Information Notice No. 91-18, Supplement 1: High-Energy Piping Failures caused by Wall Thinning

                                UNITED STATES
                           WASHINGTON, D.C.  20555

                              December 18, 1991

                                             CAUSED BY WALL THINNING


All holders of operating licenses or construction permits for nuclear power 


The U. S. Nuclear Regulatory Commission (NRC) is issuing this information 
notice to alert addressees to continuing erosion/corrosion problems 
affecting the integrity of high energy piping systems and to alert 
addressees to apparently inadequate erosion/corrosion monitoring programs.  
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 are not 
NRC requirements; therefore, no specific action or written response is 


On March 12, 1991, the NRC issued Information Notice (IN) 91-18, "High 
Energy Piping Failures Caused by Wall Thinning," to describe ruptures and 
leaks in secondary systems carrying high energy fluids at the Millstone 
Nuclear Power Station, Unit 3, the San Onofre Nuclear Generating Station, 
Unit 2, and a foreign plant.  

Following the pipe rupture at the Surry Power Station in 1986, the NRC 
issued Bulletin 87-01, "Thinning of Pipe Walls in Nuclear Power Plants," 
July 9, 1987.  In this bulletin, the staff requested licensees and 
applicants to inform the NRC about their programs for monitoring the wall 
thickness of carbon steel piping in both safety-related and nonsafety- 
related high energy fluid systems.  IN 91-18 included references related to 
this bulletin.  IN 82-22, "Failures of Turbine Exhaust Lines," July 9, 1982, 
also provides relevant information regarding pipe wall thinning in steam 

In 1989, following an audit of the erosion/corrosion programs at ten plants, 
the NRC issued Generic Letter (GL) 89-08, "Erosion/Corrosion-Induced Pipe 
Wall Thinning," May 2, 1989.  In this generic letter, the staff requested 
licensees and applicants to implement long term erosion/corrosion monitoring 
programs.  The staff made this request to obtain assurances that procedures 
or administrative controls were in place to maintain the structural 
integrity of all carbon steel systems carrying high energy fluids.  


                                                  IN 91-18, Supplement 1
                                                  December 18, 1991
                                                  Page 2 of 3

The Electric Power Research Institute (EPRI) released computer codes CHEC 
and CHECMATE in June 1987 and April 1989, respectively, to assist in 
selecting for testing those areas of the piping systems with highest 
probabilities for wall thinning.  The CHEC calculation applies to pipes 
containing a single liquid phase and the CHECMATE calculation applies to 
pipes containing both liquid and vapor phases.  

On June 11, 1987, the Technical Subcommittee Working Group on Piping 
Erosion/Corrosion of the Nuclear Management and Resources Council (NUMARC)
issued a summary report describing a method using the CHECMATE computer code 
for monitoring carbon steel components exposed to the conditions conducive 
to erosion/corrosion.  

Description of Circumstances

At Millstone Unit 2, on November 6, 1991, while the licensee, the Northeast 
Nuclear Energy Company (NNECo), was operating the plant at 100 percent of 
full power, a rupture occurred in train B of the moisture separator reheater 
(MSR) system.  An 8-inch elbow, located between the first stage MSR drain 
tank and the feedwater heater, ruptured at its extrados (Figure 1).  The 
elbow was located downstream from a 4-inch flow control valve and a 4- to 
8-inch expander.  The high energy water in the pipe (approximately 463 F, 
470 psig) flashed to steam, actuating portions of the turbine fire 
protection deluge system.  The water level in the steam generator decreased 
slightly.  The licensee had not selected the ruptured elbow for ultrasonic 
testing (UT) in its erosion/corrosion monitoring program.

The ruptured elbow was made of carbon steel with nominal wall thickness of 
0.322 inch.  Wall thickness at the failed area was eroded to 95 percent of 
the initial nominal value.  Other areas, away from the failed area, showed a 
loss of 22 percent of the wall thickness.  The identical elbow in the A 
train had a maximum wall loss of 34 percent of the initial nominal value.


The licensee has had a program for monitoring high energy fluid piping since 
1981.  The criteria for choosing components to be inspected include 
component location and service conditions as determined by the engineering 
judgement of the plant personnel.  In contrast, more relevant parameters 
that could indicate erosion or corrosion wastage include piping material and 
geometry, fluid properties (flow, temperature, and acidity), and fluid 
contents (the acidity-controlling agent and the concentration of dissolved 

In its response to GL 89-08, NNECo indicated that its Engineering Procedure 
EN-21153, "Thickness Testing of Secondary Piping," describes its monitoring 
program, established in accordance with EPRI guidelines and using the CHEC 
program to select for testing those areas of the piping systems with highest 
probabilities for wall thinning.  The licensee also selects areas on the 
basis of plant experience.  However, although the pipe wall thickness 
testing program was included in the licensee's procedures, the licensee had 
not implemented the methodology using the EPRI computer codes at Unit 2.  
This omission may account for the licensee not having tested previously the 
piping that ruptured on November 6, 1991.  

                                                  IN 91-18, Supplement 1
                                                  December 18, 1991
                                                  Page 3 of 3

The licensee did use an earlier version of the CHEC computer code in limited 
analyses at its other nuclear units.  The licensee's program did not reflect 
the use of current versions of EPRI codes (either CHEC or CHECMATE).  The 
Unit 3 rupture described in the March 12, 1991, issuance of this information 
notice prompted NNECo to commit to perform CHEC or CHECMATE analyses at all 
its units by December 31, 1991.  However, at the time of the second rupture 
on November 6, 1991, the licensee was implementing this program at its 
corporate engineering office and not at Unit 2.  The Unit 2 personnel were 
consequently relying on inspection procedures that did not possess the 
benefit of the EPRI's methodology for selecting areas of piping for UT 
inspection.  After November 6, 1991, NNECo performed a CHECMATE analysis 
that did identify this portion of the MSR system as highly susceptible to 
erosion or corrosion and thus as a candidate for UT inspection.  

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 Office of 
Nuclear Reactor Regulation (NRR) project manager.

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

Technical contacts:  K. I. Parczewski, NRR
                     (301) 504-2705

                     Vern Hodge, NRR
                     (301) 504-1861

1.  Figure 1, "Rupture of Elbow in Secondary System at 
      Millstone Unit 2 on November 6, 1991"
2.  List of Recently Issued NRC Information Notices 

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