ML20056B739

Ro:On 711111,main Steam Line High Flow Instrument Problems Occurred.Investigation of Problems Conducted

ML20056B739
Person / Time
Site:	Monticello
Issue date:	11/23/1971
From:	Duncanson R NORTHERN STATES POWER CO.
To:	Morris P US ATOMIC ENERGY COMMISSION (AEC)
References
NUDOCS 9102080463
Download: ML20056B739 (2)

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NSP NORTHERN 5TATES POWER COMPANY Minneapolis, fAinnesota 55401 N

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D MCC C" Dr. Peter A. Morris Division of Reactor Licensing 3

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United States Atomic Energy Corrnission D co c3 Num Washington D.C. 20545 82$g

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Dear Dr. Morris- -

LONTICELLO TUCLEAR GENERATilG PLANT Doekei Ib. 50-263 Li cence No. DPR-22 l Main Steam Line High Flow instrument Problems  !

l A condition has occurred recently at the Monticello Nuclear Generatine  !

Plant which we interpret to be reportable in accordance with Section 6.6.B of '

the Techni cal S peci fi catiens. T he Regi on i l i Comp 1 i ance o f fi ce has been noti fied of this occurrence.

On November 11 and 12,1971, while performing a regularly scheduled surveillance test on the steam line high flow isolation sensors, three of the sixteen sensors were found to exhibit signs of high friction or binding in their mechanisms.

Another sensor was found to exhibit a non-repeatable trip setting. The four defective instrurents were immediately replaced, the remaining twelve instruments were replaced on Ibvember 13, 1971. These instruments had been in service since September 1,1971, and were used to replace those originally installed.

The original instruments had a range of 0-200 psid, and were replaced because the roblems caused the high flow t rip previously setpoint to bereported a lower value steam thanflow reasurement p(about 15% of the original expected inst range). The new instrurents had a range of 0-70 psid, and used a snap action switch instead of a cercury bulb. The snap action switch made the inst rucent less susceptible to tripping when the instrument rack was bumped. The instrucents were found to trip as follows:  ;

I nst rurent tb. As Found Settine, T nf Rahd Finw~

116B 109-127 117C 137 117D 1 118C 12 Instruments 1170 and 117D moni tor "B" steam line, instrurent 116B monitors "A" steamline, and instrurent 118C monitors "C" steam line. The isolation logic for 4 steam line high flow is arranced in a manner that the remaining instruments would l have caused closure of the MSl V's at 113% flow.

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2 A defective switch has been returned to the factory for inspection. T he cause of the increased friction found in three of the swi tches has been identi fied  !

as a contamination of the jewel bearing in the torque tube assembly. T he t rip '

setting of instrument 116B was found to be non-repeatable. The cause of this problem appears to be in the snap-action swi tch.

i Presently, the original instruments are in service. During thi rteen j months of service there have been no failures of these or similar instruments. T he -i manufacturer has stated that contamination of the jewel bearines results in I early failures as experienced. This problem remains uni cpe to the sixteen instruments installed on September 1,1971.

The investigation of these problems is not complete. A follow-up report will be prepared when the investigation is complete. Any additional information will be made available to the Region 111 Corpliance Inspector as soon as possible.

Yours very t ruly, l

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R. . Dunca,nson, Jr. , P. E.

Manager of Power Production

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Chai rman Monticello Safety Audi t Committee i

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