ML20084U519

Final Deficiency Rept Re Failure of Channel a Traveling Incore Probe Cable & Detector.Initially Reported on 730503. Permanent Solution Will Be to Install proximity-type Limit Switch as Soon as Available

ML20084U519
Person / Time
Site:	Browns Ferry
Issue date:	07/09/1973
From:	Gilleland J TENNESSEE VALLEY AUTHORITY
To:	Kruesi F US ATOMIC ENERGY COMMISSION (AEC)
Shared Package
ML20084U431	List: ML20084U446 ML20084U462 ML20084U499 ML20084U519 ML20084U551 ML20084U609 ML20084U662 ML20084U689 ML20084U853 ML20084U869
References
10CFR-050.55E, 10CFR-50.55E, NUDOCS 8306290103
Download: ML20084U519 (4)

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. t ENNESSEE VALLEY A HO Al N CHATTANOOGA, TENNESSEE 37401 July 9, 1973

$NEOSE$N PA ATN E A Ship Mr. F. E. Kruesi, Director Directorate of Regulatory Operations T U.S. Atomic Energy Commission Washington, DC 20545

Dear Mr. Kruesi:

On May 3, 1973, TVA made initial report to AEC-DRO Inspector W. S. Little of the failure of a channel A Traveling Incore Probe (TIP) cable and detector at Browns Ferry Nuclear Plant unit 1.

l On June 1 we submitted an interim report on the failure. In l accordance with paragraph 50.55(e) of 10 CFR 50, we submit j the enclosed final report of the failure.

Very truly yours, u-(

l J. E. Gilleland l Assistant to the Manager of Power Enclosure CC (Enclosure):

Mr.' Norman C. Moseley, Director Directorate of Regulatory Operations U.S. Atomic Energy Commission Region II - Suite 818 230 Peachtree Street, NW.

Atlanta, Georgia 30303

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B306290103 740617 gDRADOCK 050008 3 0594

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,,c a a CHANNEL A TIP SYSTEM FAILURE It is believed that operation of channels A and B of the TIP system initiated from the Unit 1 control room during the time the contaiment was pressurized to 49 psig. At the time, General Electric test No. 14 for integrated leakage rate was in progress. The TIP probes were with-drawn past their shield limit switch without deactivating their limit switches. The channel A probe was pulled out of its shield and drive tube. The probe became entangled in its drive mechanism where the drive gear probably severed the cable. The channel B probe was puiled through its shield, but remained in its drive tube. The Ibnit switches were prevented from perfoming their nomal function by the pressure holding the limit switch plungers against their actuator buttons. Thus, the limit switches were incapable of performing in a normal manner because

, of the particular design details discussed below.

The system design is such that a ball valve in the guide tube tp the drywell must be open for the drive mechanism outside the drywell to insert the TIP probe. An indexing mechanism inside the drywell directs the probe

' to the desired TIP tube that penetrates into the core region. Two relief valves are provided in the indexing mechanism. One relieves the nitrogen purge and blanket gas to the containment that is supplied to the drive tube. The second equalizes pressure between the indexing mechanism and the containment whenever the containment pressure is 2 psig higher than that inside the indexing mechanism. ,, ,

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o Instrumentation in the TIP system is provided so that when the pressure is greater than 2 psig inside the containment, any inserted TIP probe is automatically signaled to be withdrawn. The limit switch in the shield should stop the withdrawal of the probe within the shield and signal the  !

ball valve to close. In case of any malfunction, a shear valve, which the operator can initiate in the control room, is provided as a backup to the ball valve. The operator in the control room has position lights that tell him if the ball valves remain open after they should be closed. Also, two area radiation detectors, which are located near the drive mechanism, will annunciate in the control room upon high radiation to alert the operator to activate the backup shear valves.

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The present limit switch has a spring-loaded plunger that releases the button whenever the probe is withdrawn. The plunger has a roller that makes contact with the detector and drive cable. Guidance for the plunger is provided by a close fitting nonvented cylinder that co:cmunicates directly with the drive tube. Thus, only a small leakage path is available to equalize precsure along the plunger, thereby making the limit switch susceptible to pressure actuation.

Operation of channels A and B of the TIP system during the containment

- leakage rate test pemitted the 49 psig to pemeate the entire drive systems through the indexing mechanism relief valves. The instrumentation initiated an automatic withdrawal of the probes, but the limit switches did not teminate the withdrawal and valve closure sequence properly.

In order to correct the design deficienc/ of the limit swf tch, a te=porary solution of milling slots in the plunger to provide pressure equalization has aircady been made and tested satisfactorily for Unit 1. ' As a pemanent solution,a proximity-type limit switch will be instal.ed as soon as it is available at the site. Units 2 and 3 will have the proximity switches installed before fuel loading occurs.

Tho system design and instrumentation logic were developed to protect the health and safety of the public in case an accident occurred while the TIP

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synt.eu was in une. Due to the preocure senaitivity of the limit switch, n very su.all path would have existed for containment leakage throu8h the guide tube. However, had the leakage contained radioactivity, two aren radiation detectors located near the drive mechanism would have annunciated in the control roca and the operator vould have operated the uhuur valves to cut the guide tube. A more careful review of denigna vill be emphasized to assure similar problems do not occur in the future.

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