ML19326C032

RO 50-313/76-1A:on 760105,required Overlap Between Operating Control Rod Groups Lost.Caused by Component Failure. Redundant Relay Addition in Circuitry Will Require Simultaneous Failure to Cause Ratchet Trip

ML19326C032
Person / Time
Site:	Arkansas Nuclear
Issue date:	04/09/1977
From:	ARKANSAS POWER & LIGHT CO.
To:
Shared Package
ML19326C027	List: ML19326C026 ML19326C032
References
NUDOCS 8004180714
Download: ML19326C032 (3)

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1. Reportahic Occurrence Report flo. 50-313/76-1A (Fo110 tup to AOR 50-313/76-1)

2. Report Date: April 9,1976 3. Occurrence Date: January 5, 1976 4 Facility: Arkansas fluelcar One-Unit 1 Russc11villo, Arkansas .

5. Identification of Occurrence: I Loss of required overlap between operating control rod groups. '

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6. Conditions Prior to Occurrence:

Steady-State Power x Reactor Power 1695 F!Wth  !

Ilot Standby Not Output 550 Fllfe Cold Shutdown Percent of Full Power 66 **

Refueling Shutdown Load Changes During Routine .

Power Operation --  !'

- Routine Startup

, . Operation Routine Shutdown Operation Other (specify)  !

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7. 11cseription of'Occurrenec: I At 16:28 hr. on January 5, 1976, group 6 control rods dropped into the core, causing an apparent violation of Tech Spec 3.5.2.5, rod group over-lap requircuent of 25'41 5?4 between sequential groups. Reactor tripped on RCS low pressure. -q I

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Attachcient "C"

.itirch 23, J l'76- NSP-10, Rev. 4 Page 2 of 4 ,

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i V Reportable' Occurrence !!cport No. 50-313/76-IA 8.' Designation of Apparent Cause of Occurrence:

4 Design Procedurc Bhnufacture IJnusual Service Condition Including Environmental Installation /

Construction Component Failure X ,

Operator Other (specify) ,

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9. Analyt,;s of Occurrence:

nere is no core s' fetya significance associated with tlie minimum overlap requireuent, rather the concern is for an overlap greater than 30's. This reduction in overlap did not create any unsafe conditions, and no core protection linits were approached. Eis incident did not effect the health and safety of the public. s.

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i Attachment "C" l

' l Fbrch 23,19i6 NSP-10,.Rev. 4 Page 3 Of 4

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U Reportab!c Occurrence !!cpot t' ;!o. 50-313/76-1 -

10. Co rrec t i ve Ac t ig:

Instrument technicians worked throughout the ensuing shutdown period tryJng to dctormine the cause of the ratchet trips. Relays controlling Control Rod Drive mechanisu cicctrical stator phase sequencing were driven and'uonitored by computer continuously all week as though they were in continuous operation driving control rods. 'Ibo positive fail- .

urcs occurred that could have resulted in a ratchet trip if the rod drive systen had been in operation at that moment, his led to the decisirn to uodify the system sligittly by adding a redundant relay in the cir-cuitry of all groups with holding coils hooked in para 11c1 and contacts hooked in series such that both relays would be required to fail sinul-taneously to cause a ratchet trip. Bis modification will in no way affect the ability of the Control Rod Drive mechanism to disengage the leadscrew and .allou control rods to fall into the core under reactor trip conditions.

l I1. Failure Data: 7 Diamond Power *

"alay !!RRIA, Part No. 56410-001 which is part of Gate Drive Assembly, Part No. 707410-1044- .

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.it t a chucut "C" l cla rch 23, 1976 NSI'- lu, Rev. . page ; or ,;

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