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[[Methods_and_Tools|METHODOLOGY]]

'''Screening and Prioritization of External Hazards'''

{| style="float:right; margin-left:16px; margin-bottom:12px; width:320px; border:2px solid #5A9E6F; border-radius:6px; background:#F5FAF7;"
|-
| colspan="2" style="background:#5A9E6F; color:#FFF; font-weight:bold; padding:6px 10px; border-radius:4px 4px 0 0;" | Quick Reference
|-
| style="padding:4px 8px; width:40%;" | '''Applicable hazards''' || style="padding:4px 8px;" | All external hazards (excl. seismic, internal fire)
|-
| style="padding:4px 8px;" | '''Team''' || style="padding:4px 8px;" | PRA analyst, meteorologist/hydrologist, external hazard specialist
|-
| style="padding:4px 8px;" | '''Primary input''' || style="padding:4px 8px;" | Comprehensive hazard catalog + site characterization data
|-
| style="padding:4px 8px;" | '''Primary output''' || style="padding:4px 8px;" | Retained hazard list + screening documentation
|-
| style="padding:4px 8px;" | '''Key guidance''' || style="padding:4px 8px;" | [https://www.epri.com/research/products/000000003002005287 EPRI 3002005287]
|}

== Overview ==
External hazard screening is the systematic process of identifying which of approximately 68 potential external hazards are credible threats to a nuclear power plant site and which require further analysis. It is not an analytical method in itself but rather a structured decision process that precedes and scopes the methods described elsewhere in this section. The EPRI screening framework [[https://www.epri.com/research/products/000000003002005287 3002005287]] starts with a broad catalog of hazards. Sites should verify that this catalog adequately reflects their specific geographic, meteorological, and industrial setting, adding any site-specific hazard phenomena not captured by the generic categories. The framework then applies qualitative and quantitative criteria and explicitly addresses combined (multi‑hazard) events.

Performing detailed PRA for every conceivable hazard would be impractical — the catalog includes on the order of 50 natural, 12 man-made, and 6 other categories. Screening provides a systematic way to focus analytical resources on the hazards that matter most. The need for robust screening was reinforced by lessons from the 2011 Fukushima Daiichi accident and by the IPEEE Program (Generic Letter 88‑20, [https://www.nrc.gov/docs/ML0722/ML072260181.pdf NUREG‑1407]).

This sets the stage for a graded approach to hazard assessent, whereby hazards of increasing severity and potential plant impact are assessed in an increasingly more detail manner.

== High‑Level Workflow ==
The screening process proceeds through progressive filtering, beginning with a comprehensive hazard list and ending with a short list of hazards requiring detailed analysis.

[[File:screening_assessment.png|center|600px|link=|External hazard screening and prioritization workflow]]

''Color key: blue = characterization, green = analysis, dark filled = endpoint, diamond = decision point, gray dashed = supplemental, light dashed = off‑ramp.''



''Note: The "CDF < 10−6/yr" quantitative screening threshold shown in the flowchart is defined in [[https://www.epri.com/research/products/000000003002005287 EPRI 3002005287]]. An alternative formulation is also acceptable: hazard frequency < 10−5/yr '''and''' conditional core damage probability (CCDP) < 0.1, which conservatively bounds the same 10−6/yr CDF contribution.''

== Key Inputs and Outputs ==

{| class="wikitable"
! Category !! Description
|-
| colspan="2" | '''Inputs'''
|-
| '''Comprehensive hazard catalog''' || Starting list of 68+ potential external hazards from EPRI 3002005287, supplemented by site‑specific geographic, meteorological, and industrial data.
|-
| '''Site characterization data''' || Geographic setting, proximity to rivers/coasts/dams/industrial facilities, historical event records, climatological data.
|-
| '''Plant design basis documents''' || Final Safety Analysis Report (FSAR), design‑basis event parameters, Standard Review Plan commitments.
|-
| '''Existing PRA model''' || Baseline internal‑events PRA for quantitative screening, including dominant accident sequences and success criteria.
|-
| colspan="2" | '''Outputs'''
|-
| '''Screened‑out hazard list''' || Hazards eliminated with documented basis (qualitative or quantitative criterion met). ''The documentation of screening justifications is a key and sometimes overlooked output.''
|-
| '''Retained hazard list''' || Hazards requiring detailed deterministic or probabilistic analysis.
|-
| '''Screening documentation''' || Complete record of the team, data sources, criteria applied, and decisions made — essential for PRA peer review and regulatory audit.
|}

== Screening Criteria ==

=== Qualitative Criteria ===
A hazard may be screened out qualitatively if any of the following conditions is met:

* The hazard cannot occur at the site (geographic impossibility — for example, tsunami at a continental interior site).
* The hazard develops sufficiently slowly that protective actions can be completed with high confidence before plant effects occur.
* The hazard's maximum credible intensity is bounded by another hazard for which the plant is already designed (for example, tornado winds bounded by the design‑basis tornado).
* The hazard cannot produce damage to any structures, systems, and components (SSC) relied upon for safe shutdown, even at maximum credible intensity.

=== Quantitative Criteria ===
If qualitative screening cannot eliminate a hazard, a bounding or conservative quantitative analysis may be performed:

{| class="wikitable"
! style="background:#D1ECF1; color:#0C5460;" | Criterion
! style="background:#D1ECF1; color:#0C5460;" | Threshold
! style="background:#D1ECF1; color:#0C5460;" | Basis
|-
| Core damage frequency (CDF) contribution || < 10−6 / year || Hazard‑specific CDF is negligible relative to the total plant CDF
|-
| Hazard frequency × CCDP || Frequency < 10−5/yr ''and'' CCDP < 0.1 || Conservative proxy: even assuming 10% conditional core damage probability, the CDF contribution is below 10−6/yr
|-
| Cliff‑edge adjustment || Lower threshold if hazard involves potential for sudden, large consequence increases near a physical or design margin || Prevents premature screening of hazards with threshold‑type failure modes
|}

These quantitative screening values were benchmarked against the risk profiles of the current LWR fleet (circa 2015) and may require adjustment for Gen III+, SMR, or advanced reactor designs [[https://www.epri.com/research/products/000000003002005287 3002005287]].

Hazards that survive individual screening are also evaluated for combined‑hazard interactions. For details on how correlated, consequential, and coincidental multi‑hazard scenarios are assessed, see [[Hazards/Compound and Cascading Hazards|Compound and Cascading Hazards]].

{| class="wikitable" style="width:100%;"
! style="background:#D1ECF1; color:#0C5460;" | Re‑screening Triggers
|-
|
* '''ASME/ANS PRA Standard''' — RA‑S Section 6 requires documented screening; hazard‑specific Parts (such as Part 7 for high winds) contain additional screening requirements tailored to individual hazards and may alter screening thresholds as they are updated.
* '''Nuclear Regulatory Commission (NRC) 50.54(f) / NTTF Recommendations''' — Post‑Fukushima re‑evaluations drove updated hazard analyses at many sites.
* '''EPRI New Information Monitoring''' — Annual surveillance may trigger re‑screening if new data changes a hazard's frequency or intensity profile.
* '''Climate Vulnerability Assessment (CVA)''' — Screening should be revisited when climate projections alter hazard frequencies or intensities [[https://www.epri.com/research/products/000000003002023814 3002023814]].
|}

== Strengths and Limitations ==

{| class="wikitable" style="width:100%;"
! style="background:#D5F5E3; color:#1E8449; width:50%;" | Strengths
! style="background:#FADBD8; color:#922B21; width:50%;" | Limitations
|-
| valign="top" |
* '''Widely adopted''' — Used across the U.S. fleet and referenced by the NRC; pilot applications at five plants (including international sites) informed the guidance.
* '''Graded approach''' — Hazards can be eliminated at the lowest effort level justified by the evidence.
* '''Addresses combined hazards''' — Framework includes ''basic'' provisions for multi‑hazard screening (see [[Hazards/Compound and Cascading Hazards|Compound and Cascading Hazards]]).
* '''Transparent documentation''' — Structured framework supports PRA peer review and regulatory audit.
| valign="top" |
Existing widely-used approaches such as 3002005287 focus on:
* '''CDF‑centric thresholds''' — May under‑represent hazards with non-CDF/LERF consequences (for example spent fuel, operational margin).
* '''Data dependence''' — Quantitative screening needs at least bounding hazard frequency and CCDP estimates, which may not be readily available.
* '''Snapshot in time''' — Outcomes reflect current plant design and hazard data; changes in climate, land use, or nearby industry may warrant re-evaluation.
Forward-looking climate vulnerability approaches work toward reducing these limitations.
|}

== Related Methods and Processes ==

{| class="wikitable"
! Method / Process !! Relationship to Screening
|-
| '''[[Methods/Probabilistic Risk Assessment|External Events PRA]]''' || Detailed quantitative risk assessment for hazards that cannot be screened out. Screening defines the scope of the PRA.
|-
| '''[[Methods/Walkdowns and Field Verification|Walkdowns & Field Verification]]''' || Physical plant inspections that follow screening to confirm exposure and collect input data for detailed analysis.
|-
| '''[[Methods/Deterministic Approaches|Deterministic Approaches]]''' || For some hazards that screen in, a deterministic resolution path (such as design‑basis evaluation, TMRE) may suffice without proceeding to full PRA.
|-
| '''New Information Monitoring''' || Annual surveillance of hazard data that may trigger re‑screening if new information changes a hazard's frequency or intensity profile.
|-
| '''[[Hazards/Compound and Cascading Hazards|Combined‑Hazard Assessment]]''' || Multi‑hazard evaluation that directly builds on the combined‑hazard screening step described above.
|}

== Key References ==
{| class="wikitable mw-collapsible mw-collapsed" style="width:100%;" data-expandtext="▶ Show references" data-collapsetext="▼ Hide references"
|-
! colspan="4" style="text-align:left; background:#eaf3fb; padding:8px 12px; font-size:100%;" | '''Key References''' — use [▶ Show references] to expand
|-
! style="text-align:left;width:80px;" | Year
! style="width:120px;" | Report Number
! style="width:320px;" | Title
! Summary
|-
| 2021 || [https://www.iaea.org/resources/safety-standards SSG‑68] || ''Design of Nuclear Installations Against External Events Excluding Earthquakes'' || International Atomic Energy Agency (IAEA) Safety Guide establishing requirements for design against external events; includes hazard identification and screening as prerequisite steps.
|-
| 2015 || [https://www.epri.com/research/products/000000003002005287 3002005287] || ''Identification of External Hazards for Analysis in Probabilistic Risk Assessment'' || Primary screening guidance. Catalogs 68 hazards, defines qualitative and quantitative screening criteria, and addresses combined‑hazard screening. Incorporates pilot feedback from five nuclear plant 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'' || International perspectives on external hazard screening and design, including three‑class External Event Classification system.
|-
| 1991 || [https://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1407/index NUREG‑1407] || ''Procedural and Submittal Guidance for the Individual Plant Examination of External Events (IPEEE) for Severe Accident Vulnerabilities'' || NRC guidance that established the original U.S. framework for systematic external‑events screening at operating plants.
|}

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----
''Return to '''[[Methods and Tools]]'''.''

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

''Date last reviewed: 2026-05-21''

Methods/Screening and Prioritization - Revision history

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