ML20030A306
| ML20030A306 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 09/30/1980 |
| From: | Noonan V Office of Nuclear Reactor Regulation |
| To: | Novak T Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML13319A640 | List:
|
| References | |
| FOIA-81-313 NUDOCS 8010090125 | |
| Download: ML20030A306 (3) | |
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8' UNITED STATES I
!). ro NUCLEAR REGULATORY COMMISSION
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SEP 3 01980 MNRANDUM FOR:
T. H. Novak, Assistant Director for Operating Reactors Division of Licensing FROM:
V. S. Noonan, Assistant Director for Materials and Qualifications Engineering Division of Engineering
SUBJECT:
ARKANSAS UNIT 1 STEAM GENERATOR LEAK - CONCERNS REGARDING DETECTABILITY OF DEFECTS l
On September 5, 1980 Arkansas Unit 1 was shut down due to a 0.3 gpm primary to secondary leak in steam generator "A".
Subsequent inspection revealed the source of the leak to be a defect in tube C74-T2 which is located near the open inspection lane. Single frequency (400 KHZ) eddy current testing (ECT) revealed the defect to be located approximately one or two inches below the uppee tubesheet. The defect is believed to be a circumferential fatigue crack similar to those which have been observed previously at the Oconee units.
This tube was removed from service by plugging.
RESULTS OF STEAM GENERATOR "A" INSPECTION ECT inspections were performed for all 560 tubes (including the leakers) located within four rows of tubes from the open inspection lane.
In addition to the leaker, a 28% indication at the 15th tube support plate was identified in tube C75-T6. The licensee attributes this 28% indication to an axially aligned defect located on the outside tube surface. No additional defect indications were found; however, approximately 60 to 70 tubes out of the 560 tubes inspec-tion exhibited distorted tubesheet signals (so-called C-1 type signals).
It should be noted that it is the steam generator "A" at Arkansas Unit 1 where the water level has been steadily increasing upwards in recent months. Al though the cause of the rising water level problem has not yet been determined, it is not believed to be related to the tube defects which I. ave been recently observed at the 15th support plate and upper tubesheet.
In addition, we have learned from I&E that"there is evidence of sludge in steam generator "A".
This is unusual for B&W units which have operated exclusively with AVT secondary water chemistry contro.l.
DISCUSSION This unit had previously becn shutdown on July 16, 1980 due-to small (approxi-mately 0.lgpm) leak in steam generator "A".
Two leaking tubes were identified, one inside and one outside the open lane region., djacent to the July,1980 The leaker which caused the September 1980 shutdown was located imediately a leaking lane tube. Although dddy current inspection was performed on all tubes adjacenttotheJuly,1980,gah4R.AndalltubesalongtheopenIcneregion,no a
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T. H. Novak SEP 3 01980 indicatio'ns other than a distorted tbbesheet signal was observed for the tube which developed the September 1980 leak.
The recent tube leaks and inspection findings at Arkansas Unit 1 suggest l
that the ECT system currently in use it that plant may not be adequate for detecting circumferential cracks, particularly in the presence of the upper tubesheet. The ECT system used for the Arkansas Unit 1 inspections is a single frequency system employing a standard differential type, circumferential1y wound coil (bobbin coil) ' robe. Recent experience at San Onofre Unit 1 indicates p
that the standard bobbin probe has very limited capability to detect circum-ferential cracks. The use of a rotating pancake coil probe (RPC probe) at San Onofre Unit 1'resulted in the finding of approximately 1400 circumferential flaws which were not detected using the standard bobbin probe.
To enhance the single frequency system's detectability of defect. signals in the presence of interfering tubesheet signals, the ECT system employed at I
Arkansas Unit 1 automatically subtracts a standard tubesheet signal ~from the total system response. Because of the lack of sensitivity of the standard bobbin probe to circumferential cracks, we have some c o cern that any deviation l
from standard tubesheet conditions might further reduce the detectability of i
these defects.
RECOMMENDATIONS The licensee should commit to demonstrating the adequacy of the ECT system currently employed at Arkansas Unit 1 by conducting supplementary inspections during its next steam generator inspection or repair outage. As a minimum, these supplementary inspections should include the approximately 60 tubes located near the open inspection lane which have been observed to contain distorted tubesheet signals.
In order to confirm that none of the possible circumferan-l tially oriented flaws are being overlooked by the current Arkansas Unit 1 ECT -
system, the supplementary inspections should include multifrequency ECT and the use of an alternate type probe (such as the RPC probe) which possesses enhanced capability to detect circumferential1y oriente flaws.
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on ials and Qualifications Engineering l
l Division of Engineering
Contact:
D. T. Huang X27377 cc: See next page l