05000245/LER-1992-001, :on 920127,discovered That Opening SBGT Circuit Breaker Would Defeat Isolation Signal to Reactor Bldg Ventilation Dampers.Caused by Inadequate Design.Sbgt Sys Operating Procedure Sp 646.6 Revised

:on 920127,discovered That Opening SBGT Circuit Breaker Would Defeat Isolation Signal to Reactor Bldg Ventilation Dampers.Caused by Inadequate Design.Sbgt Sys Operating Procedure Sp 646.6 Revised

ML20127D191
Person / Time
Site:	Millstone
Issue date:	09/02/1992
From:	Cohen S, Scace S NORTHEAST NUCLEAR ENERGY CO., NORTHEAST UTILITIES
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-92-001, LER-92-1, MP-92-958, NUDOCS 9209110266
Download: ML20127D191 (6)

LER-1992-001, on 920127,discovered That Opening SBGT Circuit Breaker Would Defeat Isolation Signal to Reactor Bldg Ventilation Dampers.Caused by Inadequate Design.Sbgt Sys Operating Procedure Sp 646.6 Revised

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i) 10 CFR 50.73(a)(2)
2451992001R00 - NRC Website

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Reference:

Facility Operating License No. DPR-21 Docket No. 50-245 Licensee Event Report 92-001-01 Gentlemen:

This letter forwards update Licensee Event Report 92-001-01 required to be submitted pursuant to 10CFR50.73(a)(2)(i).

Very truly yours, NORTHEAST NUCLEAR ENERGY COMPANY L

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Stephen E. Jeace Vice President - h1illstone Station SES/SC:ame Attachment: LER 92-001-01 cc:

T. T. Martin, Region 1 Administrator P. D. Swetland, Senior Resident inspector, hlillstone Unit Nos.1. 2 and 3 J. W. Andt rsen, NRC Acting Project Nianager Millstone Unit No.1

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v AesTaAct put to 340c spac., t... ament.y sm.en sw.-saac. wooeun on.si ou On January 27,19t>2, at 1300 hours0.015 days <br />0.361 hours <br />0.00215 weeks <br />4.9465e-4 months <br />, with 8.he plant m cold shutdown-(136 degrees Fahrenheit, 0 psi) it was

- discovered that opening a Stand-by Gas Treatment (SDGT) circuit breaker would defeat the isolation signal to two (2) of the;four (4) reactor buildmg ventilation isolation dampers thui makmg them inoperable with regard to providmg secondary containment isolation. Defeating the isolation signal to two of the four reactor buildtng

ventilation isolauon dampers resulted in a violation of Technical Specihcations secondary containment requirements. The remaining two dampers were operable and would hate isolated the reactor buildmg in the

- event of a condition requiring secondary containment nolation. The SBGT system procedures base been revised -

to provide guidance on removing one SBGT citemt from service. - No safety consequences resulted from this

. event.

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Dngiption of Evern On January 27,1992, at 1300 hours0.015 days <br />0.361 hours <br />0.00215 weeks <br />4.9465e-4 months <br />, with the plant m cold shutdown f136 degrees Fahrenheit. O pst) it was discovere that epening a Stand-by Gas Treatment (SBOT) circuit breaker would defeat the iso!ation d

signal to two m) of the four (4) reacts buildmg venulation isolauon dampers thus making them moperable with legard to providing secondary containment solanon. Defeatmg the nolanon signal to two of the four reactor buildu.; venulauon nolanon dampers resuhed in a violauon of Technical Specificanons secondary contamment regturements The remammg tuo dampers were operable and would have isolated the reactor buildmg in the esent of a condition reqmring secondar) contamment isolauon. The SBGT system procedures have been revned to proude gu dance on removmg one SBG circuit from serace. No safety consequences resuhed from tlus event.

II.

Cause of Etent The cause of this event was determined to be the inadequate design of the origmal isolation logic for the Reactor Bui! ding Ventilanon dampers for all SBGT >> stem configuration < (i.e. one SBGT breaker opened).

lil, Analysic of Ermn The isoladon of the Reactor Building Ventilanon is required to prevent a radioacuve release durmg an accident or high radiation condnion. The Reactor Building Ventilation isolates on a Group 11 isolanon (2 ps drywell pressure and low reactor water level (d)) or Hi-radiation signal fsteam tunnel exhaust radiation 50mr/hr or greater, refuel floor radiation 100mr/hr or greater and reactor buildmg ventilation exhaust radiation 11mr/hr or greater).

During an engineering technical evaluation of the supporting svstems regmred to implement an Emergency Operating Procedure (EOP) the followmg was discovered: (Reference Figures 1 and 2 attached)

1) The relay that proudes the isolation signal to the reactor building ventilation dampers also provides the SBGT iruuation signal This relay is energued to isolatelininate and receives power from the SBGT circuit breaker.

2) - The pilot solenoids to the air operated isolation dampers are normally energized and de-energize to isolate. These pilot solenoids valves receive power from Instrument AC and 125 VDC and do not fail closed when a SBGT breaker is opened These pilot solenoids vahes receive a close signal when the SBGT iniuation relay energizes. When the SBGT circuit breaker is opened, the power to the SBGT initiation relay is removed. This prevents the SBGT irunation relay from energizing and prondmg the isolation signal to 2 of the 4 Reactor Buildmg Ventilation Isolation dampers (one supply damper and one exhaust damper). These two dampers will remain open un:il the redundant supply and exhaust damper close, tnpping the Reactor Building Supply fans. The redundant supply and exhaust dampers would still receive an isolation signal and isolate the Reactor Building.

3) Defeating the tsolation signal to 2 of the 4 isolation dampers would reduce the reliabihti of the Reactor Building Ventilation isolation. However, during normal plant operation a SBGT circuit breaker is not opened except as required for mamtenance or surve:1!ance.

This event is being reported per 10CFR50.73 (a) (2) (t) (B), " Any operation or condaion proh:bned by l

Technical Specifications" based on the following:

1) The opening of a SRGT circuit breaker defeats the isolation signal to 2 of the an dampers.

These two dampers will remam open and will not receive an isolation signal un..

redundant supply and exhaust damper close, tnppmg the Reactor Buildmg supply fans.

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2) Technical Speilication's defmition of Secondary Cont:unment requires " All automatic sentilation system isolation valves be operable or secured m the isolated position" IV.

Correctshm The short term correcuse action was to revise the SBGT Sptem operaung procedure (SP 64o.6) to provide gwdance on removing one train of SBOT from service when Secondary Contamment is required per Technical Specification Requirements. The procedure revision provides guidance for removine one train of SDGT for mamienance or suneillance of the fan and blier assembly or for ma'mtenance of the SBGT control logac. The procedure directs operators to pull the thermal ove.-loads in heu of opening the circuit breaker. This prevents the isolanon signal to 2 of the 4 dampers from being defeated. If the breaker is required to be opened, the operators will be required to isolate the normal reactor buildmg ventilation ar.d run the operable SBGT tram. Additionally. Caution labels hase been mstalled on the SBGT breakers referencing the operators to SP 646.6 prior to openmg a SBGT circun breaker.

i A design modificanon to the SBGT irniation and Reactor Buildmg Ventilauon isolatton logic was evaluated and it was determined that a technical specification change was a more appropnate corrective acuan. A teclimtal specification change will be processed to add the reactor buildmg ventilation dampers, which receive an isolanon signal from the out of service SBGT train (SBGT breaker open) to the hmiting conditior af operation (LCO) for a SBGT circuit inoperable, V.

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