Frequently Asked Questions
Frequently Asked Questions
This page collects commonly asked questions about external hazards assessment for nuclear power plants and about the wiki itself. Answers are intentionally concise; follow the links for full treatments.
General Questions
What is an external hazard?
An external hazard is any event originating outside a nuclear power plant that can impair safety‑related structures, systems, or components (SSCs). Examples include high winds, flooding, wildfire, extreme temperatures, lightning, and geomagnetic disturbances. Natural hazards—such as meteorological, hydrological, and geological events—constitute the majority of external hazards, but the category also includes man-made and technological events (such as nearby industrial accidents or transportation hazards). The full catalog is on the Hazards page, including seismic events. Detailed seismic PRA methodology is also available on the companion Seismic PRA Wiki.
What hazards does this wiki cover?
The wiki addresses the external hazards tracked by the EPRI External Hazards Program. The major categories are atmospheric (wind, temperature), hydrological (flooding), geological (seismic, volcanic), biological/environmental (wildfire, drought), electrical/electromagnetic (lightning, geomagnetic disturbance), man-made (industrial accidents, transportation), and combined/cascading hazards. See the Hazards catalog for individual hazard overview pages.
Regulatory and Licensing Questions
What NRC regulations govern external hazard analysis?
The primary regulatory framework includes:
- 10 CFR 50, Appendix A, General Design Criteria — GDC 2 (natural phenomena), GDC 4 (environmental and dynamic effects)
- NUREG‑0800 (Standard Review Plan) — SRP 3.3.1 (wind), SRP 3.5.1.4 (missiles), SRP 2.4 (flooding)
- Regulatory Guides — RG 1.76 (tornado design‑basis), RG 1.59 (design‑basis floods), RG 1.200 (PRA acceptability)
See the Foundations page for a comprehensive listing of regulatory and standards references.
What is the ASME/ANS PRA Standard, and how does it relate to external hazards?
The ASME/ANS PRA Standard (RA‑S series) is the principal consensus standard governing probabilistic risk assessment for nuclear power plants. It includes dedicated parts for seismic events, hazard screening, high winds, external flooding, and other external hazards—each defining capability categories that govern the required level of analytical detail. See the Foundations page for the full parts listing and current NRC endorsement status, and the PRA page for how the standard applies in practice.
What drove the post‑Fukushima external hazard re‑evaluations?
The 2011 Fukushima Daiichi accident demonstrated that beyond‑design‑basis external events (earthquake + tsunami) could overwhelm multiple layers of defense simultaneously. In response, the NRC issued 50.54(f) letters requiring licensees to re‑evaluate seismic and flooding hazards using present‑day methods. These re‑evaluations generated new hazard data, identified sites with potential licensing‑basis exceedances, and accelerated the development of probabilistic methods for external hazards. See Historical Context and Evolution for the full timeline.
Technical and Methodology Questions
What is hazard screening?
Screening is the systematic process of identifying which of nearly 70 potential external hazards are credible at a given site and which warrant detailed analysis. Hazards may be screened out qualitatively (physically impossible or bounded) or quantitatively (contribution to core damage frequency (CDF) < 10−6/year). A hazard that is "screened out" has been demonstrated to pose negligible risk and requires no further analysis, whereas a hazard that is "screened in" remains a candidate for detailed evaluation. The EPRI screening framework is documented in 3002005287.
What is the difference between deterministic and probabilistic approaches?
Deterministic approaches compare hazard loads against design‑basis capacities using conservative, bounding assumptions. They answer the question "is the plant adequate for this specified load?" Probabilistic approaches (PRA) quantify the frequency and consequences of hazard scenarios, producing risk metrics such as core damage frequency (CDF) and large early release frequency (LERF). They answer the question "what is the risk contribution of this hazard?" In practice, the two complement each other: deterministic resolution may suffice for hazards with adequate margin; PRA is needed when margins are narrow or risk quantification is required.
What is a hazard curve?
A hazard curve is a frequency‑versus‑intensity relationship that characterizes how often a given level of an external hazard is expected to occur at a site. The horizontal axis is a hazard intensity parameter (wind speed, flood elevation, ground acceleration), and the vertical axis is the annual frequency of exceedance. Examples include tornado wind‑speed exceedance curves derived from NUREG/CR‑4461, coastal storm‑surge curves produced by the Joint Probability Method, and riverine flood curves from PFHA tools. Together with fragility functions, hazard curves form the two primary inputs to an external events PRA: the hazard curve quantifies how often a given intensity occurs, and the fragility curve quantifies the probability that plant SSCs fail at that intensity.
What is a fragility curve?
A fragility curve is a probabilistic function that describes the conditional probability of failure of a structures, systems, and components (SSC) as a function of an external hazard intensity parameter (wind speed, flood elevation, missile impact velocity). Fragilities are a critical input to external events PRA, linking the hazard characterization to the plant response model. Example: wind‑borne missile fragilities for typical NPP components are tabulated in 3002015994.
What is a PFHA?
A probabilistic flood hazard assessment (PFHA) uses analytical methods to estimate a range of floods and their associated probabilities, rather than relying on a single probable maximum flood (PMF). This approach determines the frequency at which a flood event (such as a storm surge) would be expected to exceed a particular height, producing a flood hazard curve that relates flood level to frequency for use in both design basis determination and PRA. The Joint Probability Method application for hurricane storm surge is documented in 3002012996, while deterministic design-basis flood criteria remain governed by RG 1.59.
What are combined / compound hazards?
Combined hazards are scenarios in which two or more external hazards interact. Three types are recognized: correlated (common physical driver, for example, hurricane producing wind + surge), consequential (one triggering another, for example, earthquake causing dam failure), and coincidental (independent hazards overlapping by chance).
EPRI technical point of contact: Chris Rochon (CRochon@epri.com)
Date last reviewed: 2026-05-20