Terms and Definitions
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Terms
- Annual Exceedance Probability (AEP)
- The probability that a given hazard level (for example, flood elevation, wind speed) is equaled or exceeded in any given year.
- Climate Vulnerability Assessment (CVA)
- A structured, phased evaluation of how projected non-stationary climate trends might erode operational margins or affect the resilience of a nuclear plant. The CVA framework encompasses hazard identification, exposure assessment, and vulnerability determination — linking climate projections to plant-specific SSCs and support systems rather than treating hazard characterization as an end in itself. See Climate Vulnerability Assessment.
- CMIP6 (Coupled Model Intercomparison Project Phase 6)
- The sixth phase of the international climate model intercomparison project, providing the ensemble of global climate model (GCM) outputs used by the CHIP program and other projection tools. CMIP6 supersedes CMIP5 and introduces Shared Socioeconomic Pathways (SSPs) as the primary scenario framework.
- Climate Hazard Information and Projections (CHIP)
- EPRI's program for evaluating site-specific climate data sets (for example, temperature, precipitation, sea-level rise) and future projections. Intended for use in evaluating non-stationary-climate-driven changes in external-hazard frequencies and intensities. See CHIP Climate Projections.
- Cliff-Edge Effect
- A sudden, non-linear increase in risk when a small increase in hazard intensity produces a very large change in response (for example, flood overtopping a barrier).
- Combined Hazard
- Two or more external hazards that co-occur, either because one triggers another (consequential), they share a common driver (correlated), or they occur simultaneously without causal linkage (coincidental). Also referred to as compound hazard in some sources.
- Configuration Risk Management (CRM)
- A systematic process for evaluating the impact of plant configuration changes, including screening of external-event risks and incorporation of mitigative compensatory measures.
- Core Damage Frequency (CDF)
- The estimated annual probability that an accident sequence results in damage to the reactor core.
- Defense-in-Depth
- A safety philosophy employing successive independent barriers and layers of protection to prevent and mitigate accidents.
- Beyond-Design-Basis Event (BDBE)
- An external event whose intensity exceeds the design-basis level established in the plant's licensing basis.
- Design-Basis Event (DBE)
- An external event or condition for which a nuclear facility is specifically designed and licensed to withstand without loss of safety function.
- Design-Basis Flood (DBF)
- The flood magnitude and associated conditions that a plant must be able to withstand without loss of safety function, as established in the plant's licensing basis.
- External Hazard
- Any event originating outside a nuclear power plant that can impair safety-related or risk-significant structures, systems, or components (SSCs). Examples include high winds, flooding, seismic events, wildfires, extreme temperatures, and geomagnetic disturbances.
- Exposure Matrix
- A systematic mapping of plant SSCs and support systems against identified hazard pathways, used in the CVA framework to identify potential vulnerabilities where non-stationary-climate-driven hazard changes intersect with plant equipment.
- External-Hazard Walkdown
- An on-site inspection that documents the physical layout, protective features, and operability of SSCs relative to a specific external hazard, providing input data for external-hazard probabilistic risk assessments.
- FLEX (Diverse and Flexible Coping Strategies)
- A U.S. industry program (NEI 12-06) that stages portable equipment (for example, generators, pumps) and establishes procedures to maintain safety functions when external hazards impair on-site resources. International equivalents include diverse coping strategies and portable mitigation equipment programs established under IAEA and national regulatory frameworks.
- Flood Barrier Penetration Seal (FBPS)
- A sealing element used at openings in flood barriers to maintain watertight integrity under hydrostatic pressure.
- Flood Protection System (FPS)
- The collection of engineered barriers, pumps, and drainage features that prevent inundation of safety-related equipment.
- Fragility Curve
- A probabilistic function relating the conditional probability of failure of an SSC to an external-hazard intensity parameter (for example, wind speed, flood elevation). See FAQ: What is a fragility curve?
- Hazard Curve
- A frequency-versus-intensity relationship that characterizes how often a given level of an external hazard is expected to occur at a site. Together with fragility curves, hazard curves form the two primary inputs to an external events PRA. See FAQ: What is a hazard curve?
- Hazard Screening
- The systematic process of identifying and eliminating external hazards that do not pose a credible risk to a specific site from further (more detailed) assessment, based on frequency, intensity, and plant characteristics.
- High-Wind Equipment List (HWEL)
- A plant-specific inventory of SSCs that are either directly exposed to wind loads or are vulnerable to wind-driven rain. The HWEL guides high-wind walkdowns, missile surveys, and probabilistic risk assessments.
- Initiating Event
- An event that triggers an abnormal condition requiring plant response, potentially leading to core damage if safety functions are not maintained.
- Joint Probability Method (JPM)
- A technique for estimating the annual exceedance probability of storm-surge levels by jointly sampling storm intensity, track, and tide/water-level uncertainties. See JPM.
- Large Early Release Frequency (LERF)
- The estimated annual probability of a large, early, uncontrolled release of radioactive material to the environment.
- Loss of Offsite Power (LOOP)
- An event in which the plant's external electrical supply is unavailable. External events are often associated with LOOP events.
- Margin Erosion
- A progressive reduction in the gap between design-basis capacity and projected hazard intensity as conditions (such as climate patterns) evolve over time. Identification of margin erosion is a key output of the CVA process.
- Loss of Ultimate Heat Sink (LUHS)
- The inability to reject decay heat to the environment because of failure of the ultimate heat sink (for example, seawater intake, river cooling). LUHS is a key initiator in many external flood (XF) PRA models.
- Missile Fragility
- A probabilistic description of the probability that a missile of a given impact velocity causes functional failure of a target SSC.
- Non-Stationary Climate Conditions
- The recognition that historical climate statistics (such as temperature, precipitation, wind speed distributions) may not remain constant over a plant's operating lifetime; external hazard parameters historically treated as stationary may shift in magnitude, frequency, or timing. This framing drives the need for forward-looking assessment methods such as the CVA.
- Operating Experience (OpE, OE, or OpEx)
- Documented records of actual events and near-misses at nuclear facilities, used to inform risk assessments and identify recurring vulnerabilities.
- Probabilistic Flood Hazard Assessment (PFHA)
- A stochastic approach that generates a full probability distribution of flood levels by sampling rainfall, antecedent conditions, and hydraulic model parameters. In nuclear applications, PFHA is often contrasted with deterministic PMF-based approaches.
- Probabilistic Risk Assessment (PRA)
- A quantitative, structured analysis that estimates the frequencies of core damage and large early release events, incorporating internal and external initiating events. Extensive fault trees and/or event trees provide the foudational basis for PRA models.
- RCP (Representative Concentration Pathway)
- A greenhouse-gas concentration trajectory adopted by the Intergovernmental Panel on Climate Change (IPCC) and used in CMIP5-era climate projections (for example, RCP 4.5, RCP 8.5). RCPs were introduced in the IPCC Fifth Assessment Report (AR5) and have been largely superseded by SSPs in CMIP6, though they are still referenced in older analyses.
- Probable Maximum Flood (PMF)
- Term used to describe the largest flood that could conceivably occur at a site. Modern practice treats the PMF as a deterministic envelope rather than a probabilistic quantity.
- Resilience (Climate Resilience)
- The ability to anticipate, plan for, withstand, adapt to, and recover from external hazards under non‑stationary climate conditions, maintaining safety and operational performance and learning over time.
- Source definitions:
- FERC / EPRI 3002023814 (CVA Guidance), Glossary: "The ability to withstand and reduce the magnitude and/or duration of disruptive events, which includes the capability to anticipate, absorb, adapt to, and/or rapidly recover from such an event." (3002023814 cites FERC)
- EPRI 3002025519 (Nuclear Plant Resilience), citing FERC: Same as above, with an additional operational definition: "the ability to remain operational and supply power to the electric grid."
- NCA5 (5th National Climate Assessment), Glossary: "The ability to prepare for threats and hazards, adapt to changing conditions, and withstand and recover rapidly from adverse conditions and disruptions." Climate‑specific: "The capacity of interconnected social, economic, and ecological systems to cope with a climate change event, trend, or disturbance, responding or reorganizing in ways that maintain their essential function, identity, and structure."
- IPCC AR6 WGII (2022), Annex II Glossary: "The capacity of social, economic and ecosystems to cope with a hazardous event or trend or disturbance, responding or reorganising in ways that maintain their essential function, identity and structure. Resilience is a positive attribute when it maintains capacity for adaptation, learning and/or transformation."
- UNDRR Sendai Framework (2017): "The ability of a system, community or society exposed to hazards to resist, absorb, accommodate, adapt to, transform and recover from the effects of a hazard in a timely and efficient manner, including through the preservation and restoration of its essential basic structures and functions through risk management."
- PPD‑21 (US Presidential Policy Directive, 2013): "The ability to prepare for and adapt to changing conditions and withstand and recover rapidly from disruptions."
- Canadian Standards Association, Technical Guide (Assessing Climate Change Resilience): "The ability of a system (built, natural, social or economic) to anticipate, withstand, recover, adapt to and transform in response to a climate‑related hazard."
- See also Concepts.
- Risk-Informed Decision Making
- A regulatory and operational approach integrating PRA insights with deterministic analysis, defense-in-depth, and safety margins to support licensing and operational decisions.
- Sea-Level Rise (SLR)
- The long-term increase in mean sea level driven by ocean thermal expansion, ice-sheet and glacier mass loss, and changes in land water storage. In coastal flood analyses, SLR amplifies total water level and wave energy reaching the shore.
- SSP (Shared Socioeconomic Pathway)
- A scenario framework adopted by the IPCC for CMIP6 climate projections, combining socioeconomic assumptions with radiative-forcing levels. SSPs were introduced in the IPCC Sixth Assessment Report (AR6) as successors to the RCP framework. The CHIP program uses SSP1-2.6 (low-emissions) and SSP3-7.0 (high-emissions) as bounding scenarios for nuclear plant assessments.
- Structures, Systems, and Components (SSCs)
- The physical elements of a nuclear plant. Safety-related SSCs must be protected from external hazards.
- Tornado-Generated Missile
- An object (for example, rebar, pipe, debris) accelerated by tornado winds to impact plant SSCs.
- Tornado Missile Risk Evaluator (TMRE)
- A deterministic methodology that combines a foundation of bounding analysis with selected probabilistic concepts to assess the safety significance of tornado missiles.
- Wind-Driven Rain (WDR)
- Rainfall entrained by high-speed winds that can infiltrate building envelopes and cause electrical failures in interior equipment.
Abbreviations
| Abbreviation | Full Term |
|---|---|
| AEP | Annual Exceedance Probability |
| CDF | Core Damage Frequency |
| CHIP | Climate Hazard Information and Projections |
| CMIP6 | Coupled Model Intercomparison Project Phase 6 |
| CRM | Configuration Risk Management |
| CVA | Climate Vulnerability Assessment |
| DBE | Design-Basis Event |
| DBF | Design-Basis Flood |
| FBPS | Flood Barrier Penetration Seal |
| FPS | Flood Protection System |
| HWEL | High-Wind Equipment List |
| JPM | Joint Probability Method |
| LERF | Large Early Release Frequency |
| LOOP | Loss of Offsite Power |
| LUHS | Loss of Ultimate Heat Sink |
| OE | Operating Experience |
| OpE | Operating Experience |
| OpEx | Operating Experience |
| PFHA | Probabilistic Flood Hazard Assessment |
| PMF | Probable Maximum Flood |
| PRA | Probabilistic Risk Assessment |
| RCP | Representative Concentration Pathway |
| SLR | Sea-Level Rise |
| SSCs | Structures, Systems, and Components |
| SSP | Shared Socioeconomic Pathway |
| TMRE | Tornado Missile Risk Evaluator |
Usage Notes
- PRA vs. PSA: The term probabilistic risk assessment (PRA) is standard in NRC regulatory usage. The IAEA and many international contexts use probabilistic safety assessment (PSA). Both terms refer to essentially the same analytical framework.
- Combined vs. Compound Hazard: The terms combined hazard and compound hazard are often used interchangeably. This wiki uses combined hazard. Some sources distinguish "compound" (correlated drivers) from "combined" (any co-occurrence); where such distinctions are made in source documents, they are preserved.
- Probable Maximum Flood: The term PMF originates from deterministic practice and is still used in licensing-basis documentation. Modern probabilistic approaches (PFHA) treat flood levels as continuous probability distributions rather than upper-bound envelopes.
Additional Resources
- NUREG-2122, Glossary of Risk-Related Terms in Support of Risk-Informed Decisionmaking — a comprehensive glossary of PRA and risk-assessment terminology that complements the external-hazards-specific terms defined on this page.
EPRI technical point of contact: Chris Rochon (CRochon@epri.com)
Date last reviewed: 2026-05-22