ML20085D885
| ML20085D885 | |
| Person / Time | |
|---|---|
| Site: | Calvert Cliffs  |
| Issue date: | 06/14/1995 |
| From: | Cruse C BALTIMORE GAS & ELECTRIC CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 9506160381 | |
| Download: ML20085D885 (6) | |
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c Cuc1xs II. CRUSE Baltimore Gas and Electric Company Calvert Cliffs Nuclear Power Plant Plant General Manager 1650 Calvert Cliffs Parkway Calvert Cliifs Nuclear Power Plant Lusby, Maryland 20657 410 586-2200 Ext.4101 Local 410 260-4101 Baltimore June 14,1995 U. S. Nuclear Regulatory Commission Washington, DC 20555 l
ATTENTION:
Document Control Desk
SUBJECT:
Calvert Cliffs Nuclear Power Plant Unit No. 2; Docket No. 50-318 Unit 2 Steam Generator Tube Inspection Results in the spring of 1995, Baltimore Gas and Electric Company conducted a scheduled refueling outage at Calvert Cliffs Nuclear Power Plant (CCNPP) Unit 2. An inspection of the Unit 2 Steam Generator tubes was performed during the outage yielding inspections results that fell into the C-3 category, as described in CCNPP Technical Specification 4.4.5.2.
A verbal notification was made to the Nuclear Regulatory Commission, Region I on May 16, 1995, in which the results of the inspection were provided. In accordance with Technical Specification 4.4.5.5.c., a written follow-up, providing a description of the investigations conducted to determine the cause of the tube degradation and corrective measures taken to prevent recurrence is provided below.
INSPECTION SCOPE The inspection scope for the Unit 2 Steam Generator eddy current examination consisted of a 100% bobbin coil examination of all inservice tubes from tube end to tube end and a 100% motorized rotating pancake coil (MRPC) examination of all inservice tubes at the hot leg tube sheet expansion transition zone. The top of tube sheet MRPC examination was conducted from three inches above to three inches below the expansion transition.
In addition to the top of tube sheet examination, MRPC probes were used to examine other selected areas in the steam generator. All non-quantifiable bobbin coil indications were examined with an MRPC probe i
to further characterize the flaw signal. All bobbin coil indications in the steam blanket region of the steam l
generator were examined with an MRPC probe to disposition indications. The largest volume solid support l
plate dents were examined with an MRPC probe. No steam blanket or dented support plates inspected with I
an MRPC probe revealed crack-like indications.
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9506160381 950614 PDR ADOCK 05000319 Q
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Document Control Desk June 14,1995 Page 2 All MRPC inspections were performed using the "plus point" probe technology. All crack-like indications at the hot leg top of tube sheet were reinspected with a plus point probe containing a high frequency shielded pancake coil. These inspections were performed to determine if the initiation point of the degradation was either from the tube inside diameter-Primary Water Stress Corrosion Cracking, or outside diameter-Intergranular Stress Corrosion Cracking.
To further assess the performance of the plus point MRPC probe and to allow for comparison with previous inspection results, some additional inspections, beyond the original inspection scope, were performed using a conventional three coil MRPC probe. All previous MRPC inspections had been conducted with the three coil MRPC probe. All top of tube sheet indications identified during this outage using the plus point probe that had a previous MRPC inspection history were reexamined using a conventional three coil MRPC probe. This comprised approximately 50% of the indications identified during this outage. Also, a blind sample of approximately 120 tubes was examined using the conventional three coil MRPC probe to allow for comparison with data collected using the plus point probe.
EDDY CURRENT INSPECTION RESULTS Attachment (1) contains the results of the hot leg top of tubesheet motorized rotating pancake coil (MRPC) inspection. Data showing the largest circumferential flaws is also included in the attachment. Areas other than the hot leg top of tubesheet for both steam generators were inspected with no abnormal results identified. All steam generator tubes with crack-like indications, both circumferential and axial, were removed from senice.
COMPARISON OF PLUS POINT. PLUS POINT IIF, AND TIIREE COIL MRPC RESULTS The Motorized Rotating Pancake Coil (MRPC) inspections of the Unit 2 Steam Generators consisted of:
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A production run for MRPC data acquisition using the Zetec 600 Pli plus point probe.
100% of the top of tube sheet transition arcas were examined using this probe;
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Use of the Zetec 600 liF plus point probe (with 0.080 inch diameter high frequency 1
shielded pancake coil) for positive identification in 100% of the tubes that had calls made with the 600 Pil plus point probe, and
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Use of the Zetec 600 3C three coil (conventional) probe on about 50% of the tubes that had indications called by the 600 Pli plus point probe. The tubes selected had previously been examined with the 600 3C three coil probe so data existed for comparison.
The positive identification of flaws via the 600 liF plus point probe was used to help determine flaw origination path, i.e., ID or OD of the tube.
Two-hundred and fifly-eight tubes (about 50% of tubes with calls made by the plus point probes and with a previous MRPC history) were re-examined using the 600 3C three coil probe. Of those 258 tubes,
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. June 14,1995 Page 3 i
11 (4.3%) tubes were called as NDD (No Detectable Degradation). These tubes were identified as NDD even when the analysts were given the flaw location and type identified from the plus point probe data.
We are preparing to conduct an additional study (referred to as the " blind study") which will compare two of the MRPC probes (600 HP plus point and 3C). As noted above, data from a blind sample of approximately 120 tubes (some with known indications, some without) were collected during this outage.
This study is expected to be completed by November 1995.
We believe the results achieved from using the plus point and.'C three coil probes are similar. A comparison of the growth of axial indications from 1993 3C data to 1995 plus point data was conducted l
and indicated the results were essentially identical.' We believe the two different probes are providing the same data to the analysts.
IN-SITU PRESSURE TEST RESULTS In-situ pressure tests were conducted on eight tubes in the 22 Steam Generator. The tubes selected for in-situ pressure testing contained individually the five largest circumferential indications, the longest axial indication and the deepest percent through wall axial indication. Additionally, one tube that contained both i
axial and circumferential indications was tested. The in-situ pressure tests were bounding for the largest circumferential flaws identified in the 21 Steam Generator. The in-situ tests were performed by Combustion Engineering. The test method pressurized the tube locally at the area ofinterest and was not a full tube length pressure test. We believed a localized test, versus full-length, was more representative of the expected axial loading that would be imparted on a circumferential indication during a design basis accident.
The most limiting case for CCNPP is pressurization without burst to three times difTerential operating -
pressure (3 X 1400 psi = 4200 psi). The minimum test pressure for tubes with axial indications was set at 4800 psig to account for temperature correction. For testing circumferential cracks, the minimum test i
pressure was set at 5400 psig to account for both temperature correction and possible axial loading due to tube lock events. None of the tubes tested Icaked at the minimum test pressures.
All eight tubes were tested to pressures in excess of the above values to further verify the substantial remaining stmetural integrity of the defected tubes. None of the tubes tested showed indication ofleakage at the elevated test pressures. The test pressures achieved were limited by the testing device. The test pressures reached ranged from 6400 psig to 7075 psig.
The results of the in-situ pressure test are summarized in Attachment (2).
CAUSE OF TUBE DEGRADATION AND CORRECTIVE MEASURES To fully assess the cause of the of the tube degradation and to develop effective corrective measures to mitigate, if possible, further degradation, three tubes were pulled from the 22 Steam Generator for laboratory analysis. The laboratory analysis of the tube samples is in progress. Following receipt of the analysis from the vendor and review of the results by Baltimore Gas and Electric Company personnel, a
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, June 14,1995 Page 4 thorough assessment of the degradation mechanism will be conducted and corrective measures will be generated. The results of these assessments will be provided to the Commission at a future date.
Should you have questions regarding this matter, we will be pleased to discuss them with you.
Very truly yours, for C.11. Cruse Plant General Manager pliC/MDM/bjd Attachments cc:
D. A. Brune, Esquire J. E. Silberg, Esquire L. B. Marsh, NRC D. G. Mcdonald, Jr., NRC T. T. Martin, NRC P. R. Wilson, NRC R.1. McLean, DNR J. II. Walter, PSC
4 ATTACIIMENT (1)
IlOT LEG TOP OF TUBESIIEET MRPC INSPECTION RESULTS
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SUMMARY
OF RESULTS ALL INDICATIONS 21 Steam Generator Number of tubes with circumferential indications 29
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Number of tubes with axial indications 305 Total Number of affected tubes 334 22 Steam Generator Number of tubes with circumferential indications 86 Number of tubes with axial indications 21 Total Number of affected tubes 177 NOTE:
Affected tubes are only counted once. A tube with both circumferential and axial indications is listed in the circumferential tube count. Tubes listed in axial indication tube count have axial indications only, II.
LARGEST CIRCUMFERENTIAL FLAWS.>80% AVERAGE TilROUGil WALL
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Steam Circumferential Max %
Avg %
Generator Row /Line Extent Through Wall Through Wall No. 21 43/121 330 88 %
80.7 %
No. 22 91/49 360 92 %
92.0 %
95/51 360 91 %
91.0 %
46/32 360 83 %
83.0 %
74/42 360 82 %
82.0 %
86/46 345 85 %
81.5 %
88/52 360 81 %
81.0 %
Note: Avg % Through Wall = Maximum % Through Wall x Circumferential Extent 360*
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ATTACHMENT (2)
IN-SITU PRESSURE TEST RESULTS Row /Line Indication Tarnet Pressure Max Pressure Achieved 91/49 Cire.
5400 psig 7075 psig 360*
92% Avg TW 95/51 Cire.
5400 psig 6700 psig 360 91% Avg TW 46/32 Cire.
5400 psig -
6400 psig 360*
83% Avg TW j
1 74/42 Cire.
5400 psig 7050 psig 360*
I 82% Avg TW 88/52 Cire.
5400 psig 7050 psig 360*
l 81% Avg TW 76/92 Axial 4800 psig 7000 psig 1.0" length 45% 'lw 47/39 Axial 4800 psig 7050 psig 0.4" length 96% TW l
30/28 Mixed 5400 psig 7000 peig J
Remarks For all tests, zero through wall leakage was identified at the maximum test pressure.
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Failure to achieve >7000 psig maximum test pressure was in all cases attributed to leakage from the in-situ pressure test device scaling bladder.
The target pressure for both flaw types was three times the normal operating difTerential pressure with corrections for temperature and axial loading. The axial loading correction applied only to the circumferential indications.
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