ML20086L320
| ML20086L320 | |
| Person / Time | |
|---|---|
| Site: | Cooper  |
| Issue date: | 12/09/1991 |
| From: | Horn G NEBRASKA PUBLIC POWER DISTRICT |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 9112160141 | |
| Download: ML20086L320 (2) | |
Text
GENERAL OFHCE
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P O BOX 499. COLUMOUS NEBRASKA 68/42-0499
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==g December 9,1991 U. S. Nuclear Pegulatoiy Commission Attentiom D cument Control Desk Washingwn, DC 20555 Gentlemen:
Reference:
Letter, G. R. Hom to USNRC,
Subject:
Plan for Reactor Pressure Vessel Head Stud Rejectaole Indication, dated October 25,1991 (CNSS910350)
Subject:
steactor Vessei Head Stud Examinations - Cooper Nuclear Station - NRC Docket No. 50-298, License No. DPR-46 The referenced leter was submitted per your request to describe our plan of action regarding a rejectable indication in one of the fifty-two Reactor Pressure Vessel (RPV) closure studs. The rejemble indication was discovered early in the present refueling outage as a part of planned insocction activities. His letter provides suppleniental information on the current status of this issue and our plan of action as a result of the Nondestructive Examinations (NDE) performed.
As described in the referet,ced letter, our action plan consisted of performing additional NDE in the interest of better characterizir the noted indication, as well as to be responsive to industry concerns related to RPV stud cracking. The actions included the performance of:
1.
A 0 degree L ' wave ultrasonic examination of all. fifty-two RPV studs.
2.
For the stud with the rejectable indication (Stud No. 26), a shear wave ultrasonic examination from the extensiometer (centerliae) bore.
3.
A shear wave ultrasonic examination from the extensiometer bo e of three additional studs.
These actions were started shortly after the refueling cavity was drained down in preparation for RPV re-assembly. During the above described examinations, however, conflicting data was noted. None of the L wave examinations (including a re-examination of Stud No. 26) revealed any recordable indicatione, but every shear wave evamination showed recordable indications.
At this point, a decision was made to examine all fifty-two of the studs using the shear wave technique from tne extensiometer bore. As a result of the incicased testing requirements, all of the studs except one (Stud No. 44) were found with recordable indicaticns.
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U.S. Nuclea. Regulatory Commission Page 2 December 9,1991 In contideration of the available information, parallel efforts were then initiated. A worst-case stud (Stud No. 26) was selected for destructive examination to resolue the conflicting information and, concurrently, a complete set of replacen ent studs was pursued. The destructive testing ultimately showed that the shear wave examhaion technique used by the ISI contractor yielded anomalous results. The indications were apparently caused by surface discontinuities resulting from general surface corrosion, which in this case is not detrimental to the design safety function of the stud. The contractor has indicated that he imends to perform additional research and development to improve the shear wave technique.
While the destructive examination process was proceeding, a set of replacement studs was obtained from the Shoreham facility which is currently being dismantled. The studs were received and satisfactorily pre-service inspected. Forty eight of them have been installed which, when combined with four new studs that were recently purchased, results in the complete complement of studs being replaced this outage.
Our future plans include resolving the previously mentioned anomalous indications and informing the industry via NETWORK and EPRI of the facts surrounding this asue, Should you have any questions or require further information regarding this submittal, please contact me.
. Sincerely,
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- u. f) Horn Nuclear Power Group Manager GRHkml-42W cc:
NRC Regior'l Office Regiou IV Arlington, 'I"X NRC Resident inspector Cooper Nuclear Station i
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