ML20090E478

AO 50-237/75-49:on 751009,LPCI Loop Selection Logic Circuitry Failed to Select Preferred B Loop.Caused by Out of Calibr Differential Switch 261-34 & Electrically Shorted Switch 261-34D.Mod to Provide Signal Dampening Initiated

ML20090E478
Person / Time
Site:	Dresden
Issue date:	10/17/1975
From:	Stephenson B COMMONWEALTH EDISON CO.
To:	James Keppler NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III)
References
12218, 710-75, AO-50-237-75-49, NUDOCS 8303310680
Download: ML20090E478 (2)

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- Commonwealth Edison j

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. Chicago, !!bnois 60690

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BBS Ltr. #710-75 Dresden Nuclear Power Station R. R. #1 ,

. Morris, Illincie 60450 ,

i October 17, 1975 i

/o, Mr.' James G. Keppler, Regional Director ~ ,

Directorate of Regulatory Operation-Region, III q' U. S. Nuclear Regulatory Ce.c: mission ,

799 Roosevelt Road Glen Ellyn, Illinois 60137 SUBJEDT: REPORT OF AP2!0RYAL'OCCUPSENCE PER SECTION 6.6.A 0F THE TECHNICAL SPECIFICATIONS UNIT-2 LICI LCOP SELECT FXILURE j

References:

1) Regulatory Guide 1.16 Rev. 1 Appendix A ,

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2) Notification of Region III. of U. S. Nuclear Regulatory Co=ission Telephone: Mr. Feierated, 1608 hours0.0186 days <br />0.447 hours <br />0.00266 weeks <br />6.11844e-4 months <br /> on October 10, 1975

3) Drawing Number 12E2437A & 12E2438A Report Number: 50-237/75 49.

Report Date: October 17, 1975

• Occurrence Date: Cetober 9, 1975 Facilityb' Dresdtn :iuclear Power Station, Morris, Illinois IDEN'rIFICATION OF OCCURREICE On October 9, 1975, the LPCI loop selectica logic circuitry failed to select the preferred ("B") loop.

CONDITIONS' PRIOR TO OCCURRF215E -

Unit-2 was in the cold shutdown mode.

DESCRIPTION OF OCCURRENCE- ,

.n On' September 29, 1975, the U_nik;-2 reactor scramraed oh high drywell pressure (see report, no. 50-237/75-46). A:s designed, LPcI automatically initiated when drywell pressure exceeded 2 psi. Ii' isth recirculation loops appear to be' intact, LPCI s -

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io designed to inject through the "B" loop. Since it had been determined that the drywell high pressure condition was a result of nitrogen purging rather than a 1:ss of coolant accident, confusion arose when the LPCI logic circuitry selected

• A" loop over the preferred "B" loop. The apparent failure of recirculation suction valve MO-202 4A to completely close, and the failure of the "B" recirculation [

pump to automatically trip, also contributed to confuse the issue.

Imediate investigation failed to reveal the cause of the "A" loop selection. A high-priority verification of the "as built" LPCI circuitry logic sequence was initiated. After completion of the logic diagran verification, a LPCI surveillance [

was performed at 1500 hours0.0174 days <br />0.417 hours <br />0.00248 weeks <br />5.7075e-4 months <br /> on October 9,1975 During the surveillance, the LPCI legic circuitry improperly selected "A" loop over "B" loop.

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DESIGNATION OF APPARENT CAUSE CF OCCURRENCE (Equipment Failure)

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There are four differential pressure switches used in the "one out of two, twice" LPCI loop selection logic. Follcwing the LPCI surveillance, differential pressure }

switches 261-34A and -34D were found in the closed, or "A" loop position. Further investigation revealed that switch 261-34A was cut of calibration, and switch 261-34D was electrically shorted. The failure of both switches is believed to have r:sulted from excessive cycling due to varying jet pump riser differential pressures' Each oscillation recults in some arcing between the switch contacts as associated relays are picked up and dropped out.

ANALYSIS OF CCCURRENCE This occurrence did not threaten the safety of plant personnel or the nublic.

Had a loss of ebolant accident occurred, LPCI would have functioned properly ,

through the "A" recirculation loop. In the event of the loss of the "A" loop in conjunction with such an accident, the coro spray system would have adequately protected the core. .

CORRECTIVE ACTICN Immediate correctivt action was to replace swihch 261-34D and to recalibrate switch 261-34A. A modification has been initiated to modify all four of the LPCI loop onlection differential pressure switches. The modifications should' provide come signal dampening, therefore reducing the cycling'of associated relays. The components ntcessary to modify these pressure switches on both Uni _t 2 and 3 have already been ordered, and installation will be completed as econ as practicable.

FAILURE DATA These switches have had a' history of similar malfunctions resulting from e'xcessive cycling. The switches are Barton model 286 pressure switches with f range'of 0-4 psid. . . ,, -77'// -

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