Canonical Concepts Reference
Purpose
This page lists the canonical concepts defined in the External Hazards index database. Concepts are controlled labels applied to documents, sections, and wiki pages to improve machine readability, semantic retrieval, and cross‑linking. They complement the hazard taxonomy (which classifies what can threaten a plant) by classifying how those threats are analyzed, managed, and understood.
The authoritative source for this list is the canonical_concepts table in the index DB. This page is a mirror for human reference; if the two diverge, the database is correct.
Governance Notes
- Concepts are labels, not filters. They improve grouping and traceability but must never gate retrieval or render content inaccessible.
- New concepts may be added as the knowledge system grows. Existing concepts should not be renamed; use deprecation with a replacement pointer if meaning changes.
- Concept names use natural‑language labels (not coded IDs) to maximize readability.
Risk and PRA Concepts
| Concept
|
Description
|
| Probabilistic Risk Assessment (PRA) |
A systematic, comprehensive methodology for evaluating risks associated with nuclear facilities.
|
| External Events PRA |
An extension of PRA to consider initiating events and hazards originating outside the plant boundary.
|
| Individual Plant Examination of External Events (IPEEE) |
NRC‑mandated (NUREG‑1407) systematic evaluation of each operating plant's vulnerability to severe accident sequences initiated by external events.
|
| Probabilistic Safety Assessment (PSA) |
International term (IAEA) equivalent to PRA.
|
| Core Damage Frequency (CDF) |
The estimated annual probability that an accident sequence results in damage to the reactor core.
|
| Large Early Release Frequency (LERF) |
The estimated annual probability of a large, early, uncontrolled release of radioactive material.
|
| Initiating Event |
An event that triggers an abnormal condition requiring plant response.
|
| Fault Tree Analysis (FTA) |
A top‑down, deductive failure analysis using Boolean logic.
|
| Event Tree Analysis (ETA) |
An inductive analysis method modeling initiating event progression through safety function outcomes.
|
| Human Reliability Analysis (HRA) |
Assessment of the probability and consequences of human errors in response to external hazard events.
|
| Peer Review |
Independent technical review of PRA models and results per ASME/ANS RA‑S standards.
|
| Uncertainty Analysis |
Quantification of epistemic and aleatory uncertainties in hazard parameters and risk estimates.
|
| Epistemic Uncertainty |
Uncertainty due to incomplete knowledge that can in principle be reduced through additional data.
|
| Monte Carlo Simulation |
A computational technique using repeated random sampling to estimate probability distributions.
|
| Extreme Value Analysis |
Statistical methods for estimating the probability of rare, extreme events from limited data.
|
| Sensitivity Analysis |
Systematic variation of model inputs to determine which parameters most strongly influence results.
|
| Walkdown |
A systematic physical inspection of plant structures relevant to a specific hazard.
|
| Combined Hazard |
Two or more external hazards occurring simultaneously or sequentially.
|
| Graded Hazard Analysis |
An approach that scales the depth and rigor of hazard assessment to the level of projection confidence available for each hazard type. For example, used when climate data are sparse for certain climate hazards.
|
Safety and Design Concepts
| Concept
|
Description
|
| Hazard Screening |
The systematic process of identifying and eliminating external hazards that do not pose a credible risk.
|
| Design‑Basis Event (design-basis event (DBE)) |
An external event or condition for which a nuclear facility is specifically designed and licensed to withstand.
|
| Design Basis |
The set of conditions, including external hazards, that a nuclear facility is designed to withstand.
|
| Defense‑in‑Depth |
A safety philosophy employing successive independent barriers and layers of protection.
|
| Risk‑Informed Decision Making |
Regulatory and operational approach integrating PRA insights with deterministic analysis.
|
| Licensing Basis |
The set of NRC requirements, commitments, and design‑basis conditions under which a plant is authorized to operate.
|
| Structures, Systems, and Components (SSCs) |
The physical elements of a nuclear plant categorized by safety significance.
|
| Standard Review Plan (SRP / NUREG‑0800) |
NRC guidance documents providing acceptance criteria and review procedures.
|
| Regulatory Guide (RG) |
NRC‑issued documents describing acceptable methods for meeting regulatory requirements.
|
| Configuration Risk Management |
The process of assessing and managing risk impacts from temporary plant configurations.
|
| Emergency Preparedness |
Plans, procedures, and capabilities for responding to external hazard events.
|
| Corrective Action Program (CAP) |
A systematic process for identifying, evaluating, and resolving conditions adverse to quality.
|
| Loss of Offsite Power (LOOP) |
Loss of all external AC power sources, frequently correlated with severe weather hazards.
|
| Ultimate Heat Sink (UHS) |
The body of water or atmospheric heat sink to which residual and emergency heat is rejected.
|
| Operating Experience (OpE) |
Documented records of actual events and near‑misses at nuclear facilities.
|
| Margin Assessment |
Evaluation of the gap between SSC capacity and hazard demand, encompassing margins to both design‑basis limits and operational limits such as thermal performance thresholds and equipment operating ranges.
|
Fragility and Structural Concepts
| Concept
|
Description
|
| Fragility Curve |
A probabilistic function relating conditional probability of failure to a hazard intensity parameter.
|
| Fragility Analysis |
The process of developing fragility curves for SSCs subjected to external hazard loads.
|
| Structural Integrity |
The ability of a structure to withstand applied loads without failure.
|
| Impact Testing |
Experiments to determine response and damage thresholds of structures to missile impact.
|
| Seismic Hazard Analysis (SHA) |
Probabilistic or deterministic analysis of vibratory ground motion potential at a site.
|
| Seismic PRA (SPRA) |
Probabilistic risk assessment specifically for seismic hazards.
|
| Seismic Margins Assessment (SMA) |
A methodology for evaluating plant capacity against earthquakes beyond the design basis.
|
| Soil Liquefaction |
Loss of soil strength and stiffness due to seismic shaking.
|
Flooding Concepts
| Concept
|
Description
|
| Probable Maximum Flood (PMF) |
The most severe flood reasonably possible at a site.
|
| Probable Maximum Precipitation (PMP) |
The theoretical greatest depth of precipitation for a given duration meteorologically possible at a site.
|
| Design‑Basis Flood Level (DBFL) |
The maximum flood water elevation that a plant is designed to withstand.
|
| Safe Shutdown Flood Level (SSFL) |
The flood level at which safe shutdown can still be achieved.
|
| Flood Protection Feature (FPF) |
Structures, systems, or components designed to protect the plant from external flooding.
|
| Flood Barrier |
A physical structure designed to prevent water ingress into safety‑related areas.
|
| Storm Surge |
An abnormal rise of water generated by a storm's winds and atmospheric pressure drop.
|
| Seiche |
An oscillating standing wave in an enclosed or semi‑enclosed body of water, capable of producing temporary increases in local water level.
|
| Dam Break Analysis |
Evaluation of downstream flood consequences from hypothetical dam failure.
|
| Flood Frequency Analysis |
Statistical analysis of historical flood data to estimate probability of flood events.
|
| Annual Exceedance Probability (AEP) |
The probability that a given flood level will be equaled or exceeded in any given year.
|
| Paleoflood Evidence |
Geological evidence of past flooding events used to extend the historical flood record.
|
| Hydrologic / Hydraulic Modeling |
Computational simulation of water flow, levels, and flood routing.
|
| Local Intense Precipitation (LIP) |
Extreme rainfall directly at the plant site causing ponding and localized flooding.
|
| Structured Hazard Assessment Committee Process for Flooding (SHAC‑F) |
NRC‑described structured expert elicitation process for evaluating flood hazard parameters.
|
Wind and Missile Concepts
| Concept
|
Description
|
| Tornado Design‑Basis Wind Speed |
The maximum tornado wind speed used for structural and missile design per RG 1.76.
|
| Tornado Missile Spectrum |
The set of representative objects assumed to be propelled by tornado winds for barrier design.
|
| Wind‑Borne Missile |
Any object propelled by extreme winds that could impact safety‑related SSCs.
|
| Missile Trajectory Analysis |
Computational modeling of the flight path, velocity, and impact probability of missiles.
|
| Strike Probability |
The probability that a tornado‑ or turbine‑generated missile strikes a specific target.
|
| Tornado Missile Strike Calculator (TMSC) |
EPRI software tool for computing tornado missile strike and damage probabilities.
|
| High Winds PRA (HWPRA) |
Probabilistic risk assessment specifically for high winds hazards.
|
| Tornado Missile Risk Evaluator (TMRE) |
Industry methodology for evaluating tornado missile risk. Less analytically intensive and less quantitatively detailed than a full HWPRA.
|
| Wind Fragility |
Conditional probability of failure or damage as a function of wind speed.
|
| Wind Loading |
Forces and pressures exerted on structures by wind per ASCE 7 or site‑specific analysis.
|
| Gust Factor |
A multiplier applied to mean wind speed to account for short‑duration peak gusts.
|
| Turbine Missile |
A fragment of a failed turbine rotor that may be ejected at high velocity. (Disambiguation - this is NOT a wind-related concept.)
|
| Fujita Wind‑Field Model |
A mathematical model describing the wind velocity distribution within a tornado vortex.
|
Climate and Adaptation Concepts
| Concept
|
Description
|
| Climate Vulnerability Assessment (CVA) |
A structured evaluation of how projected non‑stationary climate trends may alter external hazard frequency or severity at a specific site. See CVA page.
|
| 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.
|
| Climate Projections |
Model‑based estimates of future climate variables (temperature, precipitation, wind, sea level) used to assess long‑term hazard trends. The EPRI CHIP program provides site‑specific projections for nuclear plants.
|
| Sea Level Rise |
Long‑term increase in mean sea level affecting coastal flood hazard assessments.
|
| Climate Adaptation |
Adjustments in plant design, operations, or procedures to moderate harm from evolving external hazard conditions driven by non‑stationary climate trends.
|
| Non‑Stationary Climate Conditions |
The recognition that historical climate statistics (temperature, precipitation, wind speed distributions) may not remain constant over a plant's operating lifetime, requiring forward‑looking hazard assessment. Related to "climate change."
|
| Exposure Matrix |
A systematic mapping of plant SSCs and support systems against identified hazard pathways, used in the CVA framework to identify where climate‑driven hazard changes intersect with plant vulnerabilities.
|
| Margin Erosion |
The progressive reduction in the gap between design‑basis capacity and projected hazard intensity as climate conditions evolve over time.
|
| Adaptation Roadmap |
A multi‑year plan that sequences near‑term operational adjustments, medium‑term equipment upgrades, and long‑term capital investments to sustain plant resilience under evolving hazard conditions.
|
Change Log
| Date |
Change
|
| 2026‑03‑19 |
Initial page created; mirrored from index DB canonical_concepts table (v1.0, 2026‑03‑10).
|
| 2026‑04‑30 |
Minor revisions. Table (v1.1, 2026‑04‑30).
|
EPRI technical point of contact: Chris Rochon (CRochon@epri.com)