ML19316A370

RO 269/78-13:on 780427,primary-to-secondary Leak in 1B Steam Generator Occurred.Caused by Tube Leaks.Tube 69-1 Had Weld Crack.Tube 74-2 Had Circumferential Crack.Cracked & Suspect Tubes explosive-plugged.Investigation Continuing

ML19316A370
Person / Time
Site:	Oconee
Issue date:	06/05/1978
From:	DUKE POWER CO.
To:
Shared Package
ML19316A366	List: ML19316A365 ML19316A370 ML19316A371
References
RO-269-78-13, NUDOCS 7912110635
Download: ML19316A370 (2)

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DUKE POWER COMPANY OCONEE UNIT 2 Report Number: R0-269/78-13 Report Date: June 5,1978 W Occurrence Date: April 27,1978 Facility: Oconee Unit 2, Seneca, South Carolina Identification of Occurrence: Primary-to-Secondary Leak in IB Once Through Steam Generator (OTSG)

Conditions Prior to Occurrence: 100% FP Description of Occurrence:

At 0346 on April 20, 1978, an alarm on RIA40 indicated a possible primary-to-secondary leak in Unit 1. At 0930 the leak rate was calculated to be 0.14 gpm.

The leak rate gradually increased to 0.33 gpm on April 21, 1978. This exceeded Technical Specification 3.1.6.4 limits for leakage and a shutdown was commenced at 1205. The unit was in cold shutdown at 1600 on April 23, 1978. An initial hydrostatic test in OTSG B was performed on April 28, 1978. Pressure was held at 150 psi from 0000 on April 29 to 0800 on May 2, 1978. A possible tube leak, later identified as tube 69-1, was observed. On May 9,1978, repair work was completed and OTSG was refilled and pressurized for a second hydrostatic test.

At 0030 on May 10, 1978, a weld leak on 69-1 was verified. Tube 69-1 was repaired and OTSG was refilled for further inspection. Tubes 74-2 and 59-1 were also observed to be possibly leaking during the tests. Tube 74-2 was verified to be leaking near the Upper Tube Sheet (UTS) and Eddy Current (EC) tests verified a 45-90 circumferential crack. Tubes 70-1 and 59-1 were thought to be .

leaking but EC indications showed no degradation in excess of 10% wall thinning. I Repair work was completed and OTSG passed a final leak test on May 14, 1978.

Apparent Cause of Occurrence:

Several instances of tube leaks have occurred during the operation of the Oconee Nuclear Station (see attachment). Various failure mechanisms have been postu-lated and researched. This matter is the subject of continued investigation by both Duke and Babcock & Wilcox (0conee NSSS vendor). Several submittals and discussions have been made with regard to this subject.

The initial leakage may have been caused by one or more of the tubes plugged.

Tube 69-1 had a weld crack on the Lower Tube Sheet (LTS). The circumferential crack on 74-2 was a fatigue-failure type crack of very small magnitude. Tubes 70-1 and 59-1 were observed to be possibly leaking but no evidence of degra-dation through Eddy Current investigations was verified. Lane tube 77-27 was to be removed for analysis but extraction was unsuccessful, so the tube was stabilized and plugged.

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DUKE POWER COMPANY OCONEE UNIT 2 (Cont'd) i i

Analysis of Occurrence

j Primary-to-secondary leakage constitutes degradation of the reactor coolant i boundary. The leak test following the repair operations verified the l

reestablishment of the boundary. The leakage was well within the capacity of one HPI train so that no significant loss of coolant to the primary system could have resulted from the incident. The investigation and repair operations i verified the integrity of the RC boundary. The investigation and repair

] operations resulted in 130 individuals receiving a cumulative radiation dose of q 51.7 man-rem. The gaseous activity released during the incident was calculated to be 2.83 Ci which is insignificant when compared to normal release limits.

The effect of additional activity in the secondary system was negligible with

regard to offsite releases. This occurrence did not endanger the health and l safety of the general public.

I Corrective Action:

The accepted manner of correcting OTSG tube leakage or in preventir.g leaks in i

degraded tubes is explosive plugging. Five tubes in the IB OTSG were plugged:

j 69-1, 70-1, 74-2, 59-1 and 74-27.

l Tube 69-1 was plugged and the tube /LTS weld was repaired.

! Tube 74-2 was plugged due to crack near UTS.

Tubes 70-1, and 59-1 were plugged as a preventative measure.

Tube 74-27 was plugged and stabilized after attempts to remove it were unsuccessful.

) Investigation of a generic nature is continuing by both Duke and Babcock &

3 Wilcox.

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