Importance of Risk-Informed, 

Performance Based 

Regulations

Commissioner George Apostolakis

U.S. Nuclear Regulatory Commission

CmrApostolakis@nrc.gov

Workshop on Probabilistic Flood Hazard Assessment

January 29, 2013
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The Pre-PRA Era

• Management of uncertainty (unquantified at the 

time) was always a concern.

• Defense in depth became embedded in the 

regulations.

• Design Basis Accidents (DBAs) are postulated 

accidents that a nuclear facility must be designed 

and built to withstand without loss to the systems, 

structures, and components necessary to assure 

public health and safety. 
Some Problems with

Design Basis Accidents

• DBAs use qualitative approaches for ensuring 

system reliability (the single-failure criterion) when 

more modern quantitative approaches exist

• DBAs use stylized considerations of human 

performance (e.g., operators are assumed to take 

no action within, for example, 30 minutes of an 

accident’s initiation)

• DBAs do not reflect operating experience and 

modern understanding

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Technological Risk 

Assessment (Reactors)

• Study the system as an integrated sociotechnical system

• Probabilistic Risk Assessment (PRA) answers the 

questions:

 What can go wrong? (thousands of accident sequences 

are studied as opposed to the limited number of design 

basis accidents 

 How likely are these scenarios?

 What are their consequences?

• The significance of external events was first 

identified by PRAs 4
Probabilistic Risk 

Assessment for Reactors

• What is the core damage frequency (CDF)?

 CDF = f (IEi) x P(CD/IEi), where

 f(IEi): frequency of initiating event i

 P(CD/IEi): probability of core damage given initiating event i

• We need to consider the full range of potentially 

significant initiating events

 This range includes very unlikely (and perhaps not-yetobserved) events

 The uncertainties may be significant

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“It Cannot Be Done”

• In the early days of PRA, many engineers said that 

estimating core damage frequency for reactors 

“cannot be done”

 Yet, it was done

• When practitioners were first trying to analyze 

human errors of commission, many said it 

“cannot be done”

 Yet, it was done

• For certain external events, some experts say that 

estimating the frequencies of rare events “cannot 

be done”

 ?

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Initiating Event Classification

PLANT CONDITION IE FREQUENCY, R-y-1

1 Normal Operation

2 f > 10-1

3 10-1 > f > 10-2

4 10-2 > f > 10-4

5 10-4 > f >10-6

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NRC Reviews of Initiating 

Event Frequencies

• Risk-informing the large loss-of-coolant-accident (LLOCA) 

rule

 The Commission stated: “a frequency of 1 occurrence in 

100,000 reactor years is an appropriate mean value for the 

LOCA frequency guideline for selecting the maximum designbasis LOCA since it is complemented by the requirement that 

appropriate mitigation capabilities, including effective severe 

accident mitigation strategies, must be retained for the beyond 

design-basis LOCA category.” (US Nuclear Regulatory Commission, US 

Code of Federal Regulations, 2004)

 This statement indicates that the Commission considers the 

frequency of 10−5 per reactor year as an appropriate lower 

bound for the initiating events that should be included in the 

design basis

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Risk-informed Regulation

 “A risk-informed approach to regulatory decisionmaking represents a philosophy whereby risk 

insights are considered together with other factors 

to establish requirements that better focus licensee 

and regulatory attention on design and operational 

issues commensurate with their importance to 

public health and safety.”

[Commission’s White Paper, USNRC, 1999]
Risk-Informed Framework

 

 

Traditional 

“Deterministic” 

Approach 

• Unquantified 

probabilities 

•Design-basis accidents 

•Defense in depth and 

safety margins 

•Can impose 

unnecessary 

regulatory burden 

•Incomplete 

 

 

Risk-Based 

Approach 

 

• Quantified 

probabilities 

•Thousands of 

accident 

sequences 

•Realistic 

•Incomplete 

RiskInformed 

Approach 

•Combination 

of traditional 

and riskbased 

approaches 

through a 

deliberative 

process 

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Risk-Informed

Decision Making in 

Regulation

• Improves Safety

 New requirements (SBO, ATWS)

 Design of new reactors

 Focus on important systems and locations

• Makes regulatory system more rational

 Reduction of unnecessary burden

 Operating experience accounted for in regulations

 Consistency in regulations

• Encourages performance-based regulation

 Maintenance rule

 Fire protection

 Determination of seismic design basis motion

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Challenges

• Overcoming the deterministic mindset

• Developing new decision guidelines

• Developing new or revised codes and 

standards

• Shortage of experts

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Risk Management Task Force

(RMTF)

“To develop a strategic vision and options for 

adopting a more comprehensive and holistic 

risk-informed, performance-based regulatory 

approach for reactors, materials, waste, fuel 

cycle, and transportation that would continue to 

ensure the safe and secure use of nuclear 

material.”

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NUREG-2150, April 2012
RMTF Findings:

External Events

• The processes for establishing the external hazard 

design bases do not use consistent event frequency 

and magnitude methods

• New information that would provide the basis for 

external hazard frequency updates is not 

systematically collected, evaluated, and 

communicated

• PRA methods for assessing external hazard risks 

are available, but expertise in performing such 

studies is very limited

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Conclusions

• What is “worst case” is a matter of perception and 

is, therefore, notoriously subjective

• A risk-informed approach has many advantages 

• A process is needed to systematically address new 

data and insights from experience and analyses, 

including:

 Lessons learned from past events

 Lessons learned from other agencies and the broader 

technical community

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