic margin, seismic weakness, fragility curve