Home > Electronic Reading Room > Document Collections > ACRS > Letter Reports > 1999 > September 15, 1999
Dr. William D. TraversB
Executive Director for Operations
U.S. Nuclear Regulatory Commission
Washington, D.C. 20555-0001
Dear Dr. Travers:
SUBJECT: SAFETY EVALUATION REPORT RELATED TO ELECTRIC POWER
RESEARCH INSTITUTE RISK-INFORMED METHODS TO INSERVICE
INSPECTION OF PIPING (EPRI TR-112657, REVISION B, JULY 1999)
During the 465th meeting of the Advisory Committee on Reactor Safeguards,
September 1-3, 1999, we met with representatives of the NRC staff, Electric Power
Research Institute (EPRI), and Nuclear Energy Institute to discuss the staff's Safety
Evaluation Report (SER) on the topical report (EPRI TR-112657, Revision B) regarding
application of EPRI risk-informed methods to inservice inspection (ISI) of piping. Our
Subcommittees on Materials and Metallurgy and on Reliability and Probabilistic Risk
Assessment met on May 5, 1999, to discuss this matter. We also had the benefit of the
documents referenced.
Conclusions
We agree with the staff's conclusion that the methodology described in EPRI TR-
112657, Revision B, can be used to develop risk-informed ISI programs that will
provide an acceptable alternative to the requirements of paragraphs (a)(3) and
(g) of 10 CFR 50.55a and is consistent with the guidance in Regulatory Guides
1.174 (General Guidance) and 1.178 (ISI). The EPRI methodology is also
consistent with the requirements of American Society of Mechanical Engineers
(ASME) Code Cases N-560 (Class 1 piping systems) and N-578 (Class 1, 2, and
3 piping systems).
The EPRI methods will better focus inspections on piping with active degradation
mechanisms and relatively high risk significance than the current ASME Section
XI ISI programs and will lead to significant reductions in occupational radiation
exposure to personnel and associated inspection costs. In almost all cases, use
of EPRI methods will also result in a reduction in risk. In those cases in which
some increase in risk could occur, we believe it will be very small and well within
the guidelines in Regulatory Guide 1.174.
Although the Westinghouse Owners Group (WOG) and EPRI risk-informed ISI
methods will result in significant improvements in piping inspection programs, it
may be possible to further reduce the number and frequency of inspections in
the future with little or no increase in risk. Inspections are prioritized by
relative risk ranking regardless of the absolute level of the risk involved which,
in most cases, is very small. Consequently, excessive inspection resources may
still be expended on systems like PWR primary piping which has no known active
modes of degradation. In many cases, it can be shown that PWR primary piping
has leak-before-break behavior. In contrast, fewer inspection resources are
devoted currently to systems with less relative risk importance but with active
modes of degradation such as flow-assisted corrosion or thermal fatigue and a
much higher probability of failure.
Discussion
Although piping constitutes a significant portion of the reactor coolant system boundary,
because of its robust design and the protection afforded by other engineered safety
systems, piping failures generally make relatively small contributions to core damage
frequency (CDF) or large, early release frequency (LERF). Therefore, even "perfect"
piping ISI programs would lead to only small risk reductions.
Some ACRS members believe that, because of the low risk significance associated with
piping failures, the current approach to risk-informed ISI as expressed in the EPRI and
WOG methods and the current ASME Code Cases is overly timid and that it would be
appropriate to make more drastic changes in ISI programs. The number and frequency
of inspections could be further reduced without having a significant impact on risk.
Instead of prioritizing ISI in terms of relative risk and frequency of failure, the
inspections could be prioritized to reduce the total number of piping failures and forego
the attempts to distinguish between piping segments virtually all of which have low risk
significance. This could, for example, lead to a reduction of inspection resources
expended on systems like PWR primary piping which has no known active modes of
degradation. In many cases, it can be shown that PWR primary piping exhibits leak-
before-break behavior. On the other hand, fewer inspection resources are devoted to
systems with less relative risk importance but with active modes of degradation such as
flow-assisted corrosion or thermal fatigue and a much higher (several orders of
magnitude) probability of failure.
Other ACRS members believe that it is prudent to retain relative risk significance as an
important element in design of the ISI program, and that the EPRI and WOG methods
for the development and implementation of risk-informed ISI programs are reasonable.
Continued refinement of risk-informed ISI programs is possible so that, for example, the
augmented inspection requirements which are currently excluded from both programs
could be included in a single integrated program. We believe, however, that such
activities should not impede the timely implementation of programs resulting from the
use of EPRI and WOG methods by licensees.
Sincerely,
/s/
Dana A. Powers
Chairman
References
Memorandum dated August 12, 1999, from William H. Bateman and Richard J.
Barrett, Office of Nuclear Reactor Regulation, to John T. Larkins, Executive
Director, Advisory Committee on Reactor Safeguards, Subject: Safety
Evaluation Report Related to EPRI Risk-Informed Inservice Inspection
Evaluation Procedure (EPRI TR-112657, Revision B, July 1999).
Electric Power Research Institute, EPRI TR-112657, Revision B, WO3230, Final
Report, "Revised Risk-Informed Inservice Inspection Evaluation Procedure," July
1999.
Letter dated July 13, 1999, from Jeff Mitman, Electric Power Research Institute,
to Mike Markley, Advisory Committee on Reactor Safeguards, Subject: EPRI
Risk-Informed In-Service Inspection Procedure Discussion.
U.S. Nuclear Regulatory Commission, Regulatory Guide 1.174, "An Approach for
Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-
Specific Changes to the Licensing Basis," July 1998.
U.S. Nuclear Regulatory Commission, Regulatory Guide 1.178, "An Approach for
Plant-Specific Risk-Informed Decisionmaking Inservice Inspection of Piping,"
issued for trial use September 1998.
American Society of Mechanical Engineers, "Case N-560, Alternative
Examination Requirements for Class 1, B-J Piping Welds, Section XI, Division 1,"
August 9, 1996.
American Society of Mechanical Engineers, "Case N-578, Risk Informed
Requirements for Class 1, 2, and 3 Piping, Method B, Section XI, Division 1,"
September 2, 1997.
Westinghouse Energy Systems, WCAP-14572, Revision 1, "Westinghouse
Owners Group Application of Risk-Informed Methods to Piping Inservice
Inspection Topical Report," October 1997.
Westinghouse Energy Systems, WCAP-14572, Revision 1, Supplement 1,
"Westinghouse Structural Reliability and Risk Assessment (SRRA) Model for
Piping Risk-Informed Inservice Inspection," October 1997.
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