ML20235E256

Ro:On 710905,ECCS Initiation Problems Re Loss of Plant Air Noted.On 710909,false Initiation Signal Occurred During Surveillance Test & lo-lo Reactor Water Level Switch Failed. Caused by Human Error & Mechanical Failure

ML20235E256
Person / Time
Site:	Monticello, 05000000
Issue date:	10/05/1971
From:	Duncanson R NORTHERN STATES POWER CO.
To:	Morris P US ATOMIC ENERGY COMMISSION (AEC)
Shared Package
ML20235B311	List: IA-87-111, Submits Rept on Plant,As Discussed in ACRS 150th Meeting on 721012-14.Listed Items Can Be Resolved During Const & Facility Can Be Constructed W/Reasonable Assurance for Operation W/O Undue Risk to Public Safety IR 05000254/1972009 IR 05000358/1972003 IR 05000358/1973001 IR 05000358/1973002 IR 05000358/1973003 IR 05000358/1973005 IR 05000358/1973006 IR 05000358/1973007 IR 05000358/1974001 IR 05000358/1974002 IR 05000358/1974003 IR 05000358/1975004 ML20235B308 ML20235B364 ML20235B401 ML20235B405 ML20235B424 ML20235B456 ML20235B461 ML20235B465 ML20235B477 ML20235B488 ML20235B507 ML20235B552 ML20235B582 ML20235B585 ML20235B588 ML20235B605 ML20235B618 ML20235B623 ML20235B649 ML20235B656 ML20235B662 ML20235B670 ML20235B679 ML20235B683 ML20235B687 ML20235B702 ML20235B708 ML20235B713 ML20235B714 ML20235B718 ML20235B720 ML20235B730 ML20235B734 ML20235B748 ML20235B754 ML20235B762 ML20235B764 ... further results
References
FOIA-87-111 NUDOCS 8709280035
Download: ML20235E256 (2)

Text

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NSIB NCRTHERN STATES POWER COMPANY Minneapolis, Minnesota 55401 October 5, D g Q %y 4fh--

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Dr., Peter A. Morris d y 'O A h .

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Division of Reactor. Licensing O' 4 -

NWN h~g United States Atomic Energy Commission Opl'd N/

Washington, D. C. 20545 g. EESE d -

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Dear Dr. Morris:

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MONTICELLO NUCLEAR GENERATING PLAf#

v g y Docket No. 50-263 License No. DPR-22 Reporting of Unusual Occurrences Three conditions have occurred recently at the Monticello Nuclear Generating Plant which we interpret to be reportable in accordance with Section 6.6.0 of the Technical Specifications. The Region ill Compliance Inspector has been notified of these occurrences.

1. Loss of Plant Air Resulting in ECCS Initiation. and Related Events On September 5,1971, a reactor scram, isolation, and ECCS initiation resulted from a low pressure condition in the plant compressed air systm.

The cause of the low air pressure condition has been traced to a failed compressor control, combined with operator action of improperly placing the standby air compressor controls in the "0FF" position. The low air pressure condition caused the condensate deineralizer effluent valves to close, resulting in a low suction pressure trip of the reactor. feed-water pmps. The reactor scrammed on low reactor water level. The reactor water level decreased to just below the " low-low" level trip setting, causing an isolation, and an ECCS initiation. Reactor water level was returned to the normal operating level by automatic operation of the HPCI and RCIC Systems,and manual operation of a reactor feedwater pmp.

Analysis of computer records and the steam line pressure recorder chart indicated that the main steam isolation valves had closed and then reopened for no known reason.

Testing was performed on the isolation system to verify that the MSlV's cannot be reopened until the isolation is manually reset. Based on test results and findings of the occurrence investigation, the Operations Committee concluded that an operator unknowingly reset the isolation signal and caused the MSIV's to reopen.

The air compressor loading controls have been repaired. The procedural errors have been discussed with the individuals involved. A cover has been placed on the steam line isolation " reset" switch to prevent inadvertent operation. A memo has been distributed to the plant operators reminding them of the proper procedure for resetting this isolation.

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2. ECCS Initiation Resultino from Instrtrnent Surveillance Testino an initiation signal was generated during surveillance On testing September that caused 9,1971,he t standby diesel generators to start, and the RCIC System to operate.

Prior to removal of a simulated trip signal for a " low-low" reactor water level switch, leads for a voli/ ohmmeter were clipped in place in preparation for testing of another similar switch. 'The clips were inadvertently short thereby simulating a trip of a second " low-low" reactor water circuited'tch level swi and initiating the "one of two-twice" logic trip circuit for the RCIC System and the. diesel generators.

The surveillance test procedure has been revised to include the detailed steps of testing. These procedures have been discussed with the instrument' men to assure that they understand the written procedure, and the importance of correctly following the procedure.

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3. Unsafe Failure of ECCS Initiatiq Low-Low Reactor Water Level Switch On September 9,1971, surveillance testing revealed a failed mercury wetted, The magnetic' switch in a low-low reactor water level instrument channel.

failed switch is one of four that provide signals to the Automatic Pressure Relief System, bo h Core Spray Systems, and the RCl0 System. The switches are arranged in one of two-twice logic. Had a valid low-low level condition existed the redur. dant instrument would have operated as required.

The failed switch has been replaced and the surveillance' test successfully completed. The failed switch has been sent to the manufacturer to determine the cause of the failure.

1 Unusual Occurrence reports have been written for these three occurrences and will be available to the Region 111 Compliance inspector for review during his next visit..

! i Yours very truly,  ;

' Ak P.E. l R. O. Supt.

Gen. D6ncanson, of Power Jr.hlants-Mechanical  !

Chairman-Monticello Safety Audit Committee ROD /mmm 4

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