ML19317F384

RO 270/77-04:on 770324,dropped Control Rod Group 6 Caused Rod Overlap in Excess of Tech Spec Limit.Caused by Gate Drive Failure of Phase CC in B Power Supply.Fuse Replaced on Rod Group

ML19317F384
Person / Time
Site:	Oconee
Issue date:	04/07/1977
From:	DUKE POWER CO.
To:
Shared Package
ML19317F380	List: ML19317F379 ML19317F384
References
RO-270-77-04, RO-270-77-4, NUDOCS 8001140637
Download: ML19317F384 (2)

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'OCONEE UNIT 2-i: .

.c _. Report No.': '_RO-270/ 7 7-4 -~

' Report Date:{A'pril.7, 1977- ~ '

< ~ 0ccurrence Date: ' March 24,.'1977 Facility: Oconee Unite 2, Seneca, South' Carolina-u Identification ~of> Occurrence: Rod overlap-in excess of Technical Specification

-limit due to a dropped control. rod group; Conditions Prior'to Occurrence: ~Uniti at 78 percent full power

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. Description'of Occurrence: ~

p a On March 24,z1977, group 6 control rods dropped into the core from a 67%

. withdrawn position reducing the reactor. power level to'44 percent" full power.

A condition'of 5% control _ rod: overlap between rod group 5 and 6 resulted.

Oconee Technical Specification 3.5.2.5 specifies an overlap of 25 1 5%

between.two adjacent operating control rod groups.

The Integrated Control. System-Immediately-relatched the group 6 rods and

. began withdrawing =the rods in order;to maintain the power level a.t 78%.

r. -Rod 3 in group 6.did not relatch and this asymmetric. rod' caused power to.

be limited'to 55.. percent;-full-power. The power supply.for rod 3 was de--

energized by removing the fuses. No problems were found so the fuses were L

reinstalled and~ rod 3 was relatched and realigned with'the. rest of group.6.

Control rod. group 6 ;was'then placed on auxiliary power and reactor power escalation commenced. : Repairs to control rod group 6 power supply'were I

completed.within four hours and the-group was returned to its normal power

supply.

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[ Apparent Cause-of Occurrence:

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h This occurrence resulted from the failure of the gate drive of phase "CC"~

ll ~ ine the '?B'? power supply. .Normally, the "A" power supply would have held.

[:. , .the rods.in position', however,.the "A".. power supply to the "CC"' p'ase h

[_ f uiomentarily - failed. . Investigation of the."A": power supply revealed no pro-

, 1 blems. It isipostulated that l prolonged energization of Lone magnetic: phase n

, -in"the,"A"; power / supply resulted'in tso opposing phases being simultaneously nenergized, therefore canceling each other and result'ing in the drop and

.'immediate- relatch of the rods.

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r mi ' Analysis of-Occurrence:

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['< ;ThisLincidentirssulted!inl control rodjgroup;6~ dropping Into the core and.a

[> fcondition of:;approximately Supercentjcontrol rod overlap existing until group.

, C6 control fods'.wereire -positioned .and the required overlap was re-established.

K' .Thisfreductionin control" rod' overlap'did not' affect the: safe operation of I lthe reactor andinojcore protectionflimit'sEwere approached. It'is'therefore

[, Jconcluded that thisLoccurrence did.notJ. affect:the health 2and. safety of thec

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C yrective Action: l d on rod 3 of group ,.

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With four hours following the incident fuses were rep ac 1 verified operable.- drive stator power In addition, a modification to improve the control rodimplemented on each unit  ;

supply . reliability has been initiated and will be during.:its respective refueling outage.

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