[Federal Register Volume 59, Number 235 (Thursday, December 8, 1994)]
[Unknown Section]
[Page 0]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 94-30147]


[[Page Unknown]]

[Federal Register: December 8, 1994]


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NUCLEAR REGULATORY COMMISSION
 

Use of Probabilistic Risk Assessment Methods in Nuclear 
Regulatory Activities; Proposed Policy Statement

AGENCY: Nuclear Regulatory Commission.

ACTION: Proposed policy statement.

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SUMMARY: The Nuclear Regulatory Commission (NRC) is proposing a policy 
statement regarding the use of probabilistic risk assessment (PRA) in 
nuclear regulatory matters. The Commission believes that an overall 
policy on the use of PRA in nuclear regulatory activities should be 
established so that the many potential applications of PRA technology 
can be implemented in a consistent and predictable manner that promotes 
regulatory stability and efficiency and enhances safety. The proposed 
policy statement would improve the regulatory process through improved 
risk-effective safety decision-making, through more efficient use of 
agency resources, and through a reduction in unnecessary burdens on 
licensees. The use of PRA technology would be increased in all 
regulatory matters to the extent supported by the state-of-the-art in 
PRA methods and data and in a manner that complements the NRC's 
deterministic approach and supports the NRC's traditional defense-in-
depth philosophy.

DATES: Submit comments by February 7, 1995. Comments received after 
this date will be considered if it is practical to do so, but the 
Commission is able only to ensure consideration for comments received 
on or before this date.

ADDRESSES: Send comments to: Secretary, U.S. Nuclear Regulatory 
Commission, Washington, DC 20555, Attention: Docketing and Service 
Branch.
    Deliver comments to: One White Flint North, 11555 Rockville Pike, 
Rockville, Maryland 20852, between 7:45 am and 4:15 pm Federal 
workdays.
    Copies of comments received may be examined at: NRC Public Document 
Room, 2120 L Street NW. (Lower Level), Washington, DC.

FOR FURTHER INFORMATION CONTACT: Thomas G. Hiltz, Office of Nuclear 
Reactor Regulation, U.S. Nuclear Regulatory Commission, Washington, DC 
20555. Telephone (301) 504-1105.

SUPPLEMENTARY INFORMATION:

I. Background
II. Deterministic and Probabilistic Approaches to Regulation
III. The Commission Policy
IV. Availability of Documents

I. Background

    The NRC has generally regulated the use of nuclear material based 
on deterministic approaches. Deterministic approaches to regulation 
consider a set of challenges to safety and determine how those 
challenges should be defended. A probabilistic approach to regulation 
enhances and extends this traditional, deterministic approach, by 1) 
allowing consideration of a broader set of potential challenges to 
safety, 2) providing a logical means for prioritizing these challenges 
based on risk significance, and 3) allowing consideration of a broader 
set of resources to defend against these challenges.
    Until the accident at Three Mile Island (TMI) in 1979, the Atomic 
Energy Commission (now the NRC), only used probabilistic criteria in 
certain specialized areas of licensing reviews. For example, human-made 
hazards(e.g., nearby hazardous materials and aircraft) and natural 
hazards (e.g., tornadoes, floods, and earthquakes) were typically 
addressed in terms of probabilistic arguments and initiating 
frequencies to assess site suitability. The Standard Review Plan 
(NUREG-0800) for licensing reactors and some of the Regulatory Guides 
supporting NUREG-0800 provided review and evaluation guidance with 
respect to these probabilistic considerations.
    The TMI accident substantially changed the character of the 
analysis of severe accidents worldwide. It led to a substantial 
research program on severe accident phenomenology. In addition, both 
major investigations of the accident (the Kemeny and Rogovin studies) 
recommended that PRA techniques be used more widely to augment the 
traditional nonprobabilistic methods of analyzing nuclear plant safety. 
In 1984, the NRC completed a study (NUREG- 1050) that addressed the 
state-of-the-art in risk analysis techniques.
    In early 1991, the NRC published NUREG-1150, ``Severe Accident 
Risks: An Assessment for Five U.S. Nuclear Power Plants.'' In NUREG-
1150, the NRC used improved PRA techniques to assess the risk 
associated with five nuclear power plants. This study was a significant 
turning point in the use of risk-based concepts in the regulatory 
process and enabled the Commission to greatly improve its methods for 
assessing containment performance after core damage and accident 
progression. The methods developed for and results from these studies 
provided a valuable foundation in quantitative risk techniques.
    PRA methods have been applied successfully in several regulatory 
activities and have proved to be a valuable complement to deterministic 
engineering approaches. This application of PRA represents an extension 
and enhancement of traditional regulation rather than a separate and 
different technology. Several recent Commission policies or regulations 
have been based, in part, on PRA methods and insights. These include 
the Backfit Rule (Sec. 50.109, ``Backfitting''), the Policy Statement 
on ``Safety Goals for the Operation of Nuclear Power Plants,'' (51 FR 
30028), the Commission's ``Policy Statement on Severe Reactor Accidents 
Regarding Future Designs and Existing Plants'' (50 FR 32138), and the 
Commission's ``Final Policy Statement on Technical Specifications 
Improvement for Nuclear Power Reactors'' (58 FR 39132). PRA methods 
also were used effectively during the anticipated transient without 
scram (ATWS) and station blackout (SBO) rulemaking, and supported the 
generic issue prioritization and resolution process. Additional 
benefits have been found in the use of risk-based inspection guides to 
focus NRC inspector efforts and make more efficient use of NRC 
inspection resources.
    Currently, the NRC is using PRA techniques to assess the safety 
importance of operating reactor events and is using these techniques as 
an integral part of the design certification review process for 
advanced reactor designs. In addition, the Individual Plant Examination 
(IPE) program and the Individual Plant Examination - External Events 
(IPEEE) program (an effort resulting from the implementation of the 
Commission's ``Policy Statement on Severe Reactor Accidents Regarding 
Future Designs and Existing Plants'') have resulted in commercial 
reactor licensees using risk-assessment methods to identify any 
vulnerabilities needing attention.
    The Commission has been developing performance assessment methods 
for low-level and high-level waste since the mid-1970s and these 
activities intensified using performance assessments techniques in the 
late 1980s and early 1990s. This has involved the development of 
conceptual models and computer codes to model the disposal of waste. 
Because waste-disposal systems are passive, certain analysis methods 
used for active systems in PRA studies for power reactors had to be 
adapted to provide scenario analysis for the performance assessment of 
the geologic repository at Yucca Mountain, Nevada. In regard to high-
level waste, the NRC staff participates in a variety of international 
activities (e.g., the Performance Assessment Advisory Group of the 
Organization for Economic Cooperation and Development, Nuclear Energy 
Agency) to ensure that consistent performance assessment methods are 
used to the degree appropriate.
    The Commission believes that an overall policy on the use of PRA in 
nuclear regulatory activities should be established so that the many 
potential applications of PRA methodology can be implemented in a 
consistent and predictable manner that promotes regulatory stability 
and efficiency and enhances safety. On August 18, 1994, the NRC staff 
proposed a PRA policy statement to the Commission in SECY-94-218, 
``Proposed Policy Statement on the Use of Probabilistic Risk Assessment 
Methods in Nuclear Regulatory Activities.'' In its Staff Requirements 
Memorandum of October 4, 1994, the Commission directed the staff to 
revise the proposed PRA policy statement and publish the proposed PRA 
policy statement for public comment in the Federal Register.

II. Deterministic and Probabilistic Approaches to Regulation

(A) Extension and Enhancement of Traditional Regulation

    The NRC established its regulatory requirements to ensure that a 
facility is designed, constructed, and licensed to operate without 
undue risk to the health and safety of the public. These requirements 
are largely based on deterministic engineering criteria. Simply stated, 
this deterministic approach establishes requirements for engineering 
margin and for quality assurance in design, manufacture and 
construction. In addition, it assumes that adverse conditions can exist 
(e.g., equipment failures and human errors) and establishes a specific 
set of design basis events. It then requires that the licensed facility 
design include safety systems capable of preventing and/or mitigating 
the consequences of those design basis events to protect the public 
health and safety.
    The deterministic approach contains implied elements of probability 
(qualitative risk considerations), from the selection of accidents to 
be analyzed (e.g., reactor vessel rupture is considered too improbable 
to be included) to the system level requirements for emergency core 
cooling (e.g., safety train redundancy and protection against single 
failure).
    In contrast to the deterministic approach, PRA addresses all 
credible initiating events by assessing the event frequency. Mitigating 
system reliability is then assessed, including the potential for common 
cause failures. The probabilistic treatment therefore goes beyond the 
single failure requirements used in the deterministic approach. The 
probabilistic approach to regulation is, therefore, considered an 
extension and enhancement of traditional regulation by considering risk 
in a more coherent and complete manner. A natural result of the 
increased use of PRA methods and techniques would be the focusing of 
regulations on those items most important to safety by eliminating 
unnecessary conservatism. Where appropriate, PRA can also be used to 
support additional regulatory requirements. Deterministic-based 
regulations have been successful in protecting the public health and 
safety and PRA techniques are most valuable when they serve to focus 
the traditional, deterministic-based, regulations and support the 
defense-in-depth philosophy.
    Beyond its deterministic criteria, the NRC has formulated guidance, 
as in the safety goal policy statement, that utilizes quantitative, 
probabilistic risk objectives. The safety goal policy statement 
establishes these top-level objectives to help assure safe operation of 
nuclear power plants. The safety goals are intended to be generically 
applied by the NRC as opposed to plant- specific applications. For the 
purpose of implementation of the safety goals, subsidiary numerical 
objectives on core damage frequency and containment performance have 
been established. The safety goals provide guidance on where plant risk 
is considered to be sufficiently low such that further regulatory 
action is not necessary. Also, as noted above, the Commission has been 
using PRA in performing regulatory analysis for backfit of cost-
beneficial safety improvements at operating reactors (as required by 10 
CFR 50.109) for a number of years.

(B) Uncertainties and Limitations of Deterministic and Probabilistic 
Approaches

    The treatment of uncertainties is an important issue for regulatory 
decisions. Uncertainties exist in any regulatory approach and these 
uncertainties are derived from knowledge limitations. These 
uncertainties and limitations existed during the development of 
deterministic regulations and attempts were made to accommodate these 
limitations by imposing prescriptive, and what was hoped to be, 
conservative regulatory requirements. A probabilistic approach has 
exposed some of these limitations and provided an improved framework to 
better focus and assess their significance and assist in developing a 
strategy to accommodate them in the regulatory process.
    Human performance is an important consideration in both 
deterministic and probabilistic approaches. Assessing the influence of 
errors of commission and organizational and management issues on human 
reliability is an example that illustrates where current PRA methods 
are not fully developed. While this lack of knowledge contributes to 
the uncertainty in estimated risks, the PRA framework offers a powerful 
tool for logically and systematically evaluating the sensitivity and 
importance to risk of these uncertainties. PRA techniques and models to 
address errors of commission and the influence of organizational 
factors on human reliability are currently being developed.
    It is important to note that not all of the Commission's regulatory 
activities lend themselves to a risk analysis approach that utilizes 
the same PRA tools (e.g., fault tree methods). In general, fault tree 
methods can be more suitable for power reactor events that typically 
involve complex systems. Events associated with industrial and medical 
uses of nuclear materials generally involve simple systems, involve 
radiation overexposures, and result from human error, not equipment 
failure. Because of the characteristics of medical and industrial 
events, as discussed above, analysis of these events using relatively 
simple techniques can yield meaningful results. Power reactor events, 
however, generally involve complex systems and human interactions, can 
potentially involve more than one adverse consequence, and often result 
from equipment failures. Therefore, power reactor events can require 
greater use of more complex risk analysis techniques, such as fault 
tree analysis, to yield meaningful insights.
    Given the dissimilarities in the nature and consequences of the use 
of nuclear materials in reactors, industrial situations, and medical 
applications, the Commission recognizes that a single approach for 
incorporating risk analyses into the regulatory process is not 
appropriate. However, PRA methods and insights will be broadly applied 
within the NRC to ensure that the best use is made of available 
techniques to foster consistency in NRC risk-based decision-making.

(C) Defense-in-Depth Philosophy

    In the defense-in-depth philosophy, the Commission recognizes that 
complete reliance for safety cannot be placed on any single element of 
the design, maintenance, or operation of a nuclear power plant. Thus, 
the expanded use of PRA technology will continue to support the NRC's 
defense-in-depth philosophy by allowing quantification of the levels of 
protection and by helping to identify and address weaknesses or overly 
conservative regulatory requirements in the physical and functional 
barriers.

III. The Commission Policy

    Although PRA methods and information have thus far been used 
successfully in nuclear regulatory activities, there have been concerns 
that PRA methods are not consistently applied throughout the agency, 
that sufficient agency PRA/statistics expertise is not available, and 
that the Commission is not deriving full benefit from the large agency 
and industry investment in the developed risk assessment methods. 
Therefore, the Commission believes that an overall policy on the use of 
PRA in nuclear regulatory activities should be established so that the 
many potential applications of PRA can be implemented in a consistent 
and predictable manner that promotes regulatory stability and 
efficiency. This policy statement sets forth the Commission's intention 
to encourage the use of PRA and to expand the scope of PRA applications 
in all nuclear regulatory matters to the extent supported by the state-
of-the-art in terms of methods and data. Implementation of the proposed 
policy statement would improve the regulatory process in three areas: 
foremost, through improved risk-effective safety decision making; 
through more efficient use of agency resources; and through a reduction 
in unnecessary burdens on licensees.
    Therefore, the Commission proposes the following policy statement 
regarding the expanded NRC use of PRA:
    (1) The use of PRA technology should be increased in all regulatory 
matters to the extent supported by the state-of-the-art in PRA methods 
and data and in a manner that complements the NRC's deterministic 
approach and supports the NRC's traditional defense-in-depth 
philosophy.
    (2) PRA and associated analyses (e.g., sensitivity studies, 
uncertainty analyses, and importance measures) should be used in 
regulatory matters, where practical within the bounds of the state-of-
the-art, to reduce unnecessary conservatism associated with current 
regulatory requirements, regulatory guides, license commitments, and 
staff practices. Where appropriate, PRA should be used to support 
additional regulatory requirements. Appropriate procedures for 
including PRA in the process for changing regulatory requirements 
should be developed and followed. It is, of course, understood that the 
intent of this policy is that existing rules and regulations shall be 
complied with unless these rules and regulations are revised.
    (3) PRA evaluations in support of regulatory decisions should be as 
realistic as possible and appropriate supporting data should be 
publicly available for review.
    (4) The Commission's safety goals for nuclear power plants and 
subsidiary numerical objectives are to be used with appropriate 
consideration of uncertainties in making regulatory judgments in the 
context of backfitting new generic requirements on nuclear power plant 
licensees.

Policy Implications

    There are several important regulatory or resource implications 
that follow from the goal of increased use of PRA techniques in 
regulatory activities. First, the NRC staff, licensees, and Commission 
must be prepared to consider changes to regulations, to guidance 
documents, to the licensing process, and to the inspection program. 
Second, the NRC staff and Commission must be committed to a shift in 
the application of resources over a period of time based on risk 
findings. Third, the NRC staff must undertake a training and 
development program, which may include recruiting personnel with PRA 
experience, to provide the PRA expertise necessary to implement these 
goals. Additionally, the NRC staff must continue to develop PRA methods 
and regulatory decision-making tools and must significantly enhance the 
collection of equipment and human reliability data for all of the 
agency's risk assessment applications, including those associated with 
the use, transportation, and storage of nuclear materials.
    This proposed policy statement affirms the Commission's view that 
PRA methods can be used to derive valuable insights, perspective and 
general conclusions as a result of an integrated and comprehensive 
examination of the design of nuclear facilities, facility response to 
initiating events, the expected interactions among facility structures, 
systems and components, and between the facility and its operating 
staff.

IV. Availability of Documents

    Copies of documents cited in this section are available for 
inspection and/or for reproduction for a fee in the NRC Public Document 
Room, 2120 L Street NW, (Lower Level), Washington, DC 20037. Copies of 
NUREGs cited in this document may be purchased from the Superintendent 
of Documents, U.S. Government Printing Office, P.O. Box 37082, 
Washington, DC 20013-7082. Copies are also available for purchase from 
the National Technical Information Service, 5285 Port Royal Road, 
Springfield, VA 22161.
    In addition, copies of (1) SECY-94-218, ``Proposed Policy Statement 
on the Use of Probabilistic Risk Assessment Methods in Nuclear 
Regulatory Activities,'' (2) SECY-94-219, ``Proposed Agency-Wide 
Implementation Plan for Probabilistic Risk Assessment (PRA),'' (3) the 
Commission's Staff Requirements Memorandum of September 13, 1994 
concerning the August 30, 1994 Commission meeting on SECY-94-218 and 
SECY-94-219, and (4) the Commission's Staff Requirements Memorandum of 
October 4, 1994 on SECY-94-218 can be obtained electronically by 
accessing the NRC electronic bulletin board system (BBS) Tech Specs 
Plus. These four WordPerfect 5.1 documents are located in the 
BBS MISC library directory under the single filename ``PRAPLAN.ZIP''. 
The BBS operates 24 hours a day and can be accessed through a toll-free 
number, 1-800- 679-5784, at modem speeds up to 9600 baud with 
communication parameters set at 8 data bits, no parity, 1 stop bit, 
full duplex, and using ANSI terminal emulation.

    Dated at Rockville, Maryland, this 1st day of December 1994.

    For the Nuclear Regulatory Commission.
Gary M. Holahan,
Director, Division of Systems Safety and Analysis Office of Nuclear 
Reactor Regulation.
[FR Doc. 94-30147 Filed 12-7-94; 8:45 am]
BILLING CODE 7590-01-P