DESIGN BASIS AND BEYOND DESIGN BASIS EVENTS
As discussed in the preceding section, the design bases for the currently operating plants were established in 60s, 70s, and 80s, using the knowledge and state-of-the-art technology available then. Historically, for seismic and flooding, the design bases were established using deterministic approaches taking into account the historical events and making implicit adjustments for the uncertainties in our knowledge. Our understanding of the natural phenomena continue to evolve through advances in the state-of-the-art, new technology, and from the actual experiences. Some of the major advances that have occurred include development of the probabilistic hazard assessment methods, better instrumentation and investigative tools, investigations of actual events, enhanced modeling, and paleo-seismology and paleo-flood evaluations. Along with the enhanced understanding, the recent seismic events have shown that the design basis ground motions used in the past could have significantly different characteristics when compared to current day predictions of the ground motions at a site. Although, the design bases have a very low probability of exceedance, the recent experiences have shown that they can be exceeded. The following sections discuss in more detail evolution of the hazard understanding and observations from the recent experiences. The emergence of probabilistic risk assessment (PRA) technology also provide a framework to assess the potential risk from the natural phenomena events and establish risk-informed/performance based approaches to establish design bases and design methods. This framework is also discussed in Section 8.3.4.
Collectively, the above elements highlight the need for considering events beyond a design basis event and periodical assessment of a design basis. In fact, the regulatory framework in the US has processes which allow for evaluation of new knowledge. The US industry in 90s under a program called, Individual Plant Examination for External Events (IPEEE), examined to some extent, a plant’s capability to withstand events larger than the design basis events (this is further discussed in Section 8.4). The NRC Near-Term Task Force (NTTF) in its report  has recommended re-evaluation of the seismic and flooding hazards using the present day licensing methods for the new reactor applications for the currently operating reactors. The NTTF recommendations are aimed at improvement of overall plant safety against important external hazards that can severely affect how a plant is to be maintained in a safe shutdown condition when subjected to initiating events that are beyond the design basis and pose common cause challenges to plant protection. The containment structure is the last barrier to uncontrolled release of radioactivity; consequently, a systematic examination of the integrity of the containment barrier is necessary under the overall NTTF initiative. The NTTF has also recommended periodical assessment of the natural hazards. These aspects are discussed in more detail in Section 8.5.