Lessons learned from the seismic reevaluation of San Onofre Nuclear Station, Unit 1

A seismic reevaluation program was conducted for the San Onofre Nuclear Generating Station, Unit No. 1 (SONGS 1) as part of the Systematic Evaluation Program (SEP) initiated in 1977 by the United States Nuclear Regulatory Commission (NRC). SONGS 1 was originally built to codes and standards in effect during the time of its construction from 1964 to 1967. Since that time, there has been significant evolution in the codes and standards applied to nuclear power plant construction in the USA, particularly with respect to seismic design criteria. SEP was created by the NRC to provide (1) an assessment of the significance of differences between current technical positions on safety issues and those that existed when a particular plant was licensed, (2) a basis for deciding on how these differences should be resolved in an integrated plant review, and (3) a documented evaluation of plant safety. Several of the 137 SEP safety issues, or topics, concerned the plants’ capability to withstand extreme environmental conditions and, in particular, earthquakes. The SEP seismic review for SONGS 1 was exacerbated by the results of an evaluation of an existing capable fault near the site during the design review for Units 2 and 3, which resulted in a design ground acceleration of 0.67g. Such a design criterion is one of the highest in the USA, and is especially severe for a plant that has been operating for nearly 20 years. Southern California Edison Company (SCE), the licensee for SONGS 1, realized that a uniform application of existing seismic criteria and methods would not be feasible for the upgrading of SONGS 1 to such a high seismic requirement, when compared to the original design criterion of 0.25g ground acceleration with equivalent-static analyses. Instead, SCE elected to supplement existing seismic criteria and analysis methods by developing criteria and methods closer to the state of the art in seismic evaluation techniques. The NRC supported this effort with a substantial review program to evaluate the new criteria and methods to ensure that they provide an adequate margin of safety. SCE developed site specific ground response spectra based on the Housner response spectra. These were modified as a result of NRC review to account for uncertainties in a limited frequency range, and were applied in a conventional manner to the development of time histories for structural analysis. Frequency-dependent soil structure interaction analyses were performed for the Reactor Building. Frequency-independent soil stiffness and damping values were used in conventional analyses of the Turbine Building. Conventional acceptance criteria were applied to concrete structures. Supplemental acceptance criteria were developed for the steel structures; these criteria allowed a limited amount of ductility in isolated members and were applied only if the connections were maintained in the elastic range. Consideration of the effect of the ductility on the total structural response was required. Application of these criteria and methods required only a moderate level of reinforcement of steel structures in order to assure adequate structural integrity. The development, review and approval of supplemental criteria and methods was extended to the reevaluation of piping and equipment, with an emphasis on piping. Multiple level response spectra analyses were applied in limited cases where enveloped response spectra analyses were overly conservative. Damping was based on ASME Code Case N-411 (PURC damping). Supplemental acceptance criteria were developed for piping based on specified strain limits. The method of calculating strain involved application of a correction factor to an elastically calculated stress based on ASME Simplified Elastic-Plastic Analysis. If strain criteria were applied, additional criteria were also imposed. Supplemental criteria were similarly developed for pipe supports. The development, application and evaluation of the methods and criteria for the SONGS 1 seismic reevaluation program required substantial resources for both SCE and the NRC. Confirmatory analyses and case-by-case analysis reviews were conducted to assure an adequate margin of safety with appropriate consideration for the uncertainties in the techniques used. Nevertheless, these efforts have resulted in effective and efficient seismic improvements for this older operating nuclear power plant.