Methods/Screening and Prioritization

METHODOLOGY

Screening and Prioritization of External Hazards

Quick Reference
Applicable hazards	All external hazards (excl. seismic, internal fire)
Team	PRA analyst, meteorologist/hydrologist, external hazard specialist
Primary input	Comprehensive hazard catalog + site characterization data
Primary output	Retained hazard list + screening documentation
Key guidance	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 [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, 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.

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 [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

Category	Description
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.
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:

Criterion	Threshold	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 [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 Compound and Cascading Hazards.

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 [3002023814].

Strengths and Limitations

Strengths

Limitations

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 Compound and Cascading Hazards). Transparent documentation — Structured framework supports PRA peer review and regulatory audit.

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

Method / Process	Relationship to Screening
External Events PRA	Detailed quantitative risk assessment for hazards that cannot be screened out. Screening defines the scope of the PRA.
Walkdowns & Field Verification	Physical plant inspections that follow screening to confirm exposure and collect input data for detailed analysis.
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.
Combined‑Hazard Assessment	Multi‑hazard evaluation that directly builds on the combined‑hazard screening step described above.

Key References

Key References — use [▶ Show references] to expand
Year	Report Number	Title	Summary
2021	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	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	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	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.

---

Return to Methods and Tools.

EPRI technical point of contact: Chris Rochon (CRochon@epri.com)

Date last reviewed: 2026-05-21