[Federal Register Volume 60, Number 158 (Wednesday, August 16, 1995)]
[Notices]
[Pages 42622-42629]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 95-20237]
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NUCLEAR REGULATORY COMMISSION
Use of Probabilistic Risk Assessment Methods in Nuclear
Regulatory Activities; Final Policy Statement
AGENCY: Nuclear Regulatory Commission.
ACTION: Final policy statement.
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SUMMARY: This statement presents the policy that the Nuclear Regulatory
Commission (NRC) will follow in the use of probabilistic risk
assessment (PRA) methods in nuclear regulatory matters. The Commission
believes that an overall policy on the use of PRA methods 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 would promote regulatory stability and efficiency. In
addition, the Commission believes that the use of PRA technology in NRC
regulatory activities should be increased 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. The pertinent comments
received from the published draft policy statement are reflected in
this final policy statement. This policy statement will be implemented
through the execution of the NRC's PRA Implementation Plan.
EFFECTIVE DATE: August 16, 1995.
ADDRESSES: The proposed policy statement and the comments received may
be examined at: NRC Public Document Room, 2120 L Street, NW. (Lower
Level), Washington, DC.
FOR FURTHER INFORMATION CONTACT: Anthony Hsia, Office of Nuclear
Reactor Regulation, U.S. Nuclear Regulatory Commission, Washington, DC
20555. Telephone (301) 415-1075.
SUPPLEMENTARY INFORMATION:
I. Background.
II. Summary of Public Comments and NRC Responses.
III. Deterministic and Probabilistic Approaches to Regulation.
IV. The Commission Policy.
V. 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 mitigated. 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,
[[Page 42623]]
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; August 21, 1986), the Commission's ``Policy Statement on Severe
Reactor Accidents Regarding Future Designs and Existing Plants'' (50 FR
32138; August 8, 1985), and the Commission's ``Final Policy Statement
on Technical Specifications Improvement for Nuclear Power Reactors''
(58 FR 39132; July 22, 1993). 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.
Probabilistic analyses were extensively used in the development of the
recently proposed rule change to reactor siting criteria in 10 CFR Part
100 (59 FR 52255; October 17, 1994). The proposed rule change invoked
the use of a probabilistic approach to estimate the Safe Shutdown
Earthquake Ground Motion for a nuclear reactor site, instead of the
purely deterministic method currently specified in Appendix A to 10 CFR
Part 100.
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 potential 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. In May 1994, the NRC staff
forwarded a draft PRA policy statement to the Advisory Committee on
Reactor Safeguards (ACRS) for review and briefed ACRS on the same
subject. 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 that Commission paper, the staff proposed
that an overall policy on the use of probabilistic risk assessment
(PRA) methods in nuclear regulatory activities should be established
and that the use of PRA technology in NRC regulatory activities should
be increased. Comments from the ACRS regarding the policy statement as
documented in a letter dated May 11, 1994, were incorporated. On August
19, 1994, the staff forwarded SECY-94-219, ``Proposed Agency-Wide
Implementation Plan for Probabilistic Risk Assessment (PRA),'' to the
Commission. On August 30, 1994, the staff discussed the PRA policy
statement and the PRA implementation plan in a public meeting with the
Commission. On September 13 and October 4, 1994, the Secretary issued
two staff requirements memoranda (SRMs) providing Commission guidance
regarding the draft policy statement. In these SRMs, the Commission
directed the staff to revise the proposed PRA policy statement, publish
the policy statement for public comment in the Federal Register, and
conduct a public workshop on the PRA implementation plan.
As directed by the Commission, the staff conducted a public
workshop on December 2, 1994, to discuss the PRA implementation plan.
The purpose of the workshop was to inform the public of NRC activities
related to increasing the use of PRA methods and techniques in
regulatory applications and to receive public comments on these
activities. After the staff incorporated the comments from the SRMs,
the proposed policy statement ``Use of Probabilistic Risk Assessment
Methods in Nuclear Regulatory Activities'' was published in the Federal
Register on December 8, 1994 (59 FR 63389). The public comment period
expired on February 7, 1995.
II. Summary of Public Comments and NRC Responses
In January and February 1995, the NRC received 17 letters
commenting on the proposed policy statement on ``Use of Probabilistic
Risk Assessment Methods in Nuclear Regulatory Activities''. These
comments were from the following organizations: Six utilities--PECO
Energy Company, Detroit Edison, Washington Public Power Supply System,
Carolina Power and Light Company, Virginia Power Company, and Centerior
Energy; three State regulatory agencies--State of Illinois Department
of Nuclear Safety, State of New Jersey Department of Environmental
Protection, State of Nevada Agency for Nuclear Projects; two industry
groups--Nuclear Energy Institute and Westinghouse Owners Group; two
engineering firms--PLG,
[[Page 42624]]
Inc. and ICF Kaiser Engineers, Inc.; University of California at Los
Angeles; Ohio Citizens For Responsible Energy; Winston and Strawn,
Counsel to the Nuclear Utility Backfitting and Reform Group; and the
Department of Energy. Copies of the letters may be examined at the NRC
Public Document Room at 2120 L Street., NW. (Lower Level), Washington,
DC.
General Comments
Twelve commenters explicitly supported the basic tenet of the
policy to increase the use of PRA technology in NRC's regulatory
activities. The other commenters did not object to the policy statement
but provided recommendations for the NRC to modify and improve the
policy statement and/or the PRA implementation plan. Five commenters
indicated that they agreed with the NEI comments on the proposed PRA
policy statement. The NRC staff has reviewed the comments and
summarized them in the following areas. The staff response to the
comments are also included in this final policy statement.
Use of PRA in Regulatory Decisions
Several comments dealt with the scope of the PRA applications
(where can PRA be used) and the implementation of the policy statement
(how can PRA be used).
One commenter felt that neither the policy statement nor the PRA
implementation plan provided consistent decision criteria for accepting
PRA results as part of the justification for licensing decisions. The
commenter was concerned that the short term effect of the policy
statement would likely be an increased burden on the licensees. For the
long term, the commenter recommended a systematic review of the rules
and regulations to identify opportunities for elimination of
unnecessary regulations. The proposed policy statement directed the
staff to use PRA and associated analyses, where appropriate, as part of
the justification for licensing decisions. The PRA implementation plan
describes how the stated policy is to be implemented. Appropriate
decision criteria will be developed and documented as part of the PRA
implementation plan. The Commission has already performed a systematic
review of the many current rules and regulations to identify
opportunities for the elimination of unnecessary regulations. In 1993,
the NRC established the Regulatory Review Group (RRG) to conduct a
structured review of power reactor regulations with special attention
on the opportunity to reduce unnecessary regulatory burdens. The RRG
recommendations to reduce the regulatory burden included the suggestion
to use more risk-based approaches in quality assurance, inservice
inspection and testing, and the concept of a PRA plan. The RRG
recommendations were documented in SECY-94-003. To better focus the
NRC's effort on the PRA related activities recommended by the RRG, the
PRA Working Group, and the Regulatory Analysis Steering Group, the PRA
implementation plan was developed in 1994. The implementation plan
included a task to develop guidelines for determining when it is
practical to use PRA technology and results in regulatory activities.
The NRC has had discussions with volunteer licensees regarding the
pilot applications of risk-based regulatory initiatives. Results from
the pilot applications will be incorporated in the NRC's guidance for
PRA applications in regulatory activities. A number of current
regulatory requirements are being considered as part of the PRA
implementation plan to determine if alternative risk-based approaches
are practical. Over time, the Commission would expect some streamlining
and refocusing of its rules and regulations as part of this process.
The Commission has implemented a continuing regulatory improvement
program which is responsive to the commenter's recommendation of a
systematic examination of marginal regulatory requirements.
Another commenter recommended that the policy statement be amended
to state that when backfitting analyses are performed, mean risk levels
be the exclusive basis of regulatory decision-making when comparisons
are made against the $1000/person-rem criterion. The Commission does
not feel this policy statement needs to address the issue regarding the
use of mean risk level as the exclusive basis for applying the $1000/
person-rem criterion because the Commission's safety goal policy
statement has already spoken to the use of mean values of risk in
connection with the cost-benefit analyses. Furthermore, this issue is
addressed in the proposed Revision 2 of NUREG/BR-0058, ``Regulatory
Analysis Guidelines of the U. S. Nuclear Regulatory Commission, Draft
Report for Comment.'' This commenter also recommended that the policy
statement should direct the staff to use the relevant plant specific
PRA in assessing the need for any backfitting action at that plant. For
generic backfits, this commenter recommended that the policy should
allow licensees to take credit for plant specific information to
justify relief from NRC imposed action. The Commission believes that
the use of the plant specific PRA in the backfit analysis to evaluate
whether there is a substantial increase in the overall protection or to
justify relief from NRC imposed action is acceptable when combined with
other relevant deterministic considerations, as appropriate.
Regarding the use of safety goals, one commenter recommended
retention of the language in SECY-94-218 to effect that safety goals
could be used in granting relief from unnecessary requirements. Another
commenter recommended that the safety goals should be used as a minimum
goal, rather than the maximum level of safety. As stated in the
proposed PRA policy statement published on December 8, 1994, the
Commission's safety goals are ``* * * intended to be generically
applied by the NRC as opposed to plant specific applications,'' and ``*
* * to be used with appropriate consideration of uncertainties in
making regulatory judgements in the context of backfitting new generic
requirements on nuclear power plant licensees.'' In the Staff
Requirement Memorandum (SRM) dated June 15, 1990, regarding the
implementation of safety goals, the Commission directed that ``Safety
goals are to be used in a more generic sense and not to make specific
licensing decisions.'' Therefore, at this time, the NRC would use the
safety goals in making regulatory decisions regarding backfitting new
generic requirements but not to make specific licensing decisions
including granting relief from unnecessary requirements. Any changes to
the safety goal policy are outside the scope of the PRA policy
statement and would, therefore, need to be pursued independently.
Referring to paragraphs 1 and 2 of the proposed policy statement, a
commenter suggested that it should include the application to NRC
enforcement decisions, including the severity levels. As noted in
NUREG-1525, ``Assessment of the NRC Enforcement Program,'' the
Commission does not support defining severity levels using PRA results.
The NRC's basis for severity level categorization clearly is safety
significance. In judging safety significance, the NRC considers (1)
Actual consequences, (2) potential consequences, and (3) regulatory
significance. It is recognized that PRA results may be helpful to
provide risk insights on the likelihood and significance of potential
consequences. The NRC plans to continue to consider the use of PRA
results where relevant as
[[Page 42625]]
part of the integrated process considering all facets surrounding the
violation in support of enforcement decisions.
Several commenters discussed the role of PRA in reducing the
unnecessary conservatisms in regulations and to support additional
regulatory requirements. One commenter's concern was that the proposed
policy statement appeared to be biased in the direction of using PRA to
support deregulation. Another commenter was concerned with the
implication that PRA could result in an additional layer of regulation.
The policy statement addressed the need to remove unnecessary
conservatism associated with regulatory requirements. It is not the
Commission's intent to replace traditional defense-in-depth concepts
with PRA, but rather to exploit the use of PRA insights to further
understand the risk and improve risk-effective safety decision-making
in regulatory matters. In doing so, the Commission is focusing its
attention and resource allocation to areas of true safety significance.
Where appropriate, PRA should be used to support additional regulatory
requirements, according to 10 CFR 50.109 (Backfit Rule).
One commenter recommended that the policy statement should
explicitly state that the use of PRA by licensees in regulatory matters
is at the discretion of each licensee. The commenter also believed that
the NRC should not prescribe how and when PRA methods should be used by
licensees in regulatory matters, but should address the potential
impact the expanded use of PRA may have on regulatory interactions with
licensees. The Commission's PRA policy statement is intended only to
encourage the NRC staff and industry to use probabilistic risk
assessment methods in regulatory matters. It is not intended to
prescribe or require any of the many potential PRA applications. Any
requirements for licensees to perform PRA analyses would be expected to
occur through formal rulemaking.
One commenter's concern was that there was a wide range of
applications for which PRA was being applied without consistency and
standards. This commenter urged the NRC to insist on quality PRAs
commensurate with the intended applications and to develop standards
which require rigorous and living PRAs by regulation for nuclear power
plant applications. The commenter also questioned whether the PRA
analyses for the IPE may be used for other applications because of a
lack of PRA standards. Another commenter expressed the concern that
strict conformance to detailed PRA standards would not be desirable,
and recommended that flexibility in PRA models should be allowed. The
Commission issued Generic Letter (GL) 88-20 with the primary purpose of
generating IPEs to identify severe accident vulnerabilities. The PRAs
which supported the IPE efforts may be useful for other applications,
however, this would have to be evaluated on a case-by-case basis under
well-defined objectives. After the Commission briefing on the IPE
program, the Commission recognized, as stated in the SRM dated April
28, 1995, that current industry IPE results do not provide a complete
basis for supporting risk-based regulatory decision-making. The SRM
suggested that ``* * * the industry should, in coordination with the
staff, initiate the actions necessary to develop PRAs that are
acceptable for risk-based regulatory use (i.e., standardized methods,
assumptions, level of detail).'' The industry is encouraged to
formulate a general approach for performing PRAs acceptable for
regulatory use. This approach should include guidance on standardizing
approaches for use of PRA techniques for specific applications,
narrowing some of the variability in the IPE results, and strengthening
its usefulness in the regulatory and safety decision-making process.
The Commission is currently considering the quality level and scope of
assessment necessary to justify use of specific PRAs for specific
regulatory applications. The Commission will require PRA quality
commensurate with the proposed application.
PRA Methodology
One commenter agreed with the NRC that the probabilistic approach
should be used to complement the deterministic approach and that PRA
numbers alone should not be used to make regulatory decisions. The
commenter also believed that uncertainties should not prevent or delay
the implementation of PRA in regulatory activities. The Commission
understands that uncertainties exist in any regulatory approach. These
uncertainties are derived from knowledge limitations that are not
created by PRA, but are often exposed by it. The PRA implementation
plan has provided a framework to assess the significance of potential
uncertainties and to develop a strategy to accommodate them in the
regulatory process.
One commenter stated that probabilistic analysis is simply an
extension of deterministic analysis. They are not separate and
distinctive concepts. The Commission agrees with this concept as the
proposed policy statement stated that ``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.'' The Commission believes that the PRA method plays a
complementary role in relationship to the deterministic method. This
was reflected in the policy statement that ``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.''
One commenter recommended that the most efficient use of NRC
resources should be to enhance or improve the existing methods, but not
to develop new ones. The Commission's principal focus will be on
improving the existing methods, but some new methods development may
also be useful.
Another commenter recommended that the PRA policy statement should
seek a uniform and standard application of PRA within the NRC, and
begin with a commitment to ensure that PRA is used consistently and is
not ignored when required by those unfamiliar or reluctant to apply it.
The Commission's PRA policy statement specifically emphasizes the need
for consistent and predictable application of PRA within the Commission
to promote regulatory stability and efficiency. The Commission believes
that this goal can be achieved through the implementation plan which
will ensure that the appropriate use of PRA is implemented by the
staff.
Schedule of PRA Activities
Two letters commented that the activities discussed in the PRA
implementation plan appeared to be on a protracted schedule and
recommended that priority and urgency be stressed and reflected in the
plan, including the use of PRA and PRA insights in the near term. The
Commission's PRA implementation plan showed the target completion dates
for all the tasks. The Commission fully realizes the need for near term
PRA applications and has included them in the implementation plan
wherever possible. These milestones include examples such as pilot
applications for risk-based initiatives and transfer of IPE insights to
NRC staff members for use in regulatory matters in the near term. The
Commission plans to periodically review the progress of the ``living''
PRA
[[Page 42626]]
implementation plan and, as appropriate, to adjust the priorities.
One letter commented that the NRC review and approval of licensing
actions that are based on PRA insights should not be contingent upon
the schedule for implementation of the plan. The plan should not be an
impediment to moving forward toward the goals outlined in the policy
statement. The Commission's implementation plan had been developed to
effectively and expeditiously establish a framework for increasing the
use of PRA technology inside the Commission. Since it is a ``living''
plan, new tasks could be added and existing tasks could be modified, as
the plan progresses. The Commission agrees that the plan should not be
an impediment to moving forward to achieve the goals stated in the
policy. The Commission welcomes risk-based regulatory initiatives from
the industry as the plan is being carried out and will adjust
resources, as appropriate.
One commenter asked how the NRC will propose to control the
utilities' application of PRA and the timeframe to implement the
consistent use of PRA within the NRC. The Commission's PRA
implementation plan describes the activities and schedule to effect a
coherent and consistent PRA application within the agency. As the plan
is implemented, the NRC expects to interact with licensees and publish
guidelines for the application of PRA in their submittal to the NRC.
PRA Training
Two commenters advocated PRA training for appropriate NRC and
licensee staff as soon as possible to ensure proper application of PRA
in regulatory matters. A PRA training program has been in place for the
NRC staff for a number of years. As part of the PRA implementation
plan, the existing training program is being enhanced. The existing PRA
training curriculum serves as the basis on which to build a more
comprehensive staff PRA training program. Six new courses have been
incorporated in the training program to address the short term needs
from the increasing use of PRA in regulatory activities. As a result of
the PRA implementation plan, the number of NRC staff participating in
the training program has increased significantly during the first half
of fiscal year 1995.
One commenter recommended that NRC's PRA training should be
extended to State agencies that can justify attendance. Historically,
attendance at NRC courses has been routinely available on a space-
available, no-cost basis to State personnel as well as for other non-
NRC personnel (such as foreign regulators, EPA, DOE, and other Federal
personnel). This has included training in the PRA area for a limited
number of State regulators. In courses that were under-subscribed by
NRC personnel, many had sufficient available space to allow acceptance
of outside personnel. Logistics for these arrangements are handled by
the NRC office responsible for interactions with the outside group
(i.e., Office of State Programs for States or Office of International
Programs for foreign personnel). NRC training currently is not
available to NRC licensees. Because of recent budgetary constraints, as
described in SECY-95-017 ``Reinventing NRC Fee Policies,'' full cost
reimbursements from States for NRC training is expected in future
years. However, NRC will continue its space-available policy for all
courses, including PRA courses.
Data Collection
Several commenters expressed concerns about the potential data
collection implications of the proposed PRA policy. They are summarized
as follows:
One commenter stated that the desire to collect detailed data
related to equipment and human reliability should not prohibit the use
of PRA for applications or support for decision-making. The collection
of plant-specific data must be commensurate with the benefit that
specific information might have on the quality or insight from the PRA.
Plant-specific information may not be statistically significant.
Furthermore, requiring all plants to collect the same information
without a focus based on plant performance, is counter to the concept
behind the Maintenance Rule.
Another commenter stated that the discussion of uncertainties in
Part II.(B) of the proposed policy statement is appropriate. However,
in the implementation of this part of the policy, care must be
exercised to restrain from requiring or implying the need for massive
plant-specific component level failure rate data collection programs.
Several commenters expressed concerns that a new or expanded nuclear
power plant experience data collection rulemaking could further burden
the licensees and the resulting benefit may well be marginal.
The Commission agrees that it should make every effort to avoid any
unnecessary regulatory burdens in connection with collecting
reliability and availability data. Specific comments on the types of
data that should or should not be collected will be addressed in
connection with proposed data collection requirements when they are
published for comment.
Radiation Medicine
One commenter recommended that NRC should abandon the use of the
linear hypothesis in estimating radiation-induced cancer and mutation
risk. The commenter further stated that the NRC's PRA implementation
plan refers to risk analysis to analyze nuclear medical devices and
that, ``* * * there are no nuclear medicine devices that have risk to
be analyzed.''
The International Commission on Radiation Protection, the United
Nations Scientific Committee on the Effects of Atomic Radiation, and
the National Academy of Sciences' Committee on the Biological Effects
of Ionizing Radiation believe that, in the absence of convincing
evidence that there is a dose threshold or that low levels of radiation
are beneficial, the assumptions regarding a linear nonthreshold dose-
effect model for cancers and genetic effects and the existence of
thresholds only for certain nonstochastic effects remain appropriate
for formulating radiation protection standards. NRC follows their
guidelines. Although some data suggest the possible use of other
models, there are still many scientists who believe there are
insufficient data to deviate from the ``linear'' hypothesis. The issue
of realism involved in continuing the use of the ``linear'' hypothesis
is expected to be a matter of debate over the coming years.
The NRC regulates radiation medicine, which includes both nuclear
medicine and radiation oncology. The intent of the policy statement
concerning medical applications is to refer to medical devices
containing byproduct material, in particular, those used in radiation
oncology. The term ``nuclear medical device'' was revised in the recent
status update on the PRA implementation plan (SECY-95-079) and
clarified in the policy statement.
Nuclear Waste
One commenter recommended that the NRC expand its use of PRA to
other areas such as radiological dose assessment during the site
decommissioning process. The NRC intends to consider expansion of PRA
techniques into additional areas with the proviso that the application
of these techniques to these facilities should be tempered according to
the complexity of the disposal system, its uncertainties and the
estimated risk.
One commenter provided comments on several aspects of the proposed
policy statement in the nuclear waste
[[Page 42627]]
area. Regarding the scope of the policy statement, the commenter
recommended that the policy statement be amended to include risk
assessment applications other than power reactors. The Commission
agrees with that comment. The use of PRA should be considered for those
applications that involve projecting system performance for very long
time periods, such as hundreds or thousands of years. The policy
statement stated that the use of PRA technology should be increased in
all regulatory matters. Another recommendation was to temper the
commitment to PRA to reflect inherent risk differences associated with
different waste management facilities. Because of inherent differences
in the regulations and practices associated with the licensing of waste
management facilities, the application of performance assessment (PRA
is called performance assessment for waste management systems)
techniques to these facilities should be tempered according to the
complexity of the disposal system, uncertainties surrounding the system
performance, and the estimated risk. The Commission also agrees with
the comments regarding uncertainties in projecting repository
performance and the use of technical expert judgment in assessing these
uncertainties, but feels the PRA policy statement is not the
appropriate forum to discuss these items applicable only to waste
management.
Regarding the suggestion of describing the reasons for using the
PRA and the application of PRA in regulatory activities, the Commission
included the reasons for using PRA in Section III of the policy
statement and added a description of the impact of PRA on the rule
changes to 10 CFR Part 100 in the background discussion.
Another commenter expressed concern that the proposed policy
statement inappropriately encouraged the use of PRA in the licensing
and regulation of nuclear waste disposal facilities. The Commission
disagrees with this comment since PRA techniques are acceptable in a
performance assessment for the geologic repository, but are only part
of the requirements for a license. The commenter was also concerned
that any new regulations proposed by the Environmental Protection
Agency (EPA) and the NRC's 10 CFR Part 60 for a high-level waste (HLW)
disposal facility proposed for Yucca Mountain will probably prohibit
use of PRA for these facilities because of Type I faults at this site.
The Commission anticipates that both probabilistic and deterministic
hazard assessment methodologies will be applied to assess the
significance of faulting at Yucca Mountain. Furthermore, the Commission
does not interpret 10 CFR Part 60 so as to preclude the use of PRA as a
basis for licensing a proposed repository at Yucca Mountain. The
commenter did not agree with NRC's characterization of the waste
disposal system as passive and believed that, at this time, there is no
alternative to the use of deterministic techniques for waste disposal
application because PRA techniques are in the embryonic stage. The
``Fault Tree Handbook'' (NUREG-0492, January 1981) refers to
``passive'' as a ``* * * mechanism (e.g., wire) whereby the output of
one `active' component becomes the input to a second `active'
component.'' ``Passive'' is generally used for ``engineered''
components that have no moving parts. Since there are no ``engineered''
components that are ``active'' (or causing motion in another engineered
component) in the post-closure phase of the potential geologic
repository at Yucca Mountain, the NRC has applied the traditional PRA
concept to the waste disposal system and referred to it as a ``passive
system.'' The remanded 1985 EPA Standard, 40 CFR 190, required a
probabilistic analysis for a geologic repository. The NRC has developed
this type of analysis since 1970 and has attained a state of maturity
for these analyses that is accepted by internationally-known
organizations (e.g., Organization for Economic Cooperation and
Development (OECD)/Nuclear Energy Agency (NEA)).
A number of editorial comments were received on the role of PRAs in
the licensing of waste disposal facilities. The NRC has incorporated
the appropriate comments in this final PRA policy statement.
III. Deterministic and Probabilistic Approaches to Regulation
(A) Extension and Enhancement of Traditional Regulation
The NRC established its regulatory requirements to ensure that a
licensed facility is designed, constructed, and operated 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 as design-basis accidents (e.g., reactor vessel rupture is
considered too improbable to be included) to the requirements for
emergency core cooling (e.g., safety train redundancy and protection
against single failure). The approach by the Commission for the use of
performance assessment to implement its regulations for disposal of
radioactive nuclear waste (10 CFR Part 60 for high-level waste disposal
and 10 CFR Part 61 for low-level waste disposal) also contains implied
elements of probability. The results of the numerous calculations
obtained from a performance assessment for a given performance measure
and for a particular type of facility (e.g., a spectrum of values for
ground-water travel time or individual dose) are expressed in terms of
statistical distributions that express the probability that a given
measure of performance will be attained. When this distribution is
compared to the appropriate deterministic standard in the Commission's
regulations, the probability of not exceeding the standard can be
obtained from the part of the distribution that falls below this
standard.
PRA addresses a broad spectrum of initiating events by assessing
the event frequency. Mitigating system reliability is then assessed,
including the potential for multiple and common cause failures. The
treatment therefore goes beyond the single failure requirements 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. Where appropriate, PRA can be used to eliminate unnecessary
conservatism and 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. In addition, PRA techniques
are appropriately used when considering regulations defined in
probabilistic terms, and for estimating
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safety of systems with very large uncertainties such as waste disposal
systems (Note that PRA is called performance assessment for these waste
disposal systems).
Beyond its deterministic criteria, the NRC has formulated guidance,
as in the safety goal policy statement, that utilizes quantitative,
probabilistic risk measures. The safety goal policy statement
establishes top-level objectives to help assure safe operation of
nuclear power plants. The safety goals are intended to be applied
generically and are not for 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 sufficiently low that further regulatory action is not necessary.
Also, as noted above, the Commission has been using PRA in performing
regulatory analysis for the proposed 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 a framework to 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. Improved 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
fault tree methods. In general, a fault tree method is best suited for
power reactor events that typically involve complex systems. Events
associated with industrial and medical uses of nuclear materials
generally involve a simple system, 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. PRA methods need to be adapted for waste disposal
systems because they are passive systems subjected to interlocking
natural and man-made processes and events that are dominated by complex
phenomenology.
Given the dissimilarities in the nature and consequences of the use
of nuclear materials in reactors, industrial situations, waste disposal
facilities, 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 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 applicable to the nuclear
industry. Defense-in-depth is a philosophy used by NRC to provide
redundancy for facilities with ``active'' safety systems, e.g., a
commercial nuclear power, as well as the philosophy of a multiple-
barrier approach against fission product releases. Such barrier
principles are mandated by the Nuclear Waste Policy Act of 1982, which
provides redundancy for a geologic repository to contain and isolate
nuclear waste from the human environment.
IV. 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 policy
statement will improve the regulatory process in three areas: Foremost,
through safety decision making enhanced by the use of PRA insights;
through more efficient use of agency resources; and through a reduction
in unnecessary burdens on licensees.
Therefore, the Commission adopts 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 the
proposal for additional regulatory requirements in accordance with 10
CFR 50.109 (Backfit Rule). Appropriate procedures for including PRA in
the process for
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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 practicable 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 on the
need for proposing and 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, license
applicants, 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 significantly enhance the PRA
expertise necessary to implement these goals. Additionally, the NRC
staff must continue to develop new and improved 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. However, it
is recognized that there may be situations with material users where it
may not be cost-effective to use PRA in their specific regulatory
applications.
This policy statement affirms the Commission's belief 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.
The Commission also recognizes, and encourages, continuation of
industry initiatives to improve PRA methods, applications and data
collection to support increased use of PRA techniques in regulatory
activities.
V. 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 WordPerfect 5.1 file for the final policy statement on
the ``Use of Probabilistic Risk Assessment Methods in Nuclear
Regulatory Activities,'' is located in the BBS MISC library directory
under the filename ``PRPOLICY.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 10th day of August, 1995.
For the Nuclear Regulatory Commission.
Andrew L. Bates,
Acting Secretary of the Commission.
[FR Doc. 95-20237 Filed 8-15-95; 8:45 am]
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