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'''Historical Context and Evolution of External Hazard Assessment'''

External hazard assessment for nuclear power plants has evolved through three broad phases: a '''foundational era''' of deterministic screening and early regulatory requirements (1970s–2000s), a '''post-Fukushima expansion''' that introduced risk-informed tools and programmatic mandates (2011–present), and an ongoing '''climate-driven modernization''' that embeds forward-looking climate projections into hazard analysis, design-basis review, and resilience planning. This page traces those phases and the key milestones within each.

== Early approaches to external hazard assessment ==

Before 2011 the nuclear industry relied on a relatively modest set of regulatory and industry frameworks to identify, screen, and analyze external hazards. The first formal requirement for tornado-generated missiles appeared in '''[https://www.nrc.gov/reading-rm/doc-collections/reg-guides/power-reactors/rg/division-1/division-1-61 Regulatory Guide 1.76]''' (1974), which introduced the design-basis tornado concept and missile-impact criteria. Throughout the 1980s the Nuclear Regulatory Commission (NRC) published comprehensive external-hazard surveys such as '''[https://www.nrc.gov/docs/ML0231/ML023180425.pdf NUREG/CR-5042]''' (1987) that catalogued seismic, flood, wind, fire, and transportation hazards and defined “beyond-design-basis” screening concepts.

The '''PRA Procedures Guide''' ('''[https://www.nrc.gov/reading-rm/doc-collections/nuregs/contract/cr2300/index NUREG/CR-2300]''', 1983) integrated external-hazard treatment into Level-1 PSA methodology and introduced event-tree and fault-tree conventions for external initiators. The '''Individual Plant Examination of External Events (IPEEE)''' program, codified in [https://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1407/index NUREG-1407] (1991) and Generic Letter 88-20 (Supplement 4), obligated every U.S. plant to document plant-specific external-hazard vulnerabilities. Engineering standards such as '''ASCE/SEI 7-95''' modernized wind-load calculations, while the NRC’s [https://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr0800/ Standard Review Plan] §§ 3.5.1.4 and 3.5.3 (2000–2007) established review expectations for missile protection and barrier design. The first industry-wide screening methodology covering 68 external hazards (seismic excluded) was published in the [https://www.epri.com/research/products/000000003002005287 ''Identification of External Hazards for Analysis in Probabilistic Risk Assessment''] report (2015, updating earlier work from 2005).

Significant operating events during this era, including Hurricane Andrew (1992), which demonstrated the vulnerability of offsite power and fire-water systems at the Turkey Point Nuclear Generating Station [[https://www.nrc.gov/docs/ML2004/ML20045E785.pdf NUREG-1474]], reinforced the need for systematic external-hazard programs. The first risk-informed turbine-missile analysis (2003) demonstrated that conventional regulatory conservatism could be relaxed through probabilistic methods [[https://www.epri.com/research/products/000000000001007637 1007637]].

The following table summarizes key regulatory and technical milestones that shaped external-hazard assessment during this time period.

{| class="wikitable"
! Period !! Milestone !! Key Content
|-
| '''1970–1974''' || '''[https://www.nrc.gov/reading-rm/doc-collections/reg-guides/power-reactors/rg/division-1/division-1-61 Regulatory Guide 1.76] (Design-Basis Tornado & Missiles)''' || First U.S. requirement that new plants design for tornado-generated missiles and wind loads; introduced regional tornado wind-speed maps, missile-impact spectra, and the “design-basis tornado” (DBT).
|-
| '''1978–1985''' || '''[https://www.nrc.gov/reading-rm/doc-collections/nuregs/contract/cr4461/index.html NUREG/CR-4461] (Tornado Climatology)''' || Statistical database of U.S. tornadoes used to derive point- and line-strike probabilities, forming the probabilistic basis for tornado-hazard curves.
|-
| '''1983''' || '''[https://www.nrc.gov/reading-rm/doc-collections/nuregs/contract/cr2300/index NUREG/CR-2300] (PRA Procedures Guide)''' || Integrated external-hazard treatment into Level-1 PSA methodology; introduced event-tree and fault-tree conventions for external initiators.
|-
| '''1987''' || '''[https://www.nrc.gov/docs/ML0231/ML023180425.pdf NUREG/CR-5042] Evaluation of External Hazards''' || First comprehensive NRC review of seismic, flood, wind, fire, and transportation hazards; defined external-hazard screening process and introduced the concept of “beyond-design-basis” (BDB) events.
|-
| '''1991''' || '''[https://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1407/index NUREG-1407] (IPEEE Guidance) & Generic Letter 88-20 (Supplement 4)''' || Formalized the Individual Plant Examination of External Events (IPEEE); required utilities to identify plant-specific external-hazard vulnerabilities (high winds, floods, fire, transportation) and to propose mitigations.
|-
| '''1995–2000''' || '''ASCE/SEI 7-95 (and later editions)''' || Updated wind-load provisions (3-second gust, exposure categories) that became the engineering basis for external wind pressure calculations used in PRA fragilities.
|-
| '''2000–2007''' || '''[https://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr0800/ Standard Review Plan (SRP)] §§ 3.5.1.4 & 3.5.3''' || Codified NRC expectations for review of missiles generated by extreme winds/tornadoes and barrier-design procedures.
|-
| '''2003''' || '''[https://www.iaea.org/publications/6633/extreme-external-events-in-the-design-and-assessment-of-nuclear-power-plants IAEA‑TECDOC‑1341]''' || IAEA synthesis of Member States’ experience with extreme external events at NPPs; proposed a three‑class External Event Classification system and surveyed deterministic vs. probabilistic approaches across 21 countries.
|-
| '''2011, 2015''' || '''[https://www.epri.com/research/products/000000003002005287 3002005287] – Identification of External Hazards for PRA''' || Industry-wide screening methodology covering 68 external hazards (seismic excluded); supplied quantitative screening criteria, hazard catalogues, and templates for plant-specific hazard identification. First issued in 2011 as 1022997; updated in 2015 as 3002005287.
|-
| '''2009''' || '''[https://oecd-nea.org/jcms/pl_18730/probabilistic-safety-analysis-psa-of-other-external-events-than-earthquake?details=true NEA/CSNI R(2009)4]''' || International survey documenting PSA practice for external events (flood, wind, fire); highlighted need for standardized external-hazard PSA guidance.
|}

These early documents established the regulatory and methodological backbone that later enabled the more climate-aware, probabilistic, and combined-hazard approaches adopted after Fukushima.

== Post-Fukushima developments ==

The 2011 Fukushima Daiichi accident triggered a rapid, industry-wide reassessment of external hazards. The NRC created the '''Near-Term Task Force (NTTF)''' in March 2011 to review the regulatory framework, issuing Recommendation 2.1 (seismic hazard reevaluation) and Recommendation 2.3 ([[Hazards/External Flooding|flood]] walkdowns and other degraded-condition assessments). Under [https://www.nrc.gov/docs/ML1205/ML12053A340.pdf 10 CFR 50.54(f)] the NRC issued Requests for Information (RFIs) to licensees, requiring seismic hazard reevaluations and flood-walkdown methodologies. INPO Event Report L1-13-10 (Recommendation 2) complemented these regulatory actions by requiring utilities to systematically monitor new external-hazard information.

The post-Fukushima response also produced a suite of programmatic and technical advances:

* '''FLEX implementation''' – The Flexible Coping Strategies (FLEX) program (NEI 12-06) added portable equipment and redundant cooling to address beyond-design-basis external events.
* '''[[Hazards/High Winds|High-wind]] tools''' – The High-Wind Equipment List (HWEL) and pilot walkdowns [[https://www.epri.com/research/products/000000003002008092 3002008092]] captured wind-borne missile and rain-induced equipment vulnerabilities. The Tornado Missile Risk Evaluator (TMRE) [[https://www.nrc.gov/docs/ML1726/ML17268A023.html NEI 17-02 Rev 1]] provided a cost-effective, risk-informed workflow for resolving tornado-missile protection nonconformances.
* '''[[Hazards/External Flooding|External flood]] PRA''' – Detailed walkdown guidance [[https://www.epri.com/research/products/000000003002015989 3002015989]], probabilistic flood hazard analysis pilots [[https://www.epri.com/research/products/000000003002003013 3002003013]], the [[Methods/Joint Probability Method|Joint Probability Method]] for storm surge [[https://www.epri.com/research/products/000000003002012996 3002012996]], and integrated external flooding PRA guidance [[https://www.epri.com/research/products/000000003002023808 3002023808]] established a comprehensive flood-risk analysis framework.
* '''PRA standard revision''' – [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] codified PRA acceptance criteria and explicitly called for climate-aware assumptions in risk-informed decisions.
* '''IAEA vulnerability assessment''' – [https://www-pub.iaea.org/MTCD/Publications/PDF/TE1834_web.pdf IAEA‑TECDOC‑1834] (2017) provided an updated international methodology for assessing the vulnerabilities of operating plants to extreme external events, incorporating Fukushima lessons into hazard screening, structures, systems, and components (SSC) selection, plant capacity assessment, and walkdown practices.

Operating experience from major flood events, including the 2011 Missouri River flooding, underscored the critical role of preparatory actions, portable equipment staging, and robust flood barriers [[https://www.nrc.gov/docs/ML2222/ML22222A111.pdf ML22222A111]]. Hurricane Andrew (1992) had earlier demonstrated similar lessons for [[Hazards/High Winds|high-wind]] hazards [[https://www.nrc.gov/docs/ML2004/ML20045E785.pdf NUREG-1474]]. Industry analyses of weather-related events from 2011 to 2020 quantified operational impacts across the fleet [[https://www.epri.com/research/products/000000003002025519 3002025519]].

== Integration of non-stationary climate considerations ==

External-hazard programs are increasingly shaped by the need to reinforce plant resiliency, protect operational performance, and revisit design-basis assumptions in light of evolving climatic conditions. Post-Fukushima initiatives (such as portable mitigation equipment programs and updated flood protection bases) have been expanded to incorporate forward-looking climate projections so that margins originally set for historical extremes can be re-evaluated against emerging temperature, precipitation, sea-level, and wind trends [[https://www.epri.com/research/products/000000003002023814 3002023814 ''Climate Vulnerability Assessment Guidance for Nuclear Power Plants'']].

The climate-integration approach stresses three inter-related goals:

* '''Resiliency''' – Evaluate opportunities to strengthen plant capabilities against a broader range of hazard scenarios, including enhancements such as staged portable mitigation equipment, upgraded flood barriers, and improved severe-weather operating procedures.
* '''Operational performance''' – Identify climate-related parameters that affect equipment reliability, loss of offsite power (LOOP) risk, and loss of ultimate heat sink (LUHS) scenarios, and integrate those parameters into PRA models and operating-procedure reviews.
* '''Design-basis re-evaluation''' – Apply climate-hazard screening (10-yr to 100-yr projections) to confirm that existing design-basis events continue to provide appropriate safety margins, and identify where additional analysis or margin enhancement may add value.

Annual information-compilation reports screen newly available climate and hazard data against design-basis assumptions, directly supporting the industry commitment under INPO IER L1-13-10 (Recommendation 2) to evaluate credible new external-hazard information through a formal process. The 2024 report [[https://www.epri.com/research/products/000000003002032026 3002032026]] confirmed that existing design bases remain appropriate, while the 2025 Catalog of Relevant Information Sources [[https://www.epri.com/research/products/000000003002032027 3002032027]] provides the curated set of credible climate-hazard data used for ongoing monitoring. Research on compound hazards [[https://www.epri.com/research/products/000000003002030518 3002030518]] extends this work to multi-hazard interactions.

International standards also inform non-stationary climate integration. [https://www.iaea.org/resources/safety-standards IAEA SSG-68] provides benchmarks for external-event design, and [https://www.iaea.org/publications/8635/meteorological-and-hydrological-hazards-in-site-evaluation-for-nuclear-installations IAEA SSG-18] addresses meteorological and hydrological hazards in site evaluation. Most recently, [https://www-pub.iaea.org/MTCD/Publications/PDF/TE-2043web.pdf IAEA‑TECDOC‑2043] (2024) provides updated guidance on evaluating design robustness and safety margins for nuclear installations against external hazards, with emphasis on cliff‑edge effects and graded defense‑in‑depth.

== Legacy considerations ==

Several foundational documents and approaches from the pre-Fukushima era remain influential but have been substantially superseded or supplemented:

* '''IPEEE (NUREG-1407)''' – The IPEEE (1991) established the first plant-specific vulnerability assessments. While the IPEEE results informed early risk insights, modern external-hazard evaluations draw on updated hazard curves, fragility methods, and the current PRA 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/2024/pdf ASME/ANS RA-S-1.1-2024]). In practice, most hazards at most sites continue to be addressed through screening or deterministic methods; full probabilistic models are developed where a more detailed assessment is warranted.
* '''Original tornado climatology ([https://www.nrc.gov/reading-rm/doc-collections/nuregs/contract/cr4461/index.html NUREG/CR-4461])''' – The initial 1986 tornado database has been revised twice (2005, 2007) to incorporate expanded tornado records, finite-structure corrections, and the Enhanced Fujita (EF) scale. Current high-wind PRAs reference the most recent revision.
* '''Deterministic screening thresholds''' – Early screening used conservative bounding assumptions (such as point-target strike probabilities). Modern practice applies probabilistic hazard curves coupled with plant-response models and climate-trend monitoring, significantly refining what is screened in or out.
* '''Pre-2011 flood design bases''' – Design-basis floods established under deterministic criteria ([https://www.nrc.gov/reading-rm/doc-collections/reg-guides/power-reactors/rg/division-1/division-1-41 RG 1.59]) are being supplemented with updated probable maximum precipitation (PMP) and storm-surge analyses that account for evolving climatic data. The probabilistic flood hazard analysis (PFHA) framework now provides a complementary risk-informed perspective that strengthens confidence in existing margins.

These legacy elements are ''not discarded''; they remain part of the licensing basis for many plants, but the analytical methods, data sources, and regulatory expectations have evolved materially since their original issuance.

== Summary ==

# '''Foundations (1970s–2000s):''' Regulatory guides ([https://www.nrc.gov/reading-rm/doc-collections/reg-guides/power-reactors/rg/division-1/division-1-61 RG 1.76], RG 1.59), statistical climatologies (NUREG/CR-4461), the PRA Procedures Guide (NUREG/CR-2300), and the IPEEE framework created a deterministic-screening paradigm for external hazards.
# '''Post-Fukushima evolution (2011–present):''' NTTF recommendations, [[Hazards/External Flooding|flood]] and [[Hazards/High Winds|high-wind]] PRA guidance, industry tools (TMRE, TMSC, HWEL, wind-driven rain considerations), and diverse coping strategies transformed external-hazard assessment into a probabilistic discipline.
# '''Non-stationary climate integration (mid-2010s–present):''' International safety guides (IAEA SSG-68, SSG-18), climate-vulnerability and resilience programs, and ongoing research on compound hazards and sea-level rise embed climate-trend uncertainties into hazard frequency, fragility, and combined-hazard screening.

== Key References ==

{|Class="wikitable"
!style="text-align:left;width:80px;"|Year
!style="width:120px;"|Report Number
!style="width:320px;"|Title
!Summary
|-
| 2025 || [https://www.epri.com/research/products/000000003002032027 3002032027] || ''External Hazards: Information Compilation and Analysis – 2025 Catalog of Relevant Information Sources'' || Curated list of credible climate-hazard data sources for ongoing monitoring.
|-
| 2025 || [https://www.epri.com/research/products/000000003002032026 3002032026] || ''External Hazards: Information Compilation and Analysis – 2024 New Information Report'' || Annual screening of emerging external-hazard data; confirms existing design bases remain appropriate as of 2024.
|-
| 2024 || [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'' || Defines criteria and methods for establishing design-basis floods, including probable maximum precipitation and storm-surge analysis.
|-
| 2024 || [https://www.epri.com/research/products/000000003002030518 3002030518] || ''Compound Hazards and the Power Sector in a Changing Climate'' || Explores interactions among climate-driven hazards (such as simultaneous flooding and high winds) and their implications for PRA modeling.
|-
| 2024 || [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 for Nuclear Power Plant Applications'' || PRA standard; codifies acceptance criteria and emphasizes climate-aware modeling.
|-
| 2024 || [https://www-pub.iaea.org/MTCD/Publications/PDF/TE-2043web.pdf IAEA‑TECDOC‑2043] || ''Evaluation of Design Robustness of Nuclear Installations Against External Hazards'' || Updated IAEA guidance on assessing design margins and robustness against external hazards; addresses cliff‑edge effects, defense‑in‑depth, and lessons from Fukushima.
|-
| 2022 || [https://www.epri.com/research/products/000000003002025519 3002025519] || ''Nuclear Plant Resilience to Weather-Related Events Between 2011 to 2020'' || Quantifies operational impacts of recent weather events across the fleet.
|-
| 2022 || [https://www.epri.com/research/products/000000003002023814 3002023814] || ''Climate Vulnerability Assessment Guidance for Nuclear Power Plants'' || Structured hazard-exposure-vulnerability methodology linked to portable mitigation equipment and design-basis programs.
|-
| 2021 || [https://www.iaea.org/resources/safety-standards SSG-68] || ''Design of Nuclear Installations Against External Events Excluding Earthquakes'' || International benchmark for external-event design.
|-
| 2017 || [https://www.nrc.gov/docs/ML1726/ML17268A023.html NEI 17-02 Rev 1] || ''Tornado Missile Risk Evaluator (TMRE) Industry Guidance Document'' || Deterministic (but similar to PRA) screening workflow for tornado-missile resolution.
|-
| 2017 || [https://www-pub.iaea.org/MTCD/Publications/PDF/TE1834_web.pdf IAEA‑TECDOC‑1834] || ''Assessment of Vulnerabilities of Operating Nuclear Power Plants to Extreme External Events'' || IAEA methodology for vulnerability assessment of operating plants incorporating Fukushima lessons; covers hazard screening, SSC selection, plant capacity assessment, and walkdowns.
|-
| 2015 || [https://www.epri.com/research/products/000000003002005287 3002005287] || ''Identification of External Hazards for Analysis in Probabilistic Risk Assessment'' || Baseline screening framework covering 68 external hazards (seismic excluded).
|-
| 2012 || [https://www.nrc.gov/docs/ML1205/ML12053A340.pdf ML12053A340] || ''NRC Request for Information Pursuant to 10 CFR 50.54(f) – NTTF Recommendations 2.1, 2.3, and 9.3'' || Foundation for post-Fukushima seismic and flood hazard reevaluations.
|-
| 2011 || [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'' || International guidance for climate-driven meteorological/hydrological hazards.
|-
| 2007 || [https://www.nrc.gov/reading-rm/doc-collections/reg-guides/power-reactors/rg/division-1/division-1-61 RG 1.76] || ''Design-Basis Tornado and Tornado Missiles for Nuclear Power Plants'' || Defines design-basis tornado parameters (wind speed, pressure drop, missile spectra) for nuclear power plant safety evaluation.
|-
| 2007 || [https://www.nrc.gov/docs/ML0708/ML070810400.pdf NUREG/CR-4461 Rev. 2] || ''Tornado Climatology of the Contiguous United States'' || Updated tornado occurrence rates, wind-speed distributions, and finite-structure corrections for tornado-hazard analysis at nuclear sites.
|-
| 2003 || [https://www.iaea.org/publications/6633/extreme-external-events-in-the-design-and-assessment-of-nuclear-power-plants IAEA‑TECDOC‑1341] || ''Extreme External Events in the Design and Assessment of Nuclear Power Plants'' || IAEA synthesis of Member States' experience with extreme external event assessment; proposed a three‑class External Event Classification system and surveyed deterministic vs. probabilistic approaches.
|-
| 1993 || [https://www.nrc.gov/docs/ML2004/ML20045E785.pdf NUREG-1474] || ''Effect of Hurricane Andrew on the Turkey Point Nuclear Generating Station'' || Documented impacts of Hurricane Andrew (1992) including loss of offsite power and damage to ancillary structures.
|-
| 1991 || [https://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1407/index NUREG-1407] || ''Procedural and Submittal Guidance for the IPEEE'' || Codified the IPEEE for severe-accident vulnerabilities.
|-
| 1987 || [https://www.nrc.gov/docs/ML0231/ML023180425.pdf NUREG/CR-5042] || ''Evaluation of External Hazards to Nuclear Power Plants in the United States'' || First comprehensive NRC survey of external hazards (seismic, flood, wind, fire, transportation); established external-hazard screening framework.
|-
| 1983 || [https://www.nrc.gov/reading-rm/doc-collections/nuregs/contract/cr2300/index NUREG/CR-2300] || ''PRA Procedures Guide'' || Integrated external-hazard treatment into Level-1 PSA methodology; introduced event-tree and fault-tree conventions for external initiators.
|}

----
''EPRI technical point of contact: Chris Rochon ([mailto:CRochon@epri.com CRochon@epri.com])''

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

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