United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 89-65: Potential for Stress Corrosion Cracking in Steam Generator Tube Plugs Supplied by Babcock and Wilcox

                                  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 
..
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