Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
December 18, 1991
NRC INFORMATION NOTICE 91-18, SUPPLEMENT 1: HIGH-ENERGY PIPING FAILURES
CAUSED BY WALL THINNING
Addressees
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose
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
required.
Background
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
lines.
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.
9112120218
.
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.
Discussion
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
oxygen).
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
Attachments:
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
.