Resolution of Generic Safety Issues: Item A-40: Seismic Design Criteria (Rev. 1) ( NUREG-0933, Main Report with Supplements 1–34 )
Structures, systems, and components important to the safety of nuclear power plants are required to withstand the effects of natural phenomena such as earthquakes. Broad requirements for earthquake resistance are specified in 10 CFR Parts 50 and 100 and detailed guidance on acceptable ways of meeting these requirements are documented in various regulatory guides. Safety analysis reports for each plant are reviewed in accordance with the review and acceptance criteria described in SRP11 Sections 2.5.2, 3.7.1, 3.7.2, and 3.7.3.
Over the years, there has been an evolution of seismic design requirements and technology. Early nuclear power plants were designed without specific seismic design requirements. In the early 1970s, the requirement for resistance to seismic events was included in the regulations. The state of knowledge has advanced rapidly and the methods of seismic design vary with the vintage of the nuclear power plant. Also, the complex process of seismic design and analysis involved many engineering disciplines: seismic, geotechnical, structural, mechanical, electrical, and nuclear. Each discipline in the design process controlled the design parameters in its domain. As the total seismic design process evolved, two questions emerged: (a) How adequate are the plants in earlier generations with respect to current safety requirements? and (b) What is the margin of safety in the overall seismic design process? This issue was originally identified in NUREG-03712 and was later determined to be a USI in NUREG-0510.186
The objectives of this issue were to investigate selected areas of the seismic design sequence to determine their conservatism for all types of sites, to investigate alternate approaches to parts of the design sequence, to quantify the overall conservatism of the design sequence, and to modify the SRP11 criteria if changes were found to be justified. Studies under USI A-40 included the following: (1) quantification of conservatism in seismic design; (2) elasto-plastic seismic analysis methods; (3) site-specific response spectra; (4) nonlinear structural dynamic analysis procedures; and (5) soil-structure interaction (SSI).
One key area in seismic design is SSI analysis which is complex and has been controversial in the past. To examine some of the areas of complexity and to obtain an expert consensus, NRC sponsored an SSI workshop in June 1986. The technical areas covered during this workshop were: (1) definition of free field ground motion; (2) ground motion input needed for site-specific SSI analysis; (3) SSI methodology; and (4) experience and experimental verification. Workshop discussions were focused on improving SRP11 criteria. Reasonable consensus was achieved in the four technical areas and was incorporated into the proposed revisions to SRP11 Sections 2.5.2, 3.7.1, 3.7.2, and 3.7.3.
Significant results have become available from the joint EPRI/NRC/Taiwan Power Company (TPC) SSI Lotung experiment in Taiwan. These results were presented in an EPRI/NRC/TPC-sponsored workshop in December 1987. The staff therefore formulated specific questions on the Lotung results and solicited comments on them during the public comment period. The resolution of public comments (NUREG/CR-5347)1238 helped the staff to finalize a position on SSI which was reflected in the revised SRP11 Section 3.7.2, published as part of the final resolution of USI A-40.
Although some older sites were designed to seismic criteria less rigorous than current requirements, significant upgrading has been or will be achieved by the Systematic Evaluation Program conducted on the oldest plants, the implementation of USI A-46, and by staff bulletins and information notices such as IE Bulletin 79-02,1239 IE Bulletin 79-14,1240 and IE Bulletin 80-11.1241 The staff has therefore concluded that backfit of the proposed seismic design provisions is not necessary except for the design of safety-related large, above-ground tanks at some plants.
The implementation of USI A-46 will result in the review of large, above-ground tanks at about 70 of the older plants. The remainder of the plants fall into two groups: (1) plants that were subject to licensing review by the staff after about 1984; and (2) plants that were reviewed by the staff during the period beginning in the latter part of the 1970s up to 1984. For the plants in the first group, the NRC staff licensing review confirmed that no further action was needed. A survey of the plants in the second group was conducted by the NRC and it was found that tanks for many of these plants were designed using the new criteria. However, the staff was unable to determine the status of large tanks at four sites (Watts Bar, Callaway, Wolf Creek, and Harris) and information request letters were issued to these licensees.
Early activities on USI A-40 consisted of specific technical studies which concentrated on improvements in seismic design criteria. A technical overview and specific recommendations for changes to seismic design criteria were documented in NUREG/CR-1161.1242 The value/impact assessment for the proposed changes was documented in NUREG/CR-3480.1243 Based on the recommendations made in NUREG/CR-1161, NUREG/CR-3480, additional staff work discussed in the regulatory analysis (NUREG-1233),1244 and resolution of public comments (NUREG/CR-5347), the staff revised SRP11 Sections 2.5.2, 3.7.1, 3.7.2, and 3.7.3. These SRP sections are for use in the review of future CPs, PDAs, FDAs, and combined CP/OL applications under 10 CFR 52. In addition to the SRP revisions, the staff will review the seismic adequacy of the large, above-ground vertical tanks at the four nuclear stations outlined above. A discussion of the basis for the selection of these sites is included in NUREG-1233. If the licensee reponses to the NRC's request indicate that these tanks do not meet the proposed criteria, plant-specific backfits will be considered under 10 CFR 50.109. The Commission was informed of the staff's resolution in SECY-89-296.1245 Thus, the issue was RESOLVED and new requirements were established.1246