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ANS 2.8

Probabilistic Evaluation of External Flood Hazards for Nuclear Facilities

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Organization: ANS
Publication Date: 1 January 2019
Status: active
Page Count: 81
scope:

Purpose

The purpose of this standard is to present requirements for performing a probabilistic flood hazard assessment (PFHA) for a nuclear facility.3) This standard also can be used to update an existing PFHA. This standard also provides guidance for conducting efficient, site-specific screening analyses to screen out noncredible external flood hazard sources and mechanisms.4) A PFHA considers sources of flooding and flooding mechanisms that pose a credible hazard to a nuclear facility.

Results from the PFHA can be used in conjunction with applicable authority or regulatory body requirements to evaluate the design-basis flood (DBF) for a nuclear facility or for individual structures, systems, and components (SSCs) important to safety. PFHA results also may be used as input to external flood probabilistic risk assessments (PRAs), for development of emergency response plans, for assessment of safety-related SSCs and site vulnerability, and to design flood protection or mitigation systems.5)

The DBF may be characterized by one or more hazards or parameters, depending on the flood events that are being modeled and site-specific circumstances. The characterization of the DBF may include, but is not limited to,

• inundation or water surface elevation (WSEL);

• wind-wave effects (wave setup, wave run-up, wave splash);

• flood flow rate (discharge) and velocity;

• tsunami wave speed and wave height;

• hurricane speed, direction, and strength;

• ice effects;

• erosion and/or sedimentation;

• floating objects or debris.

In this standard, reference will be made to the DBF without referring specifically to the fact there may be multiple coincident hazards that are a part of it. When the DBF is referred to, it is understood that all hazards (WSEL, flow velocity, debris, etc.) are included, depending on the flood and site-specific circumstances.

The result of a PFHA will be a set of flood hazard curves that quantify the frequency of exceedance of a measure of the flood hazard (e.g., WSEL) and the uncertainty in the frequency of exceedance. In this context, this standard will refer to the flood hazard curve for a SSC, facility, or site. When reference is made to a flood hazard curve(s), it is used in the context that the flood hazard and its probabilistic representation may involve the quantification for multiple measures of the hazards as noted above.

Hydrologic and hydraulic information from the DBF can be used to design new nuclear facilities or to assess the vulnerability of existing nuclear facilities. Since the design, elevation, regulatory requirements, and other details will vary given the type of facility and/or SSC, the relevant DBF event(s) will most likely be specific to the site, facility, and SSC being addressed.

3) A PFHA may be performed for individual SSCs, for a single facility, for multiple facilities at a single site, or for a site depending on the circumstances. In this standard, reference is made to a nuclear facility, where it is understood that one or another set of circumstances may be considered.

4) This standard distinguishes between flood sources and mechanisms. The flood source is viewed as the root cause of the flooding event, while the flooding mechanism refers to the collection of processes involved in propagating flooding to the site. For example, in many cases, a rainfall event is the flood source, while flooding mechanisms include all of the runoff and flood wave propagation processes that influence the characteristics and severity of flooding at the site.

5) This standard uses the term "important to safety" to encompass those structures identified by the utility or the regulator the failure of which during a flood could create a potentially risk-significant outcome. The term "safety-related" is restricted to those SSCs in a nuclear power plant that meet the U.S. Nuclear Regulatory Commission requirements for the SSC to be safety-related.

Document History

ANS 2.8
January 1, 2019
Probabilistic Evaluation of External Flood Hazards for Nuclear Facilities
Purpose The purpose of this standard is to present requirements for performing a probabilistic flood hazard assessment (PFHA) for a nuclear facility.3) This standard also can be used to update an...
2.8
January 1, 1992
Determining Design Basis Flooding at Power Reactor Sites
This document presents criteria to establish design basis flooding for nuclear safety-related features at power reactor sites. Methodology is described to evaluate the flood having virtually no risk...
2.8
January 1, 1981
DETERMINING DESIGN BASIS FLOODING AT POWER REACTOR SITES
This document presents a standard to establish design basis flooding for nuclear safety-related features at power reactor sites. Methodology is described to evaluate the flood having virtually no...

References

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