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	''Looking for the basics? Start here…''		''Looking for the basics? Start here…''

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''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 ==

* [[Hazards|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
* [[Frequently Asked Questions|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:
** [https://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr0800/ NUREG‑0800 ''Standard Review Plan''] — Chapter 3, "Design of Structures, Components, Equipment, and Systems," covers design and protection against various external hazards.
** [https://www.nrc.gov/reading-rm/doc-collections/reg-guides/power-reactors/rg/division-1/division-1-181 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 — [https://www.asme.org/codes-standards/find-codes-standards/standard-for-level-1-large-early-release-frequency-probabilistic-risk-assessment-for-nuclear-power-plant-applications/2022/pdf 2022 and 2024] editions have since been issued by ASME.
** [https://www.nrc.gov/reading-rm/doc-collections/reg-guides/power-reactors/rg/division-1/division-1-41 RG 1.59] — ''Design‑Basis Floods for Nuclear Power Plants''
** [https://www.nrc.gov/reading-rm/doc-collections/reg-guides/power-reactors/rg/division-1/division-1-101 RG 1.115] — ''Protection Against Turbine Missiles''
* [https://www.asme.org/codes-standards/find-codes-standards/standard-for-level-1-large-early-release-frequency-probabilistic-risk-assessment-for-nuclear-power-plant-applications/2024/pdf 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 [[Methods/Probabilistic Risk Assessment|PRA]] page discusses the standard in the context of external events.

=== International ===

* IAEA Safety Standards:
** [https://www.iaea.org/publications/8635/meteorological-and-hydrological-hazards-in-site-evaluation-for-nuclear-installations SSG‑18] — ''Meteorological and Hydrological Hazards in Site Evaluation for Nuclear Installations''
** [https://www.iaea.org/resources/safety-standards SSG‑68] — ''Design of Nuclear Installations Against External Events Excluding Earthquakes''
** [https://www.iaea.org/publications/15067/hazards-associated-with-human-induced-external-events-in-site-evaluation-for-nuclear-installations SSG‑79] — ''Hazards Associated with Human Induced External Events in Site Evaluation for Nuclear Installations''

:These three SSGs form a complementary set for external hazard assessment of nuclear installations (excluding seismic events, which are covered by [https://www.iaea.org/publications/14664/seismic-design-for-nuclear-installations SSG‑67]):

<blockquote>
{| class="wikitable"
! 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
|}
</blockquote>

: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:
** [https://www.iaea.org/publications/6633/extreme-external-events-in-the-design-and-assessment-of-nuclear-power-plants 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.
** [https://www-pub.iaea.org/MTCD/Publications/PDF/TE1834_web.pdf 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.
** [https://www-pub.iaea.org/MTCD/Publications/PDF/TE-2043web.pdf 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.

* [https://www.osti.gov/etdeweb/servlets/purl/20421312 SKI Report 02:27] — ''Guidance for External Events Analysis'' (Swedish Nuclear Power Inspectorate)

=== EPRI Guidance (selected) ===

{| class="wikitable"
! Year !! Report Number !! Title
|-
| 2015 || [https://www.epri.com/research/products/000000003002005287 3002005287] || ''Identification of External Hazards for Analysis in Probabilistic Risk Assessment''
|-
| colspan=3 style="text-align: center;" | '''External Flooding'''
|-
| 2022 || [https://www.epri.com/research/products/000000003002023808 3002023808] || ''External Flooding Guidance for Probabilistic Risk Assessment''
|-
| 2019 || [https://www.epri.com/research/products/000000003002015989 3002015989] || ''External Flooding Probabilistic Risk Assessment Walkdown Guidance''
|-
| colspan=3 style="text-align: center;" | '''High Winds'''
|-
| 2016 || [https://www.epri.com/research/products/000000003002008092 3002008092] || ''Process for High Winds Walkdown and Vulnerability Assessments at Nuclear Power Plants''
|-
| 2015 || [https://www.epri.com/research/products/000000003002003107 3002003107] || ''High Wind Risk Assessment Guidelines''
|-
| colspan=3 style="text-align: center;" | '''Climate'''
|-
| 2026 || [https://www.epri.com/research/products/000000003002035190 3002035190] || ''Climate Vulnerability Assessments: Industry Insights and Lessons Learned''
|-
| 2024 || [https://www.epri.com/research/products/000000003002029339 3002029339] || ''Site‑Specific Climate Hazard Information and Projections: Technical Summary''
|-
| 2022 || [https://www.epri.com/research/products/000000003002023814 3002023814] || ''Climate Vulnerability Assessment Guidance for Nuclear Power Plants''
|-
| 2025 || [https://www.epri.com/research/products/000000003002032030 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 ([mailto:CRochon@epri.com CRochon@epri.com])''

''Date last reviewed: 2026-05-20''

Foundations - Revision history

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