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NRC NEWS
U. S. NUCLEAR REGULATORY COMMISSION |
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No. 96-63 FOR IMMEDIATE RELEASE
(Friday, April 26, 1996)
NOTE TO EDITORS:
The Nuclear Regulatory Commission has received four attached
reports from its Advisory Committee on Reactor Safeguards (ACRS).
The reports, in the form of letters, provide comments on:
--Proposed revisions to NRC regulations and regulatory
guides relating to reactor site criteria.
--A probabilistic risk assessment (PRA) document, its pilot
applications, and the next steps to expand the use of PRA in the
regulatory decision-making process.
--Westinghouse Electric's best-estimate computer code for a
large-break loss-of-coolant accident.
--Continued need for U.S. membership in the Nuclear Energy
Agency, a part of the Organization for Economic Cooperation and
Development.
#
Attachments:
As stated
.
April 22, 1996
The Honorable Shirley Ann Jackson
Chairman
U.S. Nuclear Regulatory Commission
Washington, D.C. 20555-0001
Dear Chairman Jackson:
SUBJECT: PROPOSED REVISIONS TO 10 CFR PARTS 50 AND 100 AND
PROPOSED REGULATORY GUIDES RELATING TO REACTOR SITE
CRITERIA
During the 430th meeting of the Advisory Committee on Reactor
Safeguards, April 11-13, 1996, we reviewed the proposed revisions
to reactor siting regulations and associated Regulatory Guides
and Standard Review Plan sections. Our Subcommittee on Extreme
External Phenomena reviewed this matter during a meeting on April
3, 1996. During this review, we had the benefit of discussions
with representatives of the NRC staff, Westinghouse Electric
Corporation, and the Nuclear Energy Institute. We also had the
benefit of the document referenced.
The staff has proposed final revisions to 10 CFR Parts 50 and 100
and a new Appendix S to Part 50 that deal with both seismic and
source term issues for future plants and sites. Many of the
implementation details will be found in new Regulatory Guides and
in Standard Review Plan sections. The existing requirements of
10 CFR Part 100 and its Appendix A will remain in effect for
operating plants.
We recommend that the proposed final rule dealing with the
seismic aspects be issued.
The proposed final rule requires that any individual, located at
any point on the exclusion area boundary for any two-hour period
following the postulated release of the fission products, not
receive a radiation dose in excess of 25 rem total effective dose
equivalent (TEDE). Similarly, an individual located at the outer
boundary of the low population zone (LPZ), who is exposed to the
radioactive cloud resulting from the release of the postulated
fission products (during the entire period of its passage), not
receive a dose in excess of 25 rem TEDE. Consistency within the
body of NRC regulations is most desirable. We recommend that
careful definitions of the TEDE limits that are mindful of organ
dose weighting factors found in 10 CFR Part 20 be included in the
final rule.
Radiological doses are to be evaluated over a two-hour period.
The proposed final rule states that the evaluation should be over
the two-hour period of maximum dose. The Office of Nuclear
Regulatory Research (RES) has a differing view and recommends
that the proposed final rule be modified from any two-hour period
after release of fission products (referred to as the "worst" two
hours) to a period of two hours commencing with fuel failure
(referred to as the "first" two hours). RES believes that the
use of the worst two-hour period in the dose calculation is not
justified by risk considerations and could lead to increased
costs for future licensees with no commensurate gain in safety.
The staff supporting the proposed rule states that (1) the
proposed licensing framework would provide a relaxation of
engineered safety feature (ESF) performance requirements
commensurate with updated source term and radiological insights,
(2) the regulatory requirements for determination of in-containment radioactive material during the two-hour dose
evaluation period would be consistent and capable of handling
designs substantially different from those analyzed in NUREG-1465, "Accident Source Terms for Light-Water Nuclear Power
Plants," (3) the analysis would be easy to perform and
reproducible with confidence, and (4) the technical bases and
analytical methods would be defensible. While the revised dose
evaluation in 10 CFR 50.34 is intended for future plants, the
staff is concerned that a current licensee might seek to use it
to remove or disable existing fission product cleanup systems.
This could markedly change the risk profile of the plant from
that which was licensed.
We are not persuaded by the rationale provided by RES in favor of
the first two-hour dose calculation. We agree with the position
taken in the proposed final rule, and recommend that the rule and
the associated Regulatory Guides and SRP sections be issued.
Sincerely,
/s/
T. S. Kress
Chairman, ACRS
REFERENCE:
Memorandum dated March 6, 1996, from T. P. Speis, Office of
Nuclear Regulatory Research, NRC, to J. T. Larkins, ACRS,
transmitting Revisions to 10 CFR Part 100, Reactor Site
Criteria, Revisions to 10 CFR Part 50, New Appendix S to Part 50
(Final Rules) and Associated Regulatory Guides and Standard
Review Plan Sections
. April 23, 1996
The Honorable Shirley Ann Jackson
Chairman
U.S. Nuclear Regulatory Commission
Washington, D.C. 20555-0001
Dear Chairman Jackson:
SUBJECT: PROBABILISTIC RISK ASSESSMENT FRAMEWORK, PILOT
APPLICATIONS, AND NEXT STEPS TO EXPAND THE USE
OF PRA IN THE REGULATORY DECISION-MAKING PROCESS
During the 430th meeting of the Advisory Committee on Reactor
Safeguards, April 11-13, 1996, we continued our deliberations on
risk-informed and performance-oriented regulation (RIPOR). We
met with representatives of the NRC staff and the Nuclear Energy
Institute (NEI) during our 429th meeting on March 7-9, 1996. Our
Subcommittee on Probabilistic Risk Assessment (PRA) also met on
October 26-27, 1995, with representatives of the NRC staff and of
the nuclear industry, and on February 27-28, 1996, with the NRC
staff and two invited experts, Dr. D. M. Karydas (performance-based standards for fire protection) and Professor T. G.
Theofanous (on the proper formulation of safety goals and
assessment of safety margins for rare and high-consequence
hazards). We also had the benefit of the documents referenced.
This report is in response to the Staff Requirements Memorandum
dated December 27, 1995, in which the Commission requested "ACRS
views on the PRA framework document, its relationship to the
pilot applications (SECY-95-280), and the next steps in the
process to expand the use of PRA in the regulatory
decision-making process."
PRA Framework Document
The PRA framework document provides a good starting point in the
development of RIPOR. The six-step process described in the
document is a reasonable way to proceed. We agree with the staff
that the focus should be on the integration of probabilistic and
deterministic approaches to regulation.
The PRA framework document, however, does not articulate an
overall philosophy for RIPOR. We believe that such a philosophy
should be developed. Some important high-level principles that
should be included are:
1. RIPOR should consider risk from all modes of nuclear plant
operations, including full power, shutdown, and transition.
2. The Commission's safety goals should serve as the top-level
acceptance criteria.
3. Subsidiary performance-based acceptance criteria should be
determined in a consistent way and must be measurable or
calculable. The licensee should be granted flexibility in
choosing the means to meet the criteria.
4. The relationship between RIPOR and defense-in-depth should
be explained. The role of defense-in-depth in the determina-
tion of performance criteria to accommodate uncertainty and
incompleteness in risk assessments should be established.
5. Criteria for the adoption of prescriptive regulations should
be clearly delineated.
6. The acceptance criteria should be set at the highest level
of plant system hierarchy that is consistent with the other
principles noted above.
Discussion
It is indicative of the novelty of these concepts that we have
spent a considerable amount of time discussing the meaning of
"performance" among ourselves and with the staff and NEI. Some
interpret performance in a limited way; i.e., its measures are
simply the reliability and availability (or related quantities)
of plant systems and components. Others take a broader view and
interpret it as the overall performance of the licensee,
including operations, maintenance, training, and the prevailing
safety culture at the plant.
Similarly, the definition of performance criteria varies widely.
At one extreme, we have simple measures that are either directly
measurable or that involve calculations (e.g., the reliabilities
and unavailabilities mentioned above). At the other extreme,
performance criteria can be probabilistic or nonprobabilistic and
can be set at any level. Observations and statistical or
experimental evidence from the plant or other sources in
conjunction with models can be used to demonstrate that the
criteria have been met. As part of an overall philosophy, the
staff needs to resolve the ambiguity in the definition of
performance criteria.
Pilot Applications
While we support the staff's use of pilot applications, we are
concerned that there seems to be no integrated justification for
their selection. We would like to see the development of a list
of important issues that are expected to arise on the road to
RIPOR, .along with a discussion of how the selected pilot projects will
help. The staff has agreed to look into these issues.
We also recommend that, for each pilot project, attempts be made
to establish performance-based decision criteria along with the
methods that would be used for demonstrating compliance. Such an
exercise should provide useful insights regarding the overall
feasibility of a performance-oriented approach to regulation.
Next Steps to Expand the Use of PRA in the Regulatory Decision-making Process
We believe that the NRC needs to take a number of important
additional steps before a RIPOR environment can be achieved.
These are discussed below.
Safety Goals
A restatement of the Commission's safety goal policy is needed
that will allow the use of safety goals on a plant-specific
basis.
Performance-Based Regulatory Criteria
A methodology is needed to determine performance-based criteria
for regulatory action that are consistent with the top-level
safety goals, as stated in the high-level principles. A "top-down" approach will ensure that this happens. An important
element should be the preservation of the concept of defense-in-depth. The development of this methodology will also provide the
opportunity to reexamine the validity of Level 2 subsidiary
goals, which appear to be controversial at this time.
Programmatic Issues
Developing a RIPOR system should be a participative effort
between the staff and the industry. We believe that the
magnitude and significance of the task that the staff has
undertaken requires a cooperative effort. Also, we recommend
that the staff work with foreign researchers and regulatory
agencies.
Conclusion
The intellectual and practical issues that the staff must
confront in developing a RIPOR structure are significant. The
staff has made a good start, but much remains to be done. We are
pleased that the staff has agreed to meet with us periodically.
Recent meetings have demonstrated that the staff is receptive to
suggestions on how to deal with these complex issues. We applaud
this attitude. We will keep you informed as these efforts
progress.
Additional comments by ACRS Members Thomas S. Kress and Don W.
Miller are presented below.
Sincerely,
/S/
T. S. Kress
Chairman, ACRS
Additional Comments by ACRS Members Thomas S. Kress and Don W.
Miller
While we agree with most of the Committee's report on this
subject, we find it to lack coherence. The major problem we have
with the Committee report is its treatment of the concept of
"performance-based" regulation. We conceive of basically two
meanings to the word "performance" in this context: (1) the
performance of equipment (systems and components) in carrying out
the intended function, or (2) the performance of the licensee in
performing its function (operation, maintenance, inspection,
training, etc.). The first of these could further relate to
either the operability of the specific equipment (e.g., does it
turn on or off, and, in the case of a pump, for example, does it
provide the required flow) or to the reliability/availability of
the equipment. In our view, the former does not provide any
basis on which to develop a regulatory structure (there are no
meaningful acceptance criteria that relate to risk). On the
other hand, the latter can clearly be anchored in risk. This,
however, would be purely risk-based regulation. The word
"performance" in this context becomes synonymous with "risk" and
such a regulatory concept should be designated as risk-based and
should not be called performance-based.
The second possible meaning of performance, the performance of
the licensee, obviously has a nexus to risk. This connotation of
performance, however, is what we have been calling organizational
factors. To date, a methodology has not been developed by which
objective performance measures can be identified and be factored
directly into PRA to quantify risk implications. Therefore, at
this time, we do not have the capability to develop such
performance-based regulations in any coherent manner. This
would, however, be an area worth pursuing in the future with
additional research.
This leads us to our main point. At this time, we should be
striving for risk-based or risk-informed regulations and should
relegate the concept of "performance" regulation to being a
remote possibility that needs substantial research to determine
feasibility.
References:
1. Memorandum dated December 27, 1995, from J. Hoyle, Secretary
of NRC, to J. Larkins, ACRS, Subject: Staff Requirements
Memorandum dated December 27, 1995
2. Memorandum dated June 16, 1995, from A. Bates, Office of the
Secretary, NRC, to File, Subject: Staff Requirements
Memorandum dated June 16, 1995
3. Letter dated February 6, 1996, from J. Milhoan, Office of
the Executive Director for Operations, NRC, to W. Rasin,
Nuclear Energy Institute, Subject: Improving the Regulatory
Process through Risk-Based and Performance-Based Regulation
4. Letter dated January 3, 1996, from J. Taylor, Executive
Director for Operations, NRC, to Chairman Jackson, NRC,
Subject: Improvements Associated With Managing the
Utilization of Probabilistic Risk Assessment (PRA) and
Digital Instrumentation and Control Technology
5. Letter dated November 30, 1995, from Chairman Jackson, NRC,
to J. Taylor, Executive Director for Operations, NRC,
Subject: Follow-up Requests in Probabilistic Risk Assessment
and Digital Instrumentation and Control
6. SECY-95-280, "Framework for Applying Probabilistic Risk
Analysis in Reactor Regulation," dated November 27, 1995
7. Letter dated November 14, 1995, from W. Rasin, Nuclear
Energy Institute, to J. Milhoan, Office of Executive
Director for Operations, NRC, Subject: Draft report,
"Improving the Regulatory Process Through Risk-Based and
Performance-Based Regulation"
. April 19, 1996
The Honorable Shirley Ann Jackson
Chairman
U.S. Nuclear Regulatory Commission
Washington, D.C. 20555-0001
Dear Chairman Jackson:
SUBJECT: WESTINGHOUSE BEST-ESTIMATE LOSS-OF-COOLANT ACCIDENT
ANALYSIS METHODOLOGY
During the 430th meeting of the Advisory Committee on Reactor
Safeguards, April 11-13, 1996, we concluded our review of the
best-estimate, large-break, loss-of-coolant accident (LBLOCA)
analysis methodology developed by the Westinghouse Electric
Corporation. We had previously reviewed this matter during our
428th meeting, February 8-10, 1996. We also had the benefit of
the referenced documents.
In our February 23, 1996 report commenting on the results of our
initial review, we identified several technical details of the
Westinghouse LBLOCA methodology needing further attention and
also commented on the adequacy of the documentation. As a result
of subsequent discussions with representatives of Westinghouse
and the NRC staff during this meeting, we believe that these
concerns have been addressed.
ACRS Member George Apostolakis did not participate in the
Committee's deliberation of this matter.
Sincerely,
/s/
T. S. Kress
Chairman, ACRS
References:
1. Memorandum dated March 25, 1996, from N. Liparulo,
Westinghouse, to Nuclear Regulatory Commission, transmitting
information on the resolution of issues related to the
review of WCAP-12945-P (Proprietary)
2. Letter dated March 15, 1996, from J. Taylor, Executive
Director for Operations, NRC, to T. S. Kress, Chairman,
ACRS, Subject: Westinghouse Best-Estimate Loss-of-Coolant
Accident Analysis Methodology
3. Report dated February 23, 1996, from T. S. Kress, Chairman,
ACRS, to Shirley Ann Jackson, Chairman, NRC, Subject:
Westinghouse Best-Estimate Loss-of-Coolant Accident Analysis
Methodology
April 17, 1996
The Honorable Shirley Ann Jackson
Chairman
U. S. Nuclear Regulatory Commission
Washington, D.C. 20555-0001
Dear Chairman Jackson:
SUBJECT: CONTINUED NEED FOR UNITED STATES MEMBERSHIP IN THE
NUCLEAR ENERGY AGENCY
The Advisory Committee on Reactor Safeguards has recently learned
of the proposed withdrawal of the United States (U.S.) from
participation in the Nuclear Energy Agency (NEA), a part of the
Organization for Economic Cooperation and Development. The
Department of Energy (DOE) and the Nuclear Regulatory Commission
(NRC) are the primary U.S. technical participants in the NEA
activities and, hence, are the agencies that have the most
complete understanding of the benefits of membership in NEA. Our
comments will perhaps assist you as you set forth the NRC
position.
For many years, the NEA Committee on Safety of Nuclear
Installations (CSNI) has been an active and productive leader in
nuclear reactor safety research. CSNI reports cover the full
scope of reactor safety concerns and are prepared by leading
technical experts from the primary technical research
laboratories and agencies of the member countries.
Current CSNI efforts contribute to U.S. programs in extended
burnup reactor fuels, high-pressure melt ejection, direct
containment heating, accident management, and steam explosions.
Clearly, the CSNI has demonstrated the ability to keep pace with
real concerns in nuclear safety. Furthermore, these efforts have
resulted in substantial savings in U.S. research costs.
Nuclear safety is truly an international concern. The NEA is a
forum for the consideration of common technical safety issues by
the responsible regulatory agencies in the member countries and
has been useful in developing consistent "western" positions. If
the NEA did not exist, we would soon be convinced that it should
be invented.
.We believe that the suggested U.S. withdrawal from the NEA is
shortsighted. We fully support your efforts to ensure continued
U.S. participation in the NEA.
Sincerely,
/S/
T. S. Kress
Chairman, ACRS
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