Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
September 8, 1989
Information Notice No. 89-65: POTENTIAL FOR STRESS CORROSION CRACKING IN
STEAM GENERATOR TUBE PLUGS SUPPLIED BY
BABCOCK AND WILCOX
Addressees:
All holders of operating licenses or construction permits for
pressurized-water reactors (PWRs).
Purpose:
The purpose of this information notice is to inform licensees of the status of
primary water stress corrosion cracking (PWSCC) problems being experienced
with steam generator tube plugs supplied by Babcock and Wilcox (B&W) and, in
particular, to alert licensees to the results of recent inspections of B&W
supplied plugs at McGuire Unit 2. PWSCC can potentially impair the integrity
of the plugs, which serve as part of the reactor coolant system (RCS)
boundary. It is expected that recipients will review the information for
applicability to their facilities and consider actions, as appropriate, to
avoid similar problems. However, suggestions contained in this information
notice do not constitute NRC requirements; therefore, no specific action or
written response is required.
Description of Circumstances:
This information notice applies to steam generator tube plugs of the "rolled,"
"ribbed," and "taper welded" designs supplied by B&W. The staff has
previously issued generic correspondence on the subject of steam generator
tube plugs supplied by Westinghouse, namely, NRC Information Notice 89-33,
"Potential Failure of Westinghouse Steam Generator Tube Mechanical Plugs," and
NRC Bulletin 89-01, "Failure of Westinghouse Steam Generator Tube Mechanical
Plugs."
During a refueling outage inspection at McGuire Unit 2 in July 1989, eddy
current inspections with a rotating pancake coil (RPC) probe revealed PWSCC
indications in B&W rolled plugs fabricated from Inconel 600 heats W592-1,
W945-1, and 2848-2. Such cracking had previously been identified at V. C.
Summer, McGuire Unit 1, and Oconee Unit 1 for B&W rolled plugs supplied from
heat W592-1. The indications at McGuire Unit 2 in plugs supplied from heats
W945-1 and 2848-2 are the first reported PWSCC indications in B&W plugs
involving heats other than W592-1.
8909010338
. IN 89-65
September 8, 1989
Page 2 of 4
The susceptibility of B&W plugs to PWSCC was first reported to the staff by
B&W letters dated September 6, 1988, and February 3, 1989. B&W provided
additional details of this problem in a meeting with the NRC staff on April
21, 1989. At that time, known instances of cracking in B&W plugs were limited
to rolled plugs fabricated from Inconel 600 heat W592-1. Investigation by B&W
established that material from heat W592-1 exhibits a microstructure
characterized by intragranular carbides with little intergranular carbide
precipitation. B&W believes that this lack of intergranular carbide
precipitation indicates that heat W592-1 is susceptible to PWSCC attack. B&W
reported that it had also examined archived plugs from all other heats used to
fabricate plugs (except heats NX2960 and 38131-14) and concluded that all the
other heats exhibit the proper carbide distribution and therefore they should
have adequate PWSCC resistance. Plug material from heats NX2960 and 38131-14
had not been characterized at the time of the April 21, 1989, meeting with the
staff.
All cracks observed in the field in rolled plugs, as of April 21, 1989, were
circumferentially oriented and were located at the "heel" (i.e., outermost)
transition region, which is not part of the pressure boundary, rather than at
the "toe" (i.e., innermost) transition region, which is part of the plug (and
RCS) pressure boundary (See Figure 1). Because of their location, B&W does
not believe that these heel transition cracks impair plug integrity.
B&W provided the NRC staff, in a letter dated June 12, 1989, its
recommendations to utility customers. These recommendations included the
inspection of rolled plugs, both hot-and cold-leg, from heat W592-1 as well as
sample inspections of rolled plugs from other heats. B&W ribbed plugs cannot
be inspected with eddy current testing. Previously in a letter to the NRC
staff dated May 19, 1989, B&W concluded that the ribbed plug design was not
subject to a plug top release failure mode because of relatively low stresses
in the plug associated with its installation. Nevertheless in its letter of
June 12, 1989, B&W recommended the removal and replacement of all ribbed plugs
fabricated from heat W592-1 at the next scheduled refueling outage. Regarding
B&W taper welded plugs, B&W believes that the design of these plugs is such
that there is no mechanism for a gross failure (i.e., plug top release
failure). B&W noted that taper welded plugs from heat W592-1 may crack in the
future and recommended their removal at a convenient time unless some other
course of action is developed to eliminate potential future concerns regarding
the integrity of these plugs.
B&W also stated in its letter of June 12, 1989, that recent industry
experience raises a concern that examination of material microstructures might
not reveal all heats of material susceptible to PWSCC. Therefore, B&W is
planning additional corrosion tests to (1) rank the relative resistance of the
various microstructures of plug material to PWSCC, (2) establish the minimum
expected service life of plugs already installed, and (3) determine if any
other heat of plug material (besides W592-1) requires extensive examination or
near-term removal. This corrosion test program is scheduled for completion by
December 1989.
The following information concerning the recent eddy current inspections of
the plugs at McGuire Unit 2 has been provided informally to the staff by the
licensee and is therefore preliminary. The B&W plugs at McGuire Unit 2 were
. IN 89-65
September 8, 1989
Page 3 of 4
supplied from three heats: W592-1, W945-1, and 2848-2. All rolled plugs
installed on the hot-leg side and a sample of plugs installed on the cold-leg
side were inspected at McGuire Unit 2. Regarding plugs from heat W592-1, 30
on the hot-leg side were identified to have eddy current indications. These
indications involved only plugs installed on or before March 1986. No indica-
tions were found in plugs from heat W592-1 that were installed after March
1986. As a conservative measure, the licensee replaced all hot-leg plugs from
heat W592-1 installed on or before March 1986 regardless of whether these
plugs exhibited eddy current indications.
Regarding plugs from the heats other than W592-1, 16 plugs on the hot-leg side
fabricated from heat W945-1 and 3 on the hot-leg side fabricated from heat
2848-2 exhibited eddy current indications and were replaced. The plugs with
indications that were fabricated from heat W945-1 were installed in March
1986. No indications were found in plugs from this heat that were installed
after March 1986. Plugs from heat 2848-2, including those with indications,
were installed in June 1988.
No indications were found in rolled plugs located on the cold-leg side.
With the exception of three plugs, all indications found during the recent
McGuire Unit 2 inspections of the rolled plugs were located at the heel
transition region. Examination of a sample of these plugs after they were
removed from the field confirmed the presence of circumferential cracks at the
heel transition region. Three plugs (from heat W592-1) found during field
eddy current testing to contain indications at the heel transition region were
also found to contain indications at the toe transition region. However, pre-
liminary findings from examinations after the plugs were removed indicate that
these indications at the toe transition region are not associated with cracks.
It is the staff's understanding that the source of these indications is still
under investigation.
Two ribbed plugs from heat W592-1 were replaced (with rolled plugs) since they
could not be inspected by eddy current testing. Examination of these ribbed
plugs after they were removed (plus four ribbed plugs removed earlier from
McGuire Unit 1) did not reveal any cracks. No ribbed or taper welded plugs
are currently in service at McGuire Unit 2.
Discussion:
Although known instances of cracking in B&W rolled plugs have been limited to
the heel transition region (which is not part of the pressure boundary), such
cracks may be a precursor to eventual cracks at the toe transition region
(which is part of the pressure boundary). Therefore, eddy current inspections
to identify heel as well as any toe transition cracks are important for
ensuring the continued integrity of the B&W rolled plugs. Furthermore, until
additional evidence becomes available from corrosion tests and/or experience,
all Inconel 600 heats used for plugs should be considered potentially
susceptible to PWSCC.
. IN 89-65
September 8, 1989
Page 4 of 4
Although B&W plugs of the ribbed and taper welded design may have less
potential for developing PWSCC over the short term compared with rolled plugs
because of differences in design and stress level, it is important that
licensees consider how the integrity of these plugs is to be ensured over the
short and long term.
This information notice requires no specific action or written response. If
you have any questions about the information in this notice, please contact
the technical contact listed below or the appropriate NRR project manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contact: E. Murphy, NRR
(301) 492-0945
Attachments:
1. Figure 1: Rolled Plug
2. List of Recently Issued NRC Information Notices
. Attachment 2
IN 89-65
September 8, 1989
Page 1 of 1
LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
______________________________________________________________________________
Information Date of
Notice No._____Subject_______________________Issuance_______Issued to_________
89-64 Electrical Bus Bar Failures 9/7/89 All holders of OLs
or CPs for nuclear
power reactors.
89-63 Possible Submergence of 9/5/89 All holders of OLs
Electrical Circuits Located or CPs for nuclear
Above the Flood Level Because power reactors.
of Water Intrusion and Lack
of Drainage
89-62 Malfunction of Borg-Warner 8/31/89 All holders of OLs
Pressure Seal Bonnet Check or CPs for nuclear
Valves Caused By Vertical power reactors.
Misalignment of Disk
89-61 Failure of Borg-Warner Gate 8/30/89 All holders of OLs
Valves to Close Against or CPs for nuclear
Differential Pressure power reactors.
88-48, Licensee Report of Defective 8/22/89 All holders of OLs
Supp. 2 Refurbished Valves or CPs for nuclear
power reactors.
89-60 Maintenance of Teletherapy 8/18/89 All NRC Medical
Units Teletherapy
Licensees.
89-59 Suppliers of Potentially 8/16/89 All holders of OLs
Misrepresented Fasteners or CPs for nuclear
power reactors.
89-58 Disablement of Turbine-Driven 8/3/89 All holders of OLs
Auxiliary Feedwater Pump Due or CPs for PWRs.
to Closure of One of the
Parallel Steam Supply Valves
89-57 Unqualified Electrical 7/26/89 All holders of OLs
Splices in Vendor-Supplied or CPs for nuclear
Environmentally Qualified power reactors.
Equipment
______________________________________________________________________________
OL = Operating License
CP = Construction Permit
..
