05000285/LER-1980-008-03, /03L-0:on 800515,during Check of Power Supply Trip Indicating lights,AB-1 Matrix Relay of Reactor Protective Sys Failed to Trip Clutch Power Supplies.Caused by Failure of Mercury Wetted Relay in Matrix Circuit

/03L-0:on 800515,during Check of Power Supply Trip Indicating lights,AB-1 Matrix Relay of Reactor Protective Sys Failed to Trip Clutch Power Supplies.Caused by Failure of Mercury Wetted Relay in Matrix Circuit

ML19310A276
Person / Time
Site:	Fort Calhoun
Issue date:	05/19/1980
From:	Connolley J, Core M OMAHA PUBLIC POWER DISTRICT
To:	NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV)
Shared Package
ML19310A275	List: 05000285/LER-1980-008, Forwards LER 80-008/03L-0 05000285/LER-1980-008-03, /03L-0:on 800515,during Check of Power Supply Trip Indicating lights,AB-1 Matrix Relay of Reactor Protective Sys Failed to Trip Clutch Power Supplies.Caused by Failure of Mercury Wetted Relay in Matrix Circuit
References
LER-80-008-03L, LER-80-8-3L, NUDOCS 8006110015
Download: ML19310A276 (4)

LER-1980-008, /03L-0:on 800515,during Check of Power Supply Trip Indicating lights,AB-1 Matrix Relay of Reactor Protective Sys Failed to Trip Clutch Power Supplies.Caused by Failure of Mercury Wetted Relay in Matrix Circuit

Event date:
Report date:
2851980008R03 - NRC Website

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3 60 63 GOCK ET NUMSER 68 69 EV ENT QATE 74 75 REPORT D ATE 30 EVENT DESCRIPTION AND PRC8ABLE CONSEQUENCES h l o i 21 l During check of power supply trip indicating lights,.it was discovered that the AB-1 )

[o;3, l matrix relay of the Reactor Protective System failed to trip the clutch power sup-l 10 j 4 l l plies.

The failure resulted in only a slight degradation of the system since five 1

1o!3; j of the six logic units were working properly. The fault resulted in loss of one of I

taisl l the four trip relays in one of six such assemblies.

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l1 loll A mercury wetted relav, C.P. Clare Model HGZX10ll. in the matrix circuit failed to I

li li j i break contact to trip clutch oower sucolies.

The relav was removed. *he rena,cas 1

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NAME OF PCEPARER 4

LER 80-008 Omaha Public Power District Fort Calhoun Station Unit No. 1 Docket No. 05000285 ATTACHMENT NO. 1

Safety Analysis

The APS uses a two-out-of-four logic trip system.

For the RPS to automatically initiate a reactor trip, identical trip units, in-two of the four channels, must be in the tripped condition.

If two trip units are tripped, the Matrix logic system will cause the reactor to trip by interrupting power to the clutch power supplies, thereby dropping the control rods into the core.

The AB-1 relay, a mercury wetted contact relay, is used as 'part of a matrix ladder consisting of a set of twenty four contacts. There are six matric ladders employed in the RPS which provide the required com-binations of two-out-of-four trip logic (i.e. A/B, A/C, A/D, B/C, B/D, C/D).

If any set of contacts in the AB matrix ladder opens, relays AB-1, 2, 3 and 4 de-energize opening redundant contactc to the four M relays which provide power to PS 1, 2, 3 and 4, the clutch power sup-plies. Opening the contacts from relays AB-1 or AB-2 will interrupt power to PS-1 and 3.

Likewise opening contacts from relays AB-3 cut AB-4 will open interrupt power to PS-2 and PS-4.

Since the AB-2 relay (the redundant relay to AL-1) was fully oper-ational at the time of the failure of the AB-1 relay, the action of the matrix would have been performed by the AB-2 relay and would have caused the same result, that is, the clutch power supplies would have been de-energized.

LER 80-008 Omaha Public Power District Fort Calhoun Station Unit No.1 Docket No. 05000285 ATTACHMENT NO. 2 Failure Data The failed relay, C. P. Clare Model HGZX 1011, is a mercury wetted contact relay. This is the third failure of this type of relay at Fort-Calhoun Station Unit No. 1.

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y LER 80-008 Omaha Public Power District Fort Calhoun Station Unit No. 1 Docket No. 05000285 ATTACHMENT NO. 3

Corrective Action

The AB-1 matrix relay was removed frem the logic ladder and tapped to remove deposits from the contacts. Experience and investigation into the operation of mercury wetted relays shows that this an acceptable method of returning the relay to a new condition.

Although this is the third failure of its kind, there has not been a second failure of the same relay in the RPS and, therefore, the licensee feels that the problems occurs af ter a number of years of operation and tapping the relay returns the relay to a new condition. As a precaution against further failures, all of the = atrix relays were removed from the RPS and tapped in the same manner as the AB-1 relay.

To prevent failures in the future, the licensee will prepare a maintenance procedure to remove all relays during refueling shutdowns and perform the cleaning operation as described above on all mercury wetted relays in the RPS matrix systems.

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