United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 89-33: Potential Failure of Westinghouse Steam Generator Tube Mechanical Plugs

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

                                 March 23, 1989


Information Notice No. 89-33:  POTENTIAL FAILURE OF WESTINGHOUSE STEAM
                                   GENERATOR TUBE MECHANICAL PLUGS


Addressees:

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

Purpose:

This information notice is being provided to alert addressees to the potential 
for failure of Westinghouse steam generator tube mechanical plugs.  Such 
failures could lead to a large primary-to-secondary leak and possible damage 
to adjacent tubes.  It is expected that recipients will review the information 
for applica-bility to their facilities and consider actions, as appropriate, to 
avoid similar problems.  However, suggestions contained in this information 
notice do not con-stitute NRC requirements; therefore, no specific action or 
written response is required. 

Description of Circumstances:

On February 25, 1989, North Anna Unit 1 experienced a large 
primary-to-secondary leak of about 70 gallons per minute (gpm) following a 
reactor trip from 76 percent power.  Unit response to the trip was normal.  No 
safety injection (manual or automatic) was initiated or required during the 
event, and post-trip cooldown and depressurization were conducted in a stable 
manner. 

Primary-to-secondary leakage in the affected steam generator (S/G) had been
negligible (< 1.0 gallon per day) with no increasing trend before the reactor 
trip. 

The licensee's investigation performed after plant shutdown showed the leak 
was associated with a broken plug located in the hot leg of tube R3C60.  This 
tube had been plugged in 1985 because a 55 percent indication had been found 
at the first support plate location.  The broken plug was a "mechanical" plug 
supplied by Westinghouse.  The top portion of the plug was completely severed 
from the body of the plug, which was expanded against the tube and tubesheet.  
The severed portion of the plug was apparently propelled up the length of the 
tube by the primary system pressure to a location just above the U-bend 
transition where it impacted the outer curvature of the tube approximately 4 
inches above the seventh support plate. 




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Further investigation with a modified Welsh-Allyn video probe showed that the
impact of the broken plug piece had punctured the tube over an area approxi-
mately 2� inches long and 3/4 inches wide.  The broken plug piece subsequently
impacted and dented the adjacent tube R4C60 directly above.

Discussion:

Westinghouse informed several licensees on January 17, 1989 that a few 
utilities had observed dripping or wetness around tube ends plugged with 
Westinghouse mechanical plugs.  When some of these plugs were removed and 
examined, inter-granular cracks were found.  Westinghouse reported that these 
cracks appeared to be associated with minimal intergranular carbide 
precipitation, which, in turn, may be the result of a low mill annealing 
temperature. 

Preliminary information obtained informally from Westinghouse indicates that
plugs from two heats (numbers 3513 and 3962) may exhibit this susceptible
microstructure.  The broken plug at North Anna Unit 1 was fabricated from
heat number 3962.

The licensee for North Anna Unit 1 is currently determining where other plugs
from the susceptible heats may be installed at Units 1 and 2 and what remedial
measures need to be taken.

During a recent inspection of steam generator tube plugs at Millstone Unit 2 
(a Combustion Engineering-designed reactor), in response to the North Anna 
event, the licensee removed four similar Westinghouse mechanical plugs.  
Current indi-cations are that these plugs were from the suspected susceptible 
heats.  One of the plugs severed during the removal operation as a result of a 
circumferential crack. 

The NRC staff is currently investigating the potential generic implications of 
these plug failures. 

No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the technical 
contact listed below or the Regional Administrator of the appropriate regional 
office. 




                                        Charles E. Rossi, Director
                                        Division of Operational Events Assessment
                                        Office of Nuclear Reactor Regulation

Technical Contact:  E. Murphy, NRR
                    (301) 492-0945

Attachment:  List of Recently Issued NRC Information Notices
.                                                            Attachment 
                                                            IN 89-33
                                                            March 23, 1989
                                                            Page 1 of 1

                             LIST OF RECENTLY ISSUED
                             NRC INFORMATION NOTICES
_____________________________________________________________________________
Information                                  Date of 
Notice No._____Subject_______________________Issuance_______Issued to________

89-32          Surveillance Testing          3/23/89        All holders of OLs
               of Low-Temperature                           or CPs for PWRs.
               Overpressure-Protection
               Systems

89-31          Swelling and Cracking         3/22/89        All holders of OLs
               of Hafnium Control Rods                      or CPs for PWRs 
                                                            with Hafnium 
                                                            control rods. 

89-30          High Temperature              3/15/89        All holders of OLs
               Environments at                              or CPs for nuclear
               Nuclear Power Plants                         power reactors.

89-29          Potential Failure of          3/15/89        All holders of OLs
               ASEA Brown Boveri                            or CPs for nuclear
               Circuit Breakers                             power reactors.
               During Seismic Event

89-28          Weight and Center of          3/14/89        All holders of OLs
               Gravity Discrepancies                        or CPs for nuclear
               for Copes-Vulcan                             power reactors.
               Air-Operated Valves

89-27          Limitations on the Use        3/8/89         All holders of OLs
               of Waste Forms and High                      or CPs for nuclear
               Integrity Containers for                     power reactors, 
               the Disposal of Low-Level                    fuel cycle 
               Radioactive Waste                            licenses and 
                                                            certain by-product 
                                                            materials licenses.

89-26          Instrument Air Supply to      3/7/89         All holders of OLs
               Safety-Related Equipment                     or CPs for nuclear
                                                            power reactors.

89-25          Unauthorized Transfer of      3/7/89         All U.S. NRC 
               Ownership or Control of                      source, byproduct, 
               Licensed Activities                          and special 
                                                            nuclear material 
                                                            licensees.
______________________________________________________________________________
OL = Operating License
CP = Construction Permit 
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