Technical Considerations and Challenges in Structural Designs and Analyses

Since the generation III/III+ reactors utilize Part 52 process based on standard designs, the containment and civil structural designs and analyses would require somewhat different approaches from Part 50 reactors. As described in previous section, in contrast to a customized plant for which the design is done to address hazards and conditions for a specific site, the standard plants are generic in nature and are typically designed to the envelope of natural phenomenon hazard effects encompassing a broad range of site conditions envisioned for potential sites for the design. Designing power plant structures for generic plants would encounter different technical issues and challenges for the designers and reviewers alike. Generally, nuclear power plant (NPP) structures need to be designed to withstand both internally and externally initiated hazards. For internal events such as internal floods, jet impingement, blowdown loads from loss-of-coolant accidents (LOCA), etc., the design treatments for containment and other Category I structures are essentially the same between Part 50 and Part 52 facilities. However, for externally initiated events such as winds, snows, earthquakes, etc., significantly different approaches to structural analyses and designs are required for Part 52 reactors as opposed to Part 50 facilities, due to the distinct differences in the licensing processes. Moreover, externally initiated events such as earthquakes, etc., are inherently site and frequency dependent and the structural designs are better suited where the site is well defined as for Part 50 facilities. However, recognizing that the external hazards are strongly site dependent, the design for civil structures is challenging for the standard plants licensed under Part 52 process.

Recognizing that a standard plant based on Part 52 process is designed to a generic site, a well-defined set of site parameters for the generic site is essential to the design as well as the NRC review of the standard plant. For containment and Category I structures and foundations, site parameters include those which affect establishing natural phenomenon loads due to earthquakes, winds, tornadoes, hurricanes, and floods. These parameters should be postulated which bound potential sites for the plant. For instance, if a standard plant is intended for the coastal region of the eastern United States, hurricane effect based on historical data should be adequately reflected in the generic site [11]. Similar scenarios can be postulated for other natural phenomenal hazards. It is recognized that in general, the standard design based on this generic site being defined on the bounding concept would be more conservative than any particular site because it is less likely that any particular site would be exposed to the most severity of all hazards; however, the conservative aspects of the standard design are considered to be a small cost compared to the benefits of achieving the standardization for the plant.

Because the seismic response has strong dependency on the characteristics of ground motion inputs and the geotechnical properties both of which are site-specific in nature, the seismic design for structures based on the generic site is particularly challenging for the standard plant [16, 17]. The following paragraphs provide a discussion of seismic issues which were identified as challenging during the application reviews. The materials included in the discussion are mostly extracted from a co-authored paper [17].

Some background information of how the seismic design for nuclear power plants (NPPs) evolved may be beneficial in providing insights and put the challenges in the seismic design for the standard plants in the proper perspectives. Until the 1970s, the seismic analysis and design of NPPs was based on the concept of “safe-shutdown earthquake” (SSE), which is defined in Appendix S, “Earthquake Engineering Criteria for Nuclear Power Plants,” to 10 CFR Part 50 and Appendix A, “Seismic and Geologic Siting Criteria for Nuclear Power Plants,” to 10 CFR Part 100, “Reactor Site Criteria” [18] as the vibratory ground motion for which safety-related SSCs must be designed to remain functional and within applicable stress, strain, and deformation limits. The NRC subsequently published a series of regulatory guides (RGs) in support of 10 CFR Part 50, Appendix S, and 10 CFR Part 100, Appendix A, including RG 1.60, “Design Response Spectra for Seismic Design of Nuclear Power Plants” [19]. The seismic analysis process for NPPs thus required site-specific investigations involving seismology, geology, and geotechnical engineering that were used to estimate site-specific, hazard-based, peak ground accelerations (PGA). The site-specific SSE was then specified in terms of broadband smooth response spectra, analogous to the RG 1.60 spectra, anchored to the PGA and defined at the ground surface in the free field. The corresponding seismic analysis and design of safetyrelated SSCs typically followed the acceptance criteria of the SRP, which provides guidance to the NRC staff for performing its safety reviews.

To address the seismic issues associated with the 10 CFR Part 52 licensing process, pertaining to the seismic analysis and design of new NPPs, the nuclear industry initiated the New Reactor Seismic Issues Resolution Program to coordinate its effort with the NRC. The program addressed two critical issues:

  • (1) the implementation of a performance-based approach for developing site-specific ground motions and
  • (2) an evaluation methodology for considering the effects of high-frequency ground motions.

The NRC implemented the performance-based approach for developing site-specific ground motions through the publication of RG 1.208, “A Performance-Based Approach to Define the Site-Specific Earthquake Ground Motion” [20], which provides a method for developing risk-consistent site-specific ground motions based on a probabilistic seismic hazard analysis (PSHA). The performance-based approach with a stipulated target performance goal is consistent with the risk goals described in the NRC’s policy statement for the operation of NPPs.

To address the high-frequency seismic issues in DC and COL applications, the NRC developed Interim Staff Guidance (ISG)-01, “Interim Staff Guidance on Seismic Issues Associated with High Frequency Ground Motion,” issued 2009 [21], which describes an acceptable process for assessing the effect of high- frequency ground motion on the seismic design of new NPPs. ISG-01 supplements the guidance provided in SRP Section 3.7.1, “Seismic Design Parameters,” Revision 3.

To illustrate the challenges for seismic design for the standard plants, several topics important to standard designs are discussed below.

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