ML20090E273

Provides Addl Info Clarifying Apparent Cause of AOs Reported on 731128 & 1203 Re Failure of Control Rods to Insert to full-in Position Following Reactor Scram.Adequate Shutdown Margin Exists W/All Control Rods in Position 02

ML20090E273
Person / Time
Site:	Nine Mile Point
Issue date:	12/21/1973
From:	Schneider R NIAGARA MOHAWK POWER CORP.
To:	Skovholt D US ATOMIC ENERGY COMMISSION (AEC)
Shared Package
ML20090E243	List: ML20090E273 ML20090E293
References
10090, NUDOCS 8303140533
Download: ML20090E273 (4)

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NIAGARA MoH AWK POWE*R CORPORATION NfAGARA MOHAWK'

- soo cmc soutevAno west 5 s f RACU S c. N.Y. 83202 December 21, 1973 g D g' c REcdvE ,;

y DEC 28 1973 X~ A EENd*

. Mr. Donald J. Skovholt Assistant Director for Reactor Operations $ soport

"* e Division of _ Reactor Licensing United States Atomic Energy Commission 8 'h 3

Washington, D.C. 20545 Re: Provisional Operating License: DPR-17 Docket No.: 50-220

Dear Mr. Skovholt:

This letter is to provide, as comitted, additional clarifying information describing the apparent cause of the abnormal occurrences reported to you on November 28 and December 3,1973, the failures of eleven (11) and fifteen (15),

respectively, control rods to insert to 00-following a reactor. scram at Nine Mile Point Nuclear Station 1 (NMPNS 1). '

The control rod drives used at NMPNS 1 are the. locking piston type using.

water as the hydraulic fluid. The assembly consists primarily of a stop piston and seals, which is fixed in positior., a drive piston-with seals, which is cylindrical around the'stop piston and a collet-piston with rings,_which locks the drive piston in an even notch position. The drive piston assembly. has ,a.

subassembly, an index tube with 24 notches. The notches being similar to'a right triangle cut, with the 90* angle at the notch top to prevent withdrawing of the. control blade. The collet-piston has a latching mechanism made up of six fingers. Spring action in the fingers-themselves hold them'against'the index tube. When these fingers are in a notch in the-index tube the= control blade-cannot be withdrawn. Also 'if the fingers are not in a notch position butlin contact with the index tube, the furthest the control rod -can withdraw or settle -

is _ to 'the next notch position.' In other words ~1f. a drive has been inserted beyond -

. notch position 02 but not beyond position 00 the rod'will settle backLinto notch-position 02 and no:further. The stop piston.'is 'a hollow assembly with two sets of seals and hydraulic ports (Buffer holes) between thef above drive piston area .an - ,

area between ~ the' two set of seals, and the: scram exhaust valve., As pressure 'is increased under the drive piston the control rod tends to insert into the core.

!!owever the water above the drive-piston must be. evacuated as the: drive piston; ,

assembly moves:into this a'rea. The water is exhausted through 8 ports in the stop piston assembly, - (buffer holes) . However-as.the. drive = piston moves' closer to fully'. inserted position the holes are covered up by seals in the-drivefpistonS.

1 reducing.the rate at;which water may be: evacuated from,the: over piston area. -

'Ihus on; a scram this decreasing rate of water. flow causes1the drive Lto slow down

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once it passe's the02 notch l position. _

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' Mr.-Donmid J. Sk it Dactmber 21, 1973:

.U.S. Atomic Ensrgy Commission 2

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As the rate of water flow decreases the pressure exerted must increase, however it remains much less than 1000 psi or reactor pressure. If the stop piston seals have become broken, scored or jammed and do not perform their function, reactor pressure water is introduced to the .ana between the two sets of seals. This excess water is carried away through the hydraulic port between the seals through the stop piston to the hollow interior. If the leakage is approximately 6 gpm past these seals it will force water at reactor pressure through the lower stop piston seal-into the above drive piston area. Because of the number and' size of the buffer holes available at notch 04 and notch 02 (Figure 1), no problem is presented to drive movement. However beyond the 02 notch position moving toward 00 the increased stop piston seal leakage becomes critica1 such 3 that the remaining buffer hole cannot vent the required water and the drive stops between position 01 and notch 00 (Figure 2). Upon reseting the scram signal, the scram dump valve closes. Pressure quickly builds up in-the drive until reactor pressure is equalized. The drivo then settles to notch

02. It can not proceed further to notch 04 because of the latching mechanism.in the collet piston. Two important items which have been substantiated through testing by the General Electric Company can be drawn from the above sequence.

1. The drive slow down is not a problem until the drive has passed the 01 position. Sufficient buffer holes exist prior to this position.

2. The drive can only settle into the 02 notch position because of the latching mechanism in the collet-piston.

The drive design has been modified by the manufacturer (General Electric ~

Company) to mount a .0012" mesh filter on the stop piston thus preventing the entry of dirt particles to the stop piston seals. These dirt particles cause the jamming and scoring of the stop piston seals. NMPNS I will start a conversion program at the Spring 1974 refueling outage.

Therefore having established that the only positions that the control rod with broken, jammed or scored stop piston seals can settle into is notch 00 or 02 consideration must be given to the shutdown margin with all control rods at notch 02. Calculations based upon shutdown margin tests and cold Xenon free criticals show a shutdown margin, with all rods at notch position 02 and the strongest control rod withdrawn, of .1.94% AK at Xenon free cold conditions.

I In a random nine rod ' array (3x3) with the center rod withdrawn to' 48 and

adjacent rods at 02 notch position the shutdowimargin is a minimum of 2.28%AK.

If all rods were at notch 02 the shutdown margin would be 2.98%; A K.

In conclusion, it has been established that adequate shutdown margin exists with all control rods'at notch position 02. . It has been further established that the possibility of even one rod settling 'in a notch position other than 00 or

02 is highly improbable if not impossible. Therefore it is concluded that no hazard

! exists to safe operation of the plant or to the general public during the remainder 1-of this operating cycle'as a result of this problem.

Very truly yours, RRS:cm ni Enclosures - 2 figures -Vice President - Electric Operations j

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