Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
November 10, 1988
Information Notice No. 86-106, SUPPLEMENT 3: FEEDWATER LINE BREAK
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
This supplement to Information Notice (IN) 86-106 is intended to provide
addressees with additional information about a potential problem that
resulted in thinning of secondary system piping at both units of an
operating nuclear power station and the catastrophic failure of a main
feedwater suction pipe. 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.
Discussion:
IN 86-106, Supplement 1, was issued on February 13, 1987, to provide
additional information regarding the catastrophic feedwater line break at
Surry Power Station, Unit 2. Results of the licensee's failure analysis and
the NRC technical panel conclusions concerning the pipe break failure
mechanism were discussed in Supplement 1. Supplement 2 addressed system
interactions.
During the September 1988 outage, the Surry licensee discovered that pipe
wall thinning had occurred more rapidly than expected. On the suction side
of one of the main feedwater pumps, an elbow that was installed during the
1987 refueling outage lost 20 percent of its 0.500 inch wall in 1.2 years.
In addition, wall thinning is continuing in safety-related main feedwater
piping and in other non-safety-related condensate piping.
On the basis of partial inspection results, the licensee indicated that the
broad area thinning rate for the replacement piping, installed during the
last refueling outage, is roughly 60 mils/year. The maximum localized
thinning rate is 90 mils/year. These rates were higher than the 20 to 30
mils/ year rate estimated previously. The estimated rate of 20 to 30
mils/year was based on a single measurement and an assumption that wall
thinning had been
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IN 86-106, Supplement 3
November 10, 1988
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progressing linearly since initial full-power operation was achieved. This
new rate of wall thinning, which is based on a second data point, indicates
that significant wall thinning may have coincided with a reduction in
feedwater dissolved-oxygen concentration subsequent to steam generator
replacement. The lower rate of wall thinning associated with a higher
feedwater dissolved-oxygen concentration is consistent with, the low rates
of erosion/ corrosion reported in IN 88-17, "Summary of Responses to NRCB
87-01, 'Thinning of Pipe Walls in Nuclear Power Plants'," for boiling water
reactors (BWRs), which typically operate at a feedwater dissolved-oxygen
concentration of approximately 30 ppb. The licensee is continuing its
failure analysis to determine the cause(s) of the increase in the estimated
pipe wall thinning rate. Because the measured rate of pipe wall thinning is
in excess of the previously estimated rate, the scheduled frequency of
future inspections may need to be reexamined to ensure that code-allowable
wall thickness is maintained.
Additional information pertaining to erosion/corrosion in feedwater-conden-
sate system piping can be found in Information Notice No. Nos. 86-106;
86-106, Supplement 1; 87-36, "Significant Unexpected Erosion of Feedwater
Lines"; 88-17; and NRC Bulletin 87-01, "Thinning of Pipe Walls in Nuclear
Power Plants." 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: Paul Wu, NRR
(301)492-0826
Attachment: List of Recently Issued NRC Information Notices
