ML17340B070

Updated LER 80-007/03X-1:on 800507,disc Stud Nut Discovered Missing from Main Stem Check Valve 4A.Caused by Failure of Associated Locking Device.Disc Studs Replaced & Improved Locking Device Installed

ML17340B070
Person / Time
Site:	Turkey Point
Issue date:	04/10/1981
From:	Pace P FLORIDA POWER & LIGHT CO.
To:	NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II)
Shared Package
ML17340B069	List: ML17340B068 ML17340B070
References
LER-80-007-03X, LER-80-7-3X, NUDOCS 8104270313
Download: ML17340B070 (3)

Text

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44 4ANVFA~iVRSR 47 CAUSS OSSCRIPTION ANO CORRSr ZIYK ACTlCNS Q17 o The cause of the stud nut dislod in from the disc stud was failure of the associated locking device. Examination of the other two MSCVs I revealed minor disc stud damage. Disc studs taere replaced and an improved. locking device .seas installed on all three MSCVs prior to CIISl starting up from cold shutdown. HSIVs were also examined.

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., jtEPORTABLE OCCURRENCE 251-~7, UPDATE 1 LICENSEE EVENT REPORT PAGE 'H~O Additional Event Descri tion and Probable Consequences:

On Mednesday, Hay 7, 1980, in the course of performing planned maintenance scheduled as a result of a problem initi'ally detected on Unit No. 3 (see LER 250-79-31),

the disc stud nut was discovered missing from the 4A main steam check valve (MSCV).

The disc and disc stud were in the proper position and the- valve was fully operable as found. The disc stud nut was subsequently recovered from the turbine control valve. The 4A HSCV disc stud had sustained some wear damage from the loose washer which had remained in place. The 4B and 4C MSCVs were inspected and the- disc stud nuts were found in place although some distress was noted on the disc stud. Addition.

ally, the main steam isolation valves (HSIVs) were inspected.

During'a reinspection of the locking device in November 1980, to evaluate its perform-

'""ance, distress was noted on- the 4A and 4B HSCV locking device.

Additional Cause Descri tion and Corrective Actions.:

The cause of the disc stud nut dislodging from the disc stud was failure of its associated locking dev'ice. Vibration resulted in the disc stud nut'"backing off" the disc stud. The disc studs and disc stud nuts on all three HSCVs were replaced and an improved locking device was installed prior to starting up from cold shut-down. The improved locking device was also ins'tailed on all three MSIVs. This improved locking device provided for changing the nut material to permit it to be fillet welded to the disc stud.

Subsequent to discovery of additional distress (in November 19SO) in the area of the improved locking device, temporary repairs were effected. A plant change/modifi-cationwas prepared for installing newdisc studs on both the MSIVs and. MSCVs. A nut will be secured to each of the new studs by use of two pins inserted .axially between the nut. and stud. Pending approval of the'modification package and receipt of. mate-rial, the new locking device will be installed. The implementation schedule is tentatively set for July 19S1.