Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
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.
8903230075
. IN 89-33
March 23, 1989
Page 2 of 2
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
..
