Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
March 12, 1991
Information Notice No. 91-18: HIGH-ENERGY PIPING FAILURES CAUSED BY
WALL THINNING
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
This information notice is intended to alert addressees to continuing
erosion/corrosion problems affecting the integrity of high-energy piping
systems and apparently inadequate monitoring programs. The piping failures
at domestic plants indicate that, despite implementation of long-term
monitoring programs pursuant to Generic Letter 89-08, "Erosion/Corrosion-
Induced Pipe Wall Thinning," piping failures caused by wall thinning
continue to occur in operating plants. 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:
On December 31, 1990, while Unit 3 of the Millstone Nuclear Power Station
was operating at 86-percent power, two 6-inch, schedule 40 pipes, in the
moisture separator drain (MSD) system, ruptured. The high-energy water
(approximately 360 degrees F, 600 psi) flashed to steam and actuated
portions of the turbine building fire protection deluge system. Two
480-volt motor control centers and one non-vital 120-volt inverter were
rendered inoperable by the flooding, resulting in the loss of the plant
process computer and the isolation of the instrument air to the containment
building.
0n July 2, 1990, while Unit 2 of the San Onofre Nuclear Generating Station
was operating at full power, the licensee discovered a steam leak in one of
the feedwater regulating valve (FRV) bypass lines. The licensee shut down
the reactor to depressurize the line for inspection and repair. Ultrasonic
testing (UT) revealed wall thinning in an area immediately downstream of the
weld attaching the 6-inch bypass line to the 20-inch feedwater piping.
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March 12, 1991
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On March 23, 1990, at Unit 1 of the Surry Power Station, a straight section
of piping, downstream of a level control valve in the low pressure heater
drain (LPHD) system, ruptured. Measurement of the piping revealed that it
had thinned to 0.009 inch at the rupture.
On May 28, 1990, at Loviisa, Unit 1, a foreign plant, a flow-measuring
orifice flange in the main feedwater system ruptured. The rupture occurred
after one of the five main feedwater pumps tripped causing a check valve in
the line to slam shut, creating a pressure spike. The utility inspected the
flange and found that the flange had thinned to approximately 0.039 inch.
After inspecting the other flow-orifice flanges in Units 1 and 2, the
utility determined that 9 of 10 flanges had been thinned to below minimum
wall requirements.
Discussion:
For all of these events, system temperature was in the range of 280 to 445
degrees F, system pressure was 500 to 1080 psi, flow was 9 to 29 feet per
second and the piping material was carbon steel. Also, in each event, flow
turbulence was present.
The licensee for Millstone Unit 3 had noted a through-wall leak
approximately two inches from the level control valve in train A of the MSD
system and was preparing to isolate the line for repair. However, when MSD
pump A was secured, a pressure transient resulted, causing MSD trains A and
B to rupture. Information obtained from the licensee indicates that in both
trains, the ruptured piping had thinned to approximately 20 mils near the
level control valve. Although the licensee had identified the MSD system as
one of the systems to be analyzed for erosion/corrosion susceptibility, that
analysis was not performed because of a communication error. The spool
piece numbers for the MSD system were incorrectly listed under the moisture
separator reheater drain system which was exempted from analysis because of
temperature. The licensee has analyzed the MSD system using the Electric
Power Research Institute computer code CHEC and determined that the MSD
system is highly susceptible to erosion/corrosion and should have been
inspected.
At San Onofre Unit 2, the licensee's erosion/corrosion monitoring program
had excluded the FRV bypass lines from inspection for wall thinning based on
the system temperature (445 degrees F) exceeding a criterion established by
the licensee. However, the thinning of the FRV bypass lines demonstrates
that erosion/corrosion is a multi-variable phenomena and that exclusion
based on one variable may not be appropriate. The variables of piping
material, configuration, flow rate, water temperature, water chemistry (pH,
pH control agent, dissolved oxygen), and steam quality for steam/water
systems are important when evaluating piping systems for erosion/corrosion
susceptibility.
At Surry Unit 1, the pipe failure occurred in a straight section of pipe
located just downstream of a level control valve in the 2B low pressure
heater drain (LPHD) system. The licensee's erosion/corrosion monitoring
program included the LPHD system and provided for inspecting the wall
thickness of the pipe elbow located immediately downstream of the failed
piping. However, the program did not provide an inspection for the short
section of piping between
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IN 91-18
March 12, 1991
Page 3 of 4
the elbow and the level control valve. After the pipe rupture occurred in
train B, the licensee performed UT inspections of the same section in train
A of the LPHD system and found that it had thinned to approximately 0.052
inch. The design requirement for minimum wall thickness in that pipe is
0.117 inch. The licensee replaced the damaged pipe with A106 grade B
material and intends to replace that material with A335-P22 erosion
resistant material during the next outage.
The licensee performed an analysis and found that the erosion/corrosion of
the failed piping was caused by a combination of high velocity flow, a pH
level of 9.0 or less in the heater drain system, and flow turbulence caused
by valve throttling.
The feedwater pipe rupture at Loviisa Unit 1 occurred in the flange of the
flow-measuring orifice (Figure 1). The 360-degree thinning of the interior
wall of the flange started near the orifice plate and increased to the point
of the rupture. In the area of the rupture, the flange wall had thinned to
0.039 inch. A 20 inch long pipe section attached to the downstream end of
the flange had circumferential wall thinning from an initial wall thickness
of 0.7 inch to a residual wall thickness of 0.195 - 0.390 inch. Neither
this section of pipe nor the flange contained significant amounts of
alloying elements. However, the piping downstream of the 20 inch pipe,
which contained 0.20 percent chromium, 0.30 percent nickel and 0.30 percent
copper, did not exhibit wall thinning.
The utility conducted an investigation and determined that the thinning was
caused by erosion/corrosion. In 1982, the utility established a pipe
inspection program for two phase (steam/water) systems and, in 1986,
augmented the program to include single phase systems; however, the program
concentrated on pipe elbows and tee fittings. To check for other degraded
flanges, the utility inspected the flow-orifice flanges at Units 1 and 2 and
found that 9 of 10 flanges were below minimum wall requirements. The
utility replaced the flanges with the same material as the original flanges
but is considering changing to a more erosion/corrosion resistant material
as a final repair.
The NRC has issued the following related generic communications:
NRC Information Notice 86-106, "Feedwater Line Break," December 16, 1986,
and supplements 1, 2, and 3.
NRC Information Notice 87-36, "Significant Unexpected Erosion of Feedwater
Lines," August 4, 1987.
NRC Information Notice 88-17, "Summary of Responses to NRC Bulletin 87-01,
'Thinning of Pipe Walls in Nuclear Power Plants'," April 22, 1988.
NRC Bulletin 87-01, "Thinning of Pipe Walls in Nuclear Power Plants," July
9, 1987.
NRC Generic Letter 89-08, "Erosion/Corrosion-Induced Pipe Wall Thinning,"
May 4, 1989.
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IN 91-18
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This information notice requires no specific action or written response. If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate NRR project
manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: Stephen S. Koscielny, NRR
(301) 492-0726
Roger Woodruff, NRR
(301) 492-1152
Attachments:
1. Figure 1. Loviisa Unit-1 Erosion/Corrosion Areas
2. List of Recently Issued NRC Information Notices
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