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

Note: EPA no longer updates this information, but it may be useful as a reference or resource.

 Use of Probabilistic Risk Assessment Methods in Nuclear 



[Federal Register: August 16, 1995 (Volume 60, Number 158)]
[Notices]               

[Page 42622-42629]

>From the Federal Register Online via GPO Access [wais.access.gpo.gov]








NUCLEAR REGULATORY COMMISSION
 


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



AGENCY: Nuclear Regulatory Commission.



ACTION: Final policy statement.






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 ``* 


[[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 spaceavailable, 
no-cost basis to State personnel as well as for other nonNRC 
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 doseeffect 
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 designbasis 
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 



[[Page 42628]]

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 multiplebarrier 
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 stateof
-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-ofthe
-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 



[[Page 42629]]

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<Register> 5.1 documents are located in the 
BBS MISC library directory under the single filename ``PRAPLAN.ZIP''. 
The WordPerfect<Register> 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]
BILLING CODE 7590-01-P

 
 

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