United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 91-18: High-Energy Piping Failures caused by Wall Thinning

                                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. 


9103060153 
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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 
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                                                            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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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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Page Last Reviewed/Updated Tuesday, November 12, 2013