ML20127G578

Ro:On 721215,problems Noted in torus-to-drywell Vacuum Problems During Scheduled Maint Shutdown.Caused by Test Operators Error

ML20127G578
Person / Time
Site:	Monticello
Issue date:	12/22/1972
From:	Mayer L NORTHERN STATES POWER CO.
To:	Giambusso A US ATOMIC ENERGY COMMISSION (AEC)
References
NUDOCS 9211170383
Download: ML20127G578 (2)

Text

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Mr. A. Giambusso Deputy Director for Reactor Projects Q %d(j'Rrgb DEc20197F]"

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Washington, D. C. 20545 CL'

Dear Mr. Giambusso:

PONTICELLO NUCLEAR GENERATI fB PLANT Docket tb. 50-263 License tb. DPR-22 Reporting of Torus-to-Drywell Vacuum Breaker Problems A condition occurred at the Monticello Nuclear Generating Plant recently which we are reporting to your office in accordance with provisions of Section 6.6.B.3 of Appendix A, Technical Specifications, of the Provisional Operating License DPR-22. Region 11) of the Directorate of Regulatory Operations has been notified of this occurrence.

Due to recent torus-to-drywell vacuum breaker problems experienced at other operating nuclear plants, i t was decided to inspect the f.bnticello torus-to-drywell vacuum breakers for proper operation. This inspection was performed during a scheduled maintenance shutdown on December 15, 1972. One of the ten vacuum breakers was found to be approximately 1 1/4 inches open; however, the position indicating lights indicated that this valve was closed. During the inspection, an exercise test was performed and four of the vacuum breakers failed to close fully. In addition, two of the test operators failed to operate properly.

Manual exercising of the valves, indicated that excessive friction was being experienced between the shaft and shaft packing. The valves are constructed with a close tolerance teflon bushing on each end of the shaft with several rings of teflon packing outboard of the bushing. All of the teflon packing was removed and penetrating oil was applied. With the packing removed, all the vacuum breakers were leak tested by establishing a .5 psi differential pressure between the drywell and torus and surveying for leaks with a sonic probe. tb significant shaf t leakage could be detected. Some minor seat leakage was detected and as a result, the valve seating surfaces were cleaned and the metal areas were dressed up with emery cloth.

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Disassembly of the two air operators indicated that they required cleaning and toplacenent of the actuator piston sealing rir s. Following these repai rs, the operators functioned properly.

The torus-to-drywell vacuum breaker posi tion is detected by limit switches, that are actuated by small arms attached to the valve shaf t. The valve mast travel a cons;derable distance before sufficient shaft rotation has occurred to actuate the limi t swi tches. A detailed design review of these valves will be performed.

I t is anticipated that this review will result in a modification to the position detection system so that accurate position indication will be available.

Prior to re-inerting the primary containment on December 20, 1972, each of the ten torus-to-drywell vacuum breakers was manually lif ted 1/2 inch off i ts seat and released. All valves closed fully from this 1/2 inch open posi tion.

Valve exercicing wi th the air operators also resulted in free operation in both directions.

Following these tests, all torus-to-drywell vacuum breakers were veri fied shut by visual inspection. immediately af ter this inspection, a di f ferential pressure was established between the drywell and torus and the differential pressure decay was measured. The dif ferential pressure decay rate was found to be less than that for a leakage path egaivalent to a one inch ori fice. Since the present limit swi tch arrangement cannot detect a small opening of the valves, weekly dif ferential pressure decay rate tests will be performed and the results compared wi th equivalent decay rates for a one inch ori fice.

Valve operability will be tested monthly by remote cycling with the air operators and will be followed closely by a leak rate measurement to verify all valves are fully closed.

An Abnormal Occurrence Report will be available at the si te for the Regulatory Operations inspector.

Yours very truly,

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L 0 Mayer, PE Di rector of Nuclear Support Services LOV/kik