[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
