Bulletin 89-01: Supplement 1, Failure of Westinghouse Steam Generator Tube Mechanical Plugs

                                                OMB No.:  3150-0011 
                                                NRCB 89-01, Supplement 1 

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF NUCLEAR REACTOR REGULATION
                           WASHINGTON, D.C.  20555

                              November 14, 1990


NRC BULLETIN NO. 89-01, SUPPLEMENT 1:  FAILURE OF WESTINGHOUSE STEAM 
                                       GENERATOR TUBE MECHANICAL PLUGS 


Addressees: 

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

Purpose:

This bulletin supplement is intended to inform addressees of recent problems 
involving steam generator (SG) tube mechanical plugs supplied by 
Westinghouse Electric Corporation.  This supplement updates information 
provided in Bulletin 89-01, "Failure of Westinghouse Steam Generator Tube 
Mechanical Plugs."  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 bulletin supplement do not constitute NRC requirements; therefore, no 
specific action or written response is required.

Description of Circumstances:

NRC Bulletin 89-01 requested that licensees repair and/or replace mechanical 
plugs supplied by Westinghouse.  This request applied only to plugs 
fabricated from Inconel 600 heats NX-3279, NX-3513, NX-3962, and NX-4523 
(i.e., "bulletin heats") on the basis of field experience and laboratory 
studies indicating that plugs from these heats are highly susceptible to 
primary water stress corrosion cracking (PWSCC).  Such cracking led to a 
gross plug failure at the North Anna Power Station, Unit 1, resulting in a 
75-gallon-per-minute primary-to-secondary leak.  The bulletin requested that 
the subject repairs and/or replacements be accomplished according to a 
schedule consistent with an algorithm developed by Westinghouse 
(Reference 1, Revision 1), using the most conservative corrosion rate data 
from the field (observed at Millstone Unit 2 for a plug fabricated from heat 
NX-3513) as a benchmark.

After the NRC issued Bulletin 89-01, Westinghouse issued Revision 3 of 
Reference 1 providing complete listings of plug lifetimes categorized by 
plant, date of installation, and heat number.  The listed lifetimes of plugs 
fabricated from the bulletin heats were consistent with the approach 
requested in the bulletin.  Revision 3 of Reference 1 also included a 
complete listing of lifetimes for plugs fabricated from all non-bulletin 
Inconel 600 heats.  These lifetimes were estimated using the approach used 
for plugs from the bulletin heats, adjusted upward by heat-specific 
"performance factors" to reflect the 


9011070328 
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                                                NRCB 89-01, Supplement 1 
                                                November 14, 1990 
                                                Page 2 of 3 


comparative time-to-cracking performance of each non-bulletin heat with heat 
NX-3513 during Westinghouse corrosion tests.

The staff has received reports of two instances of PWSCC affecting 
non-bulletin heats of Inconel 600 during the summer and fall of 1990.  At 
the Sequoyah Nuclear Power Plant, Unit 1, five plugs fabricated from heat 
NX-5222 were removed from the field to evaluate the performance of these 
plugs in the laboratory.  One of these five plugs was found to exhibit a 
circumferential crack above the expander.  This crack consisted of two small 
cracks with a total length of approximately 15 degrees around the tube 
circumference and a maximum depth of penetration of 0.009 inch.  One of the 
remaining four plugs was found to exhibit axial cracks below the expander.  
These five plugs had accumulated only 21 percent of the calculated plug 
lifetime, as given in Revision 3 of Reference 1.  

At North Anna Unit 2, 15 plugs fabricated from heat NX-6323(HR) were 
inspected using a Westinghouse eddy current test technique.  Nine of these 
plugs were located on the hot-leg side, five of which exhibited evidence of 
minor leakage.  Eight of the nine hot-leg plugs exhibited indications of 
axial and/or circumferential cracking above the expander.  One of the plugs, 
which was removed from the field and examined, was found to contain a crack 
that extended 360 degrees around the plug circumference.  The crack varied 
in depth between 74 percent and 99 percent of the plug wall thickness.  No 
indications were found in the six cold-leg plugs that were inspected.  The 
accumulated service time on these plugs was less than 20 percent of the 
calculated lifetime, as given in Revision 3 of Reference 1.

Discussion: 

Westinghouse has reassessed its lifetime estimates for mechanical plugs 
fabricated from non-bulletin heats of Inconel 600 in light of the recent 
field information from Sequoyah Unit 1 and North Anna Unit 2.  Westinghouse 
plans to issue the results of this reassessment by mid-November 1990.  This 
reassessment will include new lifetime estimates for all plugs fabricated 
from non-bulletin heats.  Based on an October 11, 1990, meeting with 
Westinghouse, the staff expects that the new lifetime estimates will be 
substantially lower than those given in Revision 3 of Reference 1 for plugs 
fabricated from the non-bulletin heats.  

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                                                NRCB 89-01, Supplement 1 
                                                November 14, 1990 
                                                Page 3 of 3 


This bulletin supplement requires no specific action or written response.  
If you have any questions about the information in this supplement, 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)  492-0710


References:

1.   Westinghouse Reports WCAP-12244 (proprietary version) and WCAP-12245 
     (non-proprietary version), "Steam Generator Tube Plug Integrity Summary 
     Report"; Revision 1, April 1989; Revision 3, November 1989.


Attachment:
1.  List of Recently Issued NRC Bulletins 
.