ML19263E955
| ML19263E955 | |
| Person / Time | |
|---|---|
| Site: | Rancho Seco |
| Issue date: | 07/26/1979 |
| From: | Engelken R NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION V) |
| To: | Mattimoe J SACRAMENTO MUNICIPAL UTILITY DISTRICT |
| References | |
| NUDOCS 7908060160 | |
| Download: ML19263E955 (1) | |
Text
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o UNITED ST ATES 8, }*.,. < ',o,'g NUCLE AR REGULATORY COMMISSION
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1990 N. C AllF ORNI A BOUL EVARD
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,6 SUIT E 202. WALNUT CR E E K PL AZA
- ,,,e WALNU T CR E E K, CALIF ORNI A 94596 July 26, 1979 Docket No. 50-312 Sacramento Municipal Utility District
- p. o. Box 15830 Sacramento, California 95813 Attention:
Mr. John J. Mattimoe Assistant General Manager Gentlemen:
The enclosed Bulletin No. 79-17 is forwarded to you for action. A written response is required.
If you desire additional infonnation regarding this matter, please contact this office.
Sincerely,
,yl(cYm d0 R. H. Engelken Director
Enclosure:
IE Bulletin No. 79-17 cc w/ enclosure:
R. J. Rodriguez, SMUD L. G. Schwieger, SMUD y-
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UNITED STATES NUCLEAR REGULATORY COR11SSION OFFICE OF INSPECTION AND ENFORCEI1ENT WASHINGTON, D.C.
20555 July 26, 1979 IE Bulletin No. 79-17 PIPE CRACKS IN STAGNANT BORATED WATER SYSTEfiS AT PWR PLANTS Description of Circumstances:
During the period of November 1974 to February 1977 a number of cracking incidents have been experienced in safety-related stainless steel piping systems and portions of systens which contain oxygenated, stagnant or essentially stagnant borated water. Metallurgical investigations revealed these cracks occurred in the weld heat affected zone of 8-inch to 10-inch type 304 material (schedule 10 and 40), initiating on the piping I.D. surface and propagating in either an intergranular or transgranular mode typical of Stress Corrosion Cracking. Analysis indicated the probable corrodents to be chloride and oxygen contamination in the affected systems.
Plants affected up to this time were Arkansas Nuclear Unit 1, R. E. Ginna, H.B. Robinson Unit 2, Crystal River Unit 3, San Onofre Unit 1, and Surry Units 1 and 2.
The NRC issued Circular 76-06 (copy attached) in view of the apparent generic nature of the problem.
During the refueling outage of Three Mile Island Unit 1 which began in February of this year, visual inspections disclosed five (5) through-wall cracks at welds in the spent fuel cooling system piping and one (1) at a weld in the decay heat removal system. These cracks were found as a result of local boric acid build-up and later confirmed by liquid penetrant tests. This initial identification of cracking was reported to the NRC in a Licensee Event Report (LER) dated May 16, 1979. A preliminary metallurgical analysis was performed by the licensee on a section of cracked and leaking weld joint frcm the spent fuel cooling system.
The conclusion of this analysis was that cracking was due to Intergranular Stress Corrosion Cracking (IGSCC) originating on the pipe I.D.
The cracking was localized to the heat affected zone where the type 304 stainless steel is sensitized (precipitated carbides) during welding.
In addition to the main through-wall crack, incipient cracks were observed at several locations in the weld heat affected zone including the weld root fusion area where a miniscule lack of fusion had occurred. The stresses responsible for cracking are believed to be primarily residual welding stresses in as much as the calculated applied stresses were found to be less than code design limits.
There is no conclusive evidence at this time to identify those aggressive chemical species which promoted this IGSCC attack.
Further analytical efforts in this area and on other system welds are being pursued.
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