Two levels of seismic, i.e. OBE and SSE, are conventionally considered in the seismic design of nuclear power plants. OBE is formerly set to equal to one half of SSE. In Advanced Light Water Reacter User Requirements Documents (ALWR-URD), US EPRI recommented to decrease OBE to one third of SSE. In the standard design of third generation of nuclear power plants, such as AP1000 of Westinghouse and EPR of AREVA, OBE was eliminated and substituted by lower level earthquake. In AP1000 standard design, OBE was decreased to one third of SSE and explicit analysis on OBE in the seimic design analyses is not required. Literatures and reports related to the regulatory requirements of seismic design are reviewed to study the reasons and means to be taken to address the issue of elimination of OBE from the design analyses of NPP. It will provide guidelines on the issue of elimination of OBE from seimic analysis of NPP design in China.

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