US Nuclear Regulatory Commission (USNRC) 

Staff Needs In Probabilistic Flood Hazard 

Assessment (PFHA)

Dr. Fernando Ferrante, USNRC

Office of Nuclear Reactor Regulation

Division of Risk Assessment

Workshop on Probabilistic Flood Hazard Assessment (PFHA)

January 29, 2013
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Presentation Outline

• Provide a brief overview of NRC’s mission, 

flooding-related guidance

• Discuss risk assessment concepts commonly used 

at the NRC

• Present examples of risk criteria used in NRC 

licensing and oversight activities

• Present issues of interest in the area of flooding 

risk to NRC licensed facilities
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NRC’s Mission and Functions

License and regulate the Nation’s civilian use of byproduct, 

source, and special nuclear materials to ensure the adequate 

protection of public health and safety, promote the common 

defense and security, and protect the environment. 

REGULATIONS

LICENSING

ACTIONS

OVERSIGHT

OPERATIONAL 

EXPERIENCE

ANALYSIS

RESEARCH

HEARINGS
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NRC Regulations Overview

• Natural phenomena must be identified and 

assessed for potential effects on safe 

operation

• Designed to withstand effects of natural 

phenomena, such as floods

• Appropriate consideration of most severe of 

the natural phenomena that have been 

historically reported

• With sufficient margin for limited accuracy, 

quantity, and period of time for accumulated 

historical data

• Proximity of man-related hazards (e.g., dams) 

for potential hazards

STREAM FLOODING

LOCAL FLOODING

DAM FAILURES

TSUNAMI

ICE FLOODING

SURGES

SEICHES

COMBINED EVENTS

10 CFR 72.92

10 CFR 100.20

10 CFR 50 App. A

10 CFR 50 App. A

10 CFR 50 App. A
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• NRC has a policy to increase the use of risk analysis in 

regulatory matters - To extent supported by the state-of-art

 - Complementary to deterministic approaches

 - Supports defense-in-depth philosophy

• Various methodologies, tools, approaches

– Varying levels of detail, qualitative/quantitative

• Large resources/effort applied by NRC/industry on systematic 

risk assessments for nuclear reactors, i.e., Reactor 

Probabilistic Risk Assessments (PRAs)

• Risk triplet: What can go wrong (s)? How likely is it (p)? What 

are the consequences (c)?

Risk Analysis at the NRC

R = {(si, pi, ci)}, i = 1, 2,..., N
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Great Tōhoku Earthquake

Source: Tokyo Electric Power Company
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EVENT FREQUENCY = 

 ∑ [(HAZARD FREQUENCY) x (CONDITIONAL PROBABILITY

 OF FAILURE)]

NRC PRA Framework

• Measure of mitigation capability

• System analysis

• Human reliability analysis

• Consideration of dependencies 

• Probability of recovery

• Uncertainty/sensitivity analysis

• Risk ranking of contributors

• Events that challenge plant 

safety, e.g., loss of cooling, 

loss of power

• Multiple potential “initiators”

− System/operator failures

− Internal Fire

− Natural hazards

o Earthquakes

o High Winds

o Floods...
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CORE DAMAGE FREQUENCY (CDF) = 

∑ [(HAZARD FREQUENCY) x (CONDITIONAL CORE DAMAGE PROBABILITY)]

NRC PRA Reactor Framework

For operating reactors: 

CDF ≈ 10-4 to 10-5/year 

with contributors in 

the range of 10-5/year 

to 10-9/year 1982 PRA RESULTS
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10-5

10-6

10-5 10-4 CDF

ΔCDF

NRC Risk Criteria for Reactors

RED

YELLOW

WHITE

GREEN

>10-4

10-5

<10-6

ΔCDF

ACCEPTANCE GUIDELINES FOR 

RISK-INFORMED LICENSING CHANGES

SIGNIFICANCE 

DETERMINATION PROCESS

• NRC has adopted probabilistic safety goals for reactors

• Risk criteria defined for multiple regulatory activities, e.g.,

• Risk-Informed licensing decisions

• Risk significance of findings and events
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NRC Staff Needs –

Severe Flood Frequencies

• Flood intensity, duration, AND frequency 

in the NRC ranges of interest

− Defensible extrapolation methods even with 

wide uncertainties

− Need to capture wider risk spectrum, 

including more frequent/less severe floods

− Obtain risk as a function of flood elevation, 

graded impact to plant activities

− Applicability of advanced methods

EVENT FREQUENCY = 

 ∑ [(HAZARD FREQUENCY) x (CONDITIONAL PROBABILITY)]

>1000-year 

return period?

• Potential screening methods

− Screening out mechanisms or thresholds for flood contributors

− Applicable to multiple sites across US
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EVENT FREQUENCY = 

 ∑ [(HAZARD FREQUENCY) x (CONDITIONAL PROBABILITY)]

NRC Staff Needs –

Dams/Flooding Protection

• Impact due to upstream, 

downstream dams

− Dam failure rates (with uncertainty)

− Combined events (e.g., seismic)

− Uncertainty in flood timing 

− Failure of appurtenant structures 

(i.e., spillways and hydro-turbines)

− Internal piping failures

− Probabilistic treatment of long 

embankments, levees

• Probability of failure of flood 

protection structures and 

barriers (debris impact, 

degradation, erosion)

• Fragility curves for 

mechanical/electrical 

systems, components

• Feasibility of manual 

actions during extreme 

events
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Ft. Calhoun 2010 Finding

Source: OPPD Public Presentation (April 4, 2012)
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Missouri Flooding 2011

Source: OPPD Public Presentation (April 4, 2012)
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Conclusions

• Recent events indicate the importance of characterizing 

low probability/high consequence events with respect to 

USNRC activities

• Current complementary use of deterministic and 

probabilistic methods at the USNRC can be significantly 

expanded by state-of-art flood hazard analysis

• Immediate needs exist in the area of licensing and 

oversight of commercial nuclear reactors

• Recognized expertise of multiple federal agencies is 

needed to bridge gaps in improving treatment of flooding 

hazard within USNRC applications