METHODS FOR BEYOND DESIGN BASIS EVALUATIONS
Historical Perspective — Evolution of Methods
As noted above, there has been significant advancement in the PRA technology since the publication of WASH-1400. The external event methodology, particularly the SPRA methodology, has been applied in many applications. The following paragraph related to the evolution of the seismic PRA (SPRA) has been taken from .
“The methodology for performing SPRA has been in existence for over three decades, having arisen in the late 1970s from the ground-breaking NRC sponsored work with a major research program called the “Seismic Safety Margins Research Program (SSMRP)” run by Lawrence Livermore National Laboratory and involving more than a dozen experts nationally . The methodology has evolved and matured, like the rest of PRA over this 3-decade period, and has by now been applied to study several dozen nuclear power plants all around the world. SPRA has been used to support risk informed decision-making to upgrade the safety of existing plants, to help prioritize which proposed backfits are most urgent, to help regulatory agencies like the US NRC, Department of Energy (DOE), and international agencies like the International Atomic Energy Agency (IAEA) to develop regulations and regulatory guidance related to seismic risk, to support the prioritization of safety research projects, and to develop insights into the overall seismic risk from an individual plant and from the whole fleet of plants. What emerges from the large number of SPRA studies is that typically the seismic part of the overall reactor risk is a significant contributor to the overall risk, sometimes dominant, almost always important, although sometimes negligible. One of the reasons for this and a unique and important aspect of the seismic risk is that earthquakes are one of the only initiators that affect the entire site, all of the plant structures, systems, and components (SSCs), and also the surrounding areas.”
In the US, SPRAs have been performed for several NPPS beginning in late seventies and early eighties (for example, Oyster Creek , Zion , IPPS , Limerick , Millstone , and Seabrook ). These have included several Level 3 PRAs including results related to seismic induced core damage sequences, seismic related containment performance, and seismic contribution to consequences. Since then, most of the PRAs have been Level 1 studies with consideration of containment performance that affects the large-early release frequencies (LERF).
Methodology for other external hazards, such as flood and wind, is conceptually same as for the seismic initiators, but are less developed and not widely applied. Screening approaches are applied to many of the external events. Some of the earlier studies have included flood and wind initiators in a limited sense. These hazards are further discussed in Section 8.4.6.
At this point it is also necessary to mention the “Seismic Margin Analysis Methodology” (SMA) and its relationship to SPRA methodology. The SMA approaches, as outlined in the standard methodology references  and  are derived from insights and results of SPRA studies conducted in 1980s. The SMA methodology uses the system insights to focus on initiating events and systems which were found risk significant in those SPRAs. It concentrates on selected accident scenarios. Another simplification is in the determination of seismic capacities using more deterministic approaches. Perhaps the most appealing aspect of the SMA methodology is that it does not make use of the seismic hazard curves with their uncertainties, but instead requires selecting a large enough earthquake for the margin assessment. Therefore, the plant capacity is evaluated at a seismic ground motion level that is chosen to be higher than the design motion. Use of a prescribed deterministic level altogether avoids the issue of large uncertainties in the PSHA. Results from SMA studies are primarily related to the plant’s seismic capacity, and therefore risk insights are limited. It is necessary to understand the limitations of this methodology. The application of the SMA methodology is limited to light water reactors. The SMA methods do not provide risk results such as core-damage frequency and large-early release frequency.
In conjunction with the Advance Light Water Reactors program, a PRA-based margin approach has also been used in the evaluation of new reactors as discussed in Section 8.4.5.
In early to mid-1990s, the industry performed beyond design basis evaluations for external hazards under the IPEEE program. The objectives of the IPEEE for each licensee were to:
- (1) Develop an appreciation of severe accident behavior;
- (2) Understand the most likely severe accident sequences that could occur at its plant under full-power operating conditions;
- (3) Gain a qualitative understanding of the overall likelihood of core damage and fission product releases; and
- (4) If necessary, reduce the overall likelihood of core damage and fission product releases by modifying, where appropriate, hardware and procedures that would help prevent or mitigate severe accidents.
Several PRAs, SMAs, and other evaluations were conducted for some of the external hazards. NUREG/ CR-1472  summarizes insights from these studies. Pertinent insights are included in Section 8.4.4.
The ASME/ANS Standard  includes requirements for SPRA and other external hazards, and for the SMA methodology.