Information Notice No. 91-67: Problems with the Reliable Detection of Intergranular Attack (IGA) of Steam Generator Tubing
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
October 21, 1991
NRC INFORMATION NOTICE 91-67: PROBLEMS WITH THE RELIABLE DETECTION OF
INTERGRANULAR ATTACK (IGA) OF STEAM GENERATOR
TUBING
Addressees
All holders of operating licenses or construction permits for pressurized-water
reactors (PWRs).
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice
to inform licensees of recent problems experienced at the Trojan Nuclear Plant
concerning the reliable detection of general intergranular attack (IGA) of the
steam generator (SG) tubes at the tube-to-support plate (TSP) intersections.
This information notice complements NRC Information Notice 90-49, "Stress
Corrosion Cracking in PWR Steam Generator Tubes," which discussed, in part,
problems in detecting intergranular stress corrosion cracks (IGSCC) in steam
generator tubes at TSP intersections. 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.
Description of Circumstances
The Trojan Nuclear Plant is a 4-loop pressurized water reactor (PWR) designed
by Westinghouse Electric Corporation with Model 51 steam generators. During a
refueling outage in the summer of 1991, the Portland General Electric Company
(the licensee) conducted an inservice inspection of the steam generator tubes.
The licensee used an eddy current test bobbin probe to inspect all tubes in
each steam generator from the hot leg tube end to at least the top support
plate on the cold leg side. A 20% sample of tubes in one steam generator was
inspected with a bobbin probe over the full tube length. The licensee per-
formed additional inspections with a more sensitive motorized pancake coil
(MRPC) probe to confirm and better characterize indications found with the
bobbin probe. The MRPC probe was also used by the licensee to inspect all
tubes at the expansion transition region at the top of the tubesheet on the hot
leg side and all row 2 tubes in the u-bend region. (All row 1 tubes were
previously plugged). As a result of these inspections, the licensee identified
297 tubes with axial indications at the TSP intersections, 26 tubes with axial
or circumferential indications at the expansion transitions, and ten tubes with
axial or circumferential indications in the row 2 u-bends.
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IN 91-67
October 21, 1991
Page 2 of 3
The licensee removed a tube exhibiting an axial indication at the first
(i.e., lowermost) TSP location for laboratory examinations. These examinations
revealed circumferential bands of IGA at the first, second, and third TSP
locations. The IGA morphology is a relatively uniform attack of all grain
boundaries over a portion of the tubing surface. Stress is not a significant
contributor to the morphology of IGA. The licensee found the IGA at the first
TSP to range to a maximum depth of 92 percent through-wall and to extend
270 degrees around the tube circumference. The maximum depth of IGA at the
second and third TSPs was found to be 40 percent and 42 percent through-wall,
respectively. However, no eddy current test indications had been called in
the field at the second and third TSPs prior to tube removal.
The pulled tube examinations prompted the licensee to reassess the field bobbin
probe data and to determine that indications were, in fact, present at these
locations. The voltage amplitudes of these indications were below the thresh-
old criteria of the plant data analysis guidelines. These guidelines stated
that the amplitude be at least 1.5 volts before a signal would be designated a
"possible indication" (PI) and the tube subjected to a confirmatory MRPC
inspection. Accordingly, the licensee revised the guidelines for data analysis
to ensure that any flaw-like signal, regardless of amplitude, would be desig-
nated as a PI. Further, only one rather than two data channels having a
flaw-like signal response would be sufficient for reporting a signal as a PI.
An added refinement to the data analysis guidelines, based on the examinations
of the removed tube specimen, was that the residual TSP signal response (from
the 400 kHz and 500/100kHz differential mix channels) to volumetric IGA need
not exhibit a rapid transition across the support plate centerline to be
reported as a PI.
The licensee reassessed all of the bobbin probe field data and identified an
additional 2500 PIs at the TSPs using the revised guidelines for data analysis.
The licensee performed MRPC inspections and confirmed approximately 34 percent
of these 2500 PIs. The licensee evaluated the data from the MRPC at a rela-
tively high gain level to better detect flaws. Using the MRPC, the licensee
inspected 200 other TSP intersections without PIs to validate the effectiveness
of the bobbin probe as the screening method for IGA. However, the licensee
found eight of these intersections (without PIs) to contain indications. For
this reason, the licensee concluded that it should rely exclusively on the MRPC
probe for detecting the degradation at the support plates. Accordingly, the
licensee performed MRPC inspections of all TSP intersections up through the
fifth support plate on the hot leg side. These MRPC inspections confirmed
indications in approximately 10 percent of the tubes not exhibiting PIs with
the bobbin probe. Sample inspections with the MRPC were also conducted at
the sixth and seventh support plates and confirmed these support elevations to
be relatively free of indications. The licensee has identified a total of
about 2160 tubes with indications during the current outage to date. Each of
these tubes will be plugged or sleeved.
Discussion
Licensees are finding increasing numbers of small amplitude, axial and circum-
ferential indications at TSP intersections in steam generators constructed by
Westinghouse and Combustion Engineering. Licensees have examined affected
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IN 91-67
October 21, 1991
Page 3 of 3
tubing removed from North Anna Unit 1, Farley Units 1 and 2, McGuire Unit 1,
and Catawba Unit 1 and have found degradation characterized mainly as IGSCC,
sometimes associated with limited amounts of IGA. However, the Trojan findings
show that crack-like indications may also indicate IGA in which cracks have
opened between the grains, thus dominating the eddy current response of the
region affected by IGA.
The threshold at which IGA can be detected reliably with MRPC and bobbin probes
is not well understood. The licensee for Trojan has taken the conservative
position that MRPC indications at the TSPs may indicate IGA flaws greater than
the 40 percent plugging limit in the Technical Specifications, even where the
bobbin probe does not show a PI. The licensee plans to remove additional tube
specimens before restarting from the current outage. Examination of these
specimens is expected to provide additional data on the Trojan TSP degradation
mechanism and the associated eddy current signal responses and to validate the
effectiveness of the inspection methodology and data analysis criteria at
Trojan to detect degradation greater than the 40 percent plugging limit.
NRC Information Notice 90-49 discussed potential non-conservatisms associated
with the use of bobbin probe voltage amplitude criteria for identifying IGSCC
at TSP locations in excess of the plugging limit in the Technical Specifica-
tions. Although the licensee for Trojan ultimately elected not to rely on the
bobbin probe to detect IGA flaws at the TSPs, the Trojan experience with the
bobbin probe further underscores the potential non-conservatisms associated
with the use of voltage amplitude criteria for evaluating indications at the
TSPs in the absence of corroborating data from removed tube specimens.
This information notice requires no specific action or written response. If
you have any questions about the information in this notice, please contact the
technical contact 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 contact: E. Murphy, NRR
(301) 492-0710
Attachment: List of Recently Issued NRC Information Notices
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