ML17199F971

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Safety Evaluation Supporting Corrective Actions to Automatic Transfer of Power Feeds & Alignment of Transformers to Same Offsite Bus.Plant in Compliance W/Gdc 17
ML17199F971
Person / Time
Site: Dresden  
Issue date: 12/16/1986
From:
Office of Nuclear Reactor Regulation
To:
Shared Package
ML17199F970 List:
References
NUDOCS 8701080549
Download: ML17199F971 (5)
v • d • e


Text

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e UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION

~UPPORTING DRESDEN UNITS 2 AND 3 COMPLIANCE WITH GDC-i7 *

(

  • i COMMONWEALTH EDISON COMPANY

)

DRESrJEN NUCLEAR POWER STATION t UNIT NOS *. 2' AND: 3.,

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DOCKET NOS *. 50-237/249 l

l;O INTRODUCTION

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On August 16" 1985, Dresden Unit 2 scrammed on 1 ow reactor water leve 1 *.

The scram resulted fro~ a loss of offsit~ power due to a fault on Unit 1 reserve auxiliary transform~r TR-12.

  • Transformer*TR-12 was b~in~.

powered from the Silme 138KV of;fs ite power 'source as*.. the _Unit 2 reset*ve:

auxiliary transformer, TR-22.

  • When protecthe relaying sensed the *.
  • fault, the Unit 2 _13811'.V offsite power source 'isolated and powerto*

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.. - TR-22.was* lost. Subsequently, two 4KV buses, Nos. 22 and 24, lost power when*an automatic transfer from TR-22 to the.unit*auxili~~Y transformer, TR-21, did not occur~ As.a result, one of two runninCi._*._.

reactor feed pump~_trfpped and __ the standby reactor ~eedp~mp coOld ~ot.,

1

  • start due to bus underv0ltage.

The low-water level scram and the

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. complete loss :of power* to the 'unit resulted. - Both. emergency d5esel;, *,.

generators started and the unit was placed in a safe shutdown :

condition.' Ourind the.NRC staff review of this :event,, discuss-ions *.

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with the licensee~;led to questions -regarding the availability of a:*.,.,

second* access-- circuit *for offs-1te_ power.

As a-resu-lt,*the* lfcerlsee~:-.J

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aoreed :to-review the desion conformance to General. Desian Criterion,-*17-(GDC-17), *iElectric Power-Systems".

ThiS safety evaluation pr.esents*.th~,

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st~ff review of the licensee's analysis of the event and con.formancei to.:

GDC-17".

  • 2.O EVALUATION*

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  • The licensee forwarded--initial descriptions of the event and* immedi.a;te: 0:..
  • * ¢orrective actions by letters dated August 22, 1985 and September 11~

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1985 (~ER 85-034) *. Additiona 1 information in response to staff questions '-*<:

was provided by letter.to the NP.C dated December 13 and December 31,.198,5. :-

Backoround Figure 1 i 11 ust.rates the configuration of off site power at the.

switchyard.

The electrical system for the Dresden station i~shown in.

Figure 2.

During normal power operation of each unit, one division of...

  • the safety-related.electrical distribution system is energized from,,. :
  • _the plant's main aenerator via the unit* auxiliary transformer (e.g.,

~TR-22, for Unit 2J.

The design includes a feature to fast transfer.the loads on the reserve auxiliary transformer to the unit auxiliary

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transformer in the event of a power loss.

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Discussion The event caused the staff to question the acceptability of *plant... *-*

operations with Unit 2 reserve auxiliary transformer TR-22_ aligned to 138KV bus section 3 in addition* to ilinit 1 transformer TR-12 alianed to. *

. the s.arrie bus section. 'At issue:.is.fhe*increasecl vu'lnerabiTity to""loss O'f

  • the GDC-17 immediate-access offsiteipower circllit for Unit 2 (TR-22)
  • 1 caused by electrical faults on Unit: 1. equipment (TR-12) due tc "ot :
  • feeding these transformers from:. separate bus sections of the 138 KV
  • switchyerd~ The plant lic~nsinq basis (i.e~, Final Safety Analjsis

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.:.: Report.. (FSAR)} shows the Unit 1 transformer TR-12.powered from bus *sect1on *

  • 3.and the.Unit.* 2 transformer TR-22 powered from.either bus section 1 or 3, '

with the cor,nection. to section J be,ing. foormally closed". The staff's

    • .* review of-the P.ugust 1985 evenf rev'ealed that the plant is operated with
  • * * *,,.' TR-22 energized pre_fe1*entially _frcm:bus section. 3, whi,ch is the. same

. source.used for TR-12 *.. The *1 icensee responded to sta*ff concerns *by t letters dated.August 22,, December 13, and Decen:iber 31, 1985.

The licens~e*

has described its basis for conr.ludina that the* offsite power alianmen*t

  • 1 that was and is currently in use provides the best; reliability.and flexibility~
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l Licensee investigation of the 4KV breaker. control circuitry determined.;.

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that the f~eder breakers for ~KV buses 22 and 24.were net desia~ed, as I

  • ' believed, to autcmatically transfer power feed; from the normal supply, * *
.. reserve auxiliary transformer TR-2i.. Licensee, review identified a desi.gn.

": error fn the clr.sing control circuits of certain bus 22 and 24 feed*. -:.t break~rs which ~revented the automatic*power transfer from-TR-22 to TR~21 *. *

  • 1 The.design erro_r resulted from a previous modification. *Specifically, :*.
-i i the modificatio
n was intended to cause a fast tra.r.sfer of certain buses l

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  • from the_ n:-22 :as :the source of power* to TR-21*; however, the design of tbe. *..

trarsfer.looic assumed (and: required) that* !!ni 'loss. of JR-22 wouJd involve

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. fa fault which would lead to the electrical loc'kcut of the transfbrrrier *. '"-< *:*"

rurina"the -Jl.uaust 1985 event* the t~ansformer output ceased simply because'..'..

  • l( of loss of incorr,ing power, with.out!, an actuation of_ the lockout relay for

" ' the transformer. ;Therefore, the t:ransfer did 'not function.

  • This trc.nsfer.-..

feature is~ necessary to satisfy the portion: of, GDC-17 which *req1,11res that._*:..

,._. the loss of an offsite.circuit not cause the foss of the: plant.n1C,;ir

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  • generator, which could,happen'if power.is lost to certain equipment such;,*_:.. ', *.. *.*
  • *
  • as feedwa ter pumps* \\*1hic'h in. turn cause a p 1 ant: trip.

I * * * *. i..,, *.

" L The des ian error has been c~r-rected-and. the tr'ansfP.r schem*e has :been tested satisfactorily *. NRC Region III*has reviewed the modificatir.r1 and!*

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  • observed post-modification tf:sting of the automatic transfer function
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  • .r frcim TR-22 to TR-21.

The results of the Reoion !If.review were published..

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in a letter to the licensee ~ated:September 27, 1985.

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Based Ori staff and reaior:al review of licensP.e. corrective actions..

relative to the automatic transfer of power feeds from transformer TR-22 to transformer TR-21, design is consistent with *the guidelines of*

Regul_atory Guide 1.32 and the commitments rriade in the.updated FSAR. *

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The licensrP's GDC-17 analvsis statei that the Unit 2 secnnd access

  • . sb~rce of offsite pow~~ re~uired hy GDC-17 consists of a 4~V cross-ti~
      • between l'nit 2.and Unit 3 via bus-tie 24'.'"1/34-1. *rtie secorin source of pnwe~ is normally supolied from the U~it 3 ~45KV switchyard;
  • The.

sec'ond source of offsi":e oower can be activaterl from the main control rnom via two breaker c6nt~n1 switches and was available durinq the Auaust.lf, 10BS event.

The analysis states that the load c~r~ying capahi l i ty nf the bus-tie has been eva 1 uated ?nd determined t:c b_e adecuate to support the ne<:essar.v emeroency 1.o;:1ds.

The licensee load ana_1ysiSincludes r:irl.v*one division of em~r0P.ncy loads.* This desi'}n satisfies the interrr~tation of GDC-17 th~t was used when-Dresden 2 ard 3 were licensed.

j,O. CQNCIJ!SJ0.N P<isPci on the discussior.s ir the Evaluation Sect inn.0f +.his report~

the 1;cP11si:ie's aporoach to rP.solutir.r. of NRC questions reqarcing +.h_e

.. :*.. correct.1vP. actions relative t.o +.he automatic trci.nsfor r.f power f'eeds *

    • ~and rPaar~irc thP. ~liqn~ent of tran~former~ TR-?l~~nd TP-1? tn the-~am~

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(]959-1971\\.

Pased on the past ciperatira expP.rience of thts and other rlant:s that have onJ:v twn half capacity~lines, the staff does not believe that doublinvr*

  • this Tpchriical Specificatinn and address* its acceptahi1it\\* i_n a*

~eparate li~ensing action.

  • . Princir)al co_ntributor:

R. Scholl Dated:*. December 16, 1986

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