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. 

9110160088
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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
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                                                            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
.