Information Notice No. 93-52: Draft NUREG-1477, "Voltage-Based Interim Plugging Criteria for Steam Generator Tubes"

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                            WASHINGTON, D.C.  20555

                                 July 14, 1993



All holders of operating licenses or construction permits for pressurized
water reactors (PWRs).      


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to inform addressees that draft NUREG-1477, "Voltage-Based Interim
Plugging Criteria for Steam Generator Tubes," is available for review and
comment.  The draft report will be the basis for a final NUREG-1477 to be
issued on the use of voltage-based interim plugging criteria to address outer
diameter stress corrosion cracking (ODSCC) at the tube-to-tube support plate
intersections in steam generators.  This information notice contains no NRC
requirements, and no specific action or written response is required.


On July 2, 1993, the NRC published in the Federal Register (58 FR 35985) a
notice that the above draft report on interim tube plugging criteria was
available for public comment.  The conclusions and recommendations of the
draft report are preliminary and comments received will be incorporated as
applicable.  The draft report was prepared by an NRC task group established to
review the technical bases for interim approval of voltage-based plugging
criteria for application to ODSCC, to review the outstanding issues associated
with those criteria, and to prepare conclusions and recommendations for
implementing those criteria.  The report indicates that the NRC considers most
of these issues to be relevant to the long-term approval of voltage-based
plugging criteria.  The task group was composed of NRC staff from the Offices
of Nuclear Reactor Regulation and Nuclear Regulatory Research, and consultants
from NRC contractors.  The task group reviewed and evaluated tube integrity
issues, including the potential for tube rupture or leakage under postulated
accident conditions and their effect on safety.  The task group also assessed
outstanding technical issues and concerns that NRC staff members raised
regarding voltage-based plugging criteria for ODSCC.

The task group assessed tube integrity issues including (1) the ODSCC
mechanism, (2) methods for evaluating margins against tube rupture and
estimating potential primary-to-secondary leakage, (3) operational primary-to-
secondary leakage limits, and (4) inservice inspection methods and scope.  In


                                                            IN 93-52
                                                            July 14, 1993
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performing the assessment for item (2), the task group evaluated an NRC model
and an industry model.  The NRC model used a traditional mechanistic
approach for assessing tube integrity, while the industry model used an
empirical approach based on the voltage amplitude of eddy current
measurements.  Analyses performed using these models predicted significantly
different primary-to-secondary leakage rates for postulated accident
conditions.  Understanding and resolving this difference was important for the
task group to assess the safety implications with regard to offsite
radiological doses and core integrity.

To determine the safety implications of the interim plugging criteria, the
task group performed calculations to assess the effect of a range of assumed
primary-to-secondary leak rates on radiological doses and the potential for
core damage associated with them.  The task group also performed a risk
assessment to determine the safety significance of ODSCC of steam generator

The draft report documents the results of the task group effort including
preliminary conclusions and recommendations for implementing voltage-based
interim plugging criteria.  The draft report has been assigned NRC accession
number 9306030163 and is available in the NRC public document rooms.  The
Federal Register notes that the NRC staff will consider all public comments on
the draft report received within 45 days of the date of the Federal Register
notice and will revise the draft report as appropriate.  The final report will
be the technical basis for a forthcoming NRC staff generic position on the use
of voltage-based plugging criteria to address ODSCC at the tube-to-tube
support plate intersections.  The NRC staff will prepare this generic position
in accordance with applicable NRC procedures.

If you wish to comment on the draft report, address your comments to the U.S.
Nuclear Regulatory Commission, ATTN:  Document Control Desk, Washington, D.C. 
20555.  Comments should be provided within 45 days of the date of the 
Federal Register notice to be considered in the revision to the report..

                                                            IN 93-52
                                                            July 14, 1993
                                                            Page 3 of 3

This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.  

                                    ORIGINAL SIGNED BY

                                    Brian K. Grimes, Director
                                    Division of Operating Reactor Support
                                    Office of Nuclear Reactor Regulation

Technical contacts:  Jack Strosnider, NRR
                     (301) 504-2795

                     Richard Lobel, NRR
                     (301) 504-2865

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