Preliminary research on elimination of OBE in seismic design requirements for NPPs

Literatures and reports by US NRC and EPRI are reviewed and the statistical data of stress ratios of some class 1 pipes are analyzed to demonstrate the reasons of elimination of OBE from the DESIGN-BASIS of NPPs. It is reasonable to decrease the ratio of OBE to SSE, to eliminate OBE from the loading combinations, and to use OBE only as a limit to shutdown the nuclear power plants in earthquake, which can optimize the seismic design. It is shown that measures shall be taken to maintain overall safety level against earthquake. (authors)