Foundations
Looking for the basics? Start here…
What Are External Hazards?
Definition: Events originating outside the plant that can impair safety‑related SSCs (structures, systems, and components), for example, high winds, floods, earthquakes, wildfires, tornado‑generated missiles, and geomagnetic disturbances.
Scope: This wiki covers external hazards to nuclear power plants.
Why it matters: External hazards (such as earthquakes, flooding, extreme weather, and human‑induced events) are uniquely important to nuclear safety because they can simultaneously challenge multiple safety systems, structures, and units. Unlike most internal events, external hazards act as common‑cause stressors, capable of degrading several layers of defense‑in‑depth at once. International safety standards and regulatory frameworks reflect this by requiring explicit, hazard‑specific evaluation and protection measures.
Operating experience has shown that several of the most consequential nuclear events have involved external hazards, often in combination with other hazards or through unexpected escalation mechanisms. These events highlight the presence of cliff‑edge effects, where small exceedances of design assumptions (such as water overtopping a barrier or loss of off‑site power during a storm) can lead to disproportionate safety consequences.
External hazards also differ from many plant challenges in that they evolve over time. New scientific data, changes in land use, and long‑term climate trends can alter hazard characterizations over a plant’s operating life, necessitating periodic reassessment to ensure risk, safety, and operational margins remain adequate.
From a regulatory and public‑confidence perspective, external hazards are highly visible, intuitively understood, and closely scrutinized. Robust, transparent treatment of external hazards therefore serves not only as a technical necessity, but as a key indicator of overall plant resilience and long‑term safety assurance.
Explore the Wiki
- Hazard Catalog — Overview pages for individual external hazard types
- Methods and Tools — Analytical methods, software tools, and field processes for external hazard assessment
- Industry Experience — Operating experience, lessons learned, and key events by hazard type
- Historical Context and Evolution — How external hazard analysis has evolved over time
- Terms and Definitions — Key terminology and abbreviations
- FAQ — Frequently asked questions
Key Resources
The following resources serve as key entry points for understanding external hazard assessment in the nuclear industry.
US Regulatory and Standards
- NRC:
- NUREG‑0800 Standard Review Plan — Chapter 3, "Design of Structures, Components, Equipment, and Systems," covers design and protection against various external hazards.
- RG 1.200 — Acceptability of Probabilistic Risk Assessment Results for Risk‑Informed Activities
- RG 1.200 Revision 3 endorses ASME/ANS RA‑Sb‑2013 (published November 2017) as the basis for judging PRA technical acceptability in risk‑informed applications. The endorsed 2013 edition is not the most recent version of the standard — 2022 and 2024 editions have since been issued by ASME.
- RG 1.59 — Design‑Basis Floods for Nuclear Power Plants
- RG 1.115 — Protection Against Turbine Missiles
- ASME/ANS RA‑S‑1.1–2024 — Standard for Level 1 / Large Early Release Frequency Probabilistic Risk Assessment
- Part 5 — Requirements for Seismic Events At‑Power PRA
- Part 6 — Requirements for Screening of External Hazards
- Part 7 — Requirements for High Winds At‑Power PRA
- Part 8 — Requirements for External Flood At‑Power PRA
- Part 9 — Requirements for Other External Hazards At‑Power PRA
- Each part defines capability categories that govern the level of analytical detail required. The PRA page discusses the standard in the context of external events.
International
- IAEA Safety Standards:
- These three SSGs form a complementary set for external hazard assessment of nuclear installations (excluding seismic events, which are covered by SSG‑67):
SSG Scope Lifecycle Phase SSG‑18 Natural hazards — meteorological (wind, temperature, precipitation, lightning, tornadoes) and hydrological (floods, storm surge, tsunami, low water) Site evaluation — defines the hazard parameters a site must be assessed against SSG‑68 All external events excluding earthquakes — both natural and human‑induced Design — translates hazard parameters into engineering design requirements, safety margins, and beyond‑design‑basis considerations SSG‑79 Human‑induced external events — explosions, fires, aircraft crashes, hazardous material releases, transport incidents Site evaluation — identifies and screens human‑induced hazards specific to the site and its surrounding region
- In practice, SSG‑18 and SSG‑79 together define what hazards a site faces (natural and human‑induced, respectively), while SSG‑68 addresses how to design against those hazards. SSG‑18 and SSG‑79 feed hazard characterization into SSG‑68's design framework.
- Three IAEA TECDOCs complement the SSGs with practical assessment methodology:
- IAEA‑TECDOC‑1341 (2003) — Extreme External Events in the Design and Assessment of Nuclear Power Plants — International survey of external hazard assessment practices including a three‑class External Event Classification system.
- IAEA‑TECDOC‑1834 (2017) — Assessment of Vulnerabilities of Operating Nuclear Power Plants to Extreme External Events — Methodology for hazard screening, SSC selection, plant capacity assessment, and walkdowns, incorporating Fukushima lessons.
- IAEA‑TECDOC‑2043 (2024) — Evaluation of Design Robustness of Nuclear Installations Against External Hazards — Guidance on design margin assessment, cliff‑edge effect identification, and graded defense‑in‑depth for external hazards.
- SKI Report 02:27 — Guidance for External Events Analysis (Swedish Nuclear Power Inspectorate)
EPRI Guidance (selected)
| Year | Report Number | Title |
|---|---|---|
| 2015 | 3002005287 | Identification of External Hazards for Analysis in Probabilistic Risk Assessment |
| External Flooding | ||
| 2022 | 3002023808 | External Flooding Guidance for Probabilistic Risk Assessment |
| 2019 | 3002015989 | External Flooding Probabilistic Risk Assessment Walkdown Guidance |
| High Winds | ||
| 2016 | 3002008092 | Process for High Winds Walkdown and Vulnerability Assessments at Nuclear Power Plants |
| 2015 | 3002003107 | High Wind Risk Assessment Guidelines |
| Climate | ||
| 2026 | 3002035190 | Climate Vulnerability Assessments: Industry Insights and Lessons Learned |
| 2024 | 3002029339 | Site‑Specific Climate Hazard Information and Projections: Technical Summary |
| 2022 | 3002023814 | Climate Vulnerability Assessment Guidance for Nuclear Power Plants |
| 2025 | 3002032030 | Impact of Sea Level Rise on Coastal Flooding Hazard |
Terms and Definitions
For a list of key terms and definitions, see: Terms and Definitions
EPRI technical point of contact: Chris Rochon (CRochon@epri.com)
Date last reviewed: 2026-05-20