ML20029B598

LER 91-003-00:on 910203 & 16,spurious Control Bldg Isolation Signals Received from Train a Control Bldg Ventilation Inlet Radiation Monitor.Caused by Equipment Malfunction.Detector replaced.W/910304 Ltr

ML20029B598
Person / Time
Site:	Millstone
Issue date:	03/04/1991
From:	Rachel Johnson, Scace S NORTHEAST NUCLEAR ENERGY CO., NORTHEAST UTILITIES
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-91-003, LER-91-3, MP-91-195, NUDOCS 9103130091
Download: ML20029B598 (4)

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NORTHEAST UT1UTIES ""a' o"- S **a S"' Saa ""*""'

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HARTFORD. CoNNECTICU' Ot.414-0270 (203)605-5000 Re: 10CFR50.73(a)(2)(iv)

March 4, 1991 MP-91-195 U.S. Nuclear Regulatory Commission Document Control Desk Washington, D.C. 20555

Reference:

Facility Operating License No. NPF-49 Docket No. 50-423 Licensee Event Report 91-003-00 Gentlemen:

This letter forwards Licensee Event Report 91-003-00 required to be submitted within thirty (30) days pursuant to 10CFR.'U.73(a)(2)(iv), any event or condition that resulted in manual or automatic actuation of any Engineered Safety Feature (ESF).

Very truly yours, NORTHEAST NUCLEAR ENERGY COMPANY

. F

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Stephe E. S ace

'MW irector, hiillstone Station l

1 SES/RWJ:mo 1

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Attachment:

LER 91-003-00 cc: T. T. hiartin, Recion I Administrator W. J. Raymond,' Senior Resident inspector, hiillstone Unit Nos.1, 2 and 3 D. H, Jaffe, NRC Project hianacer, hiillstone Unit No. 3

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Beginning at 1427 and ending at l$04 hours, on Feb-uary 3,1491. In Mode 3 (Hot Shutdown), at 09 reactor power, 354 degrees Fahrenheit and 344 psia, six spurious Control Building Solauon (CBI) signak were recched from the A Tram control buildmg sentilation inlet radtauon monitor dunny the reactor plant shutdown for the scheduled refuehng outage. On February 16.1941 at 0645 hours0.00747 days <br />0.179 hours <br />0.00107 weeks <br />2.454225e-4 months <br />, while in Mode 6 (Refuehng). 4' degrees Fahrenheit and 20 psia, another spunous CBI signal was recened from the A Tram radiation morntor. Each time a (il signal was received, the Reactor Operators immediateh responded by obserrmg the B Tram radiation monitor was indicating normal bacQtound lesel while the A Tram radiation monitor readmg was rapidly decaying from the high alarm lesel to the normal background level. Having determined the A Tram radiation monitor was malfunctioning, the Reactor Operators initiated CBI signal blocks prior to the actuation of automaut control buitdmg pressunzauon and manualh- aligned and operated the senulauon system with full recirculated filtered air for the control room pressua en elope.

A subsequent msestigainn revealed the spurious CBI signab uere caused by radiation monitor equipment malfunction. The root cau3e of the equipment malfuncuon is undetermined at this time. A Supplemental Report will be submitted by August 30. 1091 to discuss the root cause determmation, i

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1. Dacnntion of Event

. Be,: inning at 1427 and ending at 1500 on February 3,1991, dunng the normal reactor shutdown for the 1991 scheduled refuehng outage, in Mode 3 filot Shutdown), at 0"r reactor power,354 degrees Fahrenheit and 399 psia, six spurious Control Building Isolanon (CBI) signals were recened hom the A Train Control Buildmp inlet ventilation radiation monitor. On February 16,1991 at 0646 hours0.00748 days <br />0.179 hours <br />0.00107 weeks <br />2.45803e-4 months <br />, while in L

Mode 6 (Refuehng), at 97 cegrees Fahrenheit and 20 psia, another spurious Control Buildmg isolauon (CBI) signal was received from the A Train control buildmg att ventilation inlet radiation monitor. Each ,

time a CBI signal was received, the Reactor Operators imrnediately verthed the B Train radiation monitor was indicating normal back.pround radiation level and observed that the A Train radiation monitor reading was rapidly decaying from the high alarm level to nor nal background levels. Having determmed the A Train radiation monitor was malfunctioning. the Reactor Operators initiated CBI b'ocks pnor to the actuation of the automatic control room pressurization. In accordance with plant Technical Specifications 3.3,2, the Reactor Operators manually aligned the control room emergency ventilanon system to provide full recirculated filtered att while the A Train radiation monitor was inoperable. A subsequent ,

investigation revealed the spurious CBI signals were caused by radiation monitor detector equipment -

malfunction.

II. came nf Event The root cause of the malfunctioning A Train Control Building ventilation inlet radiation monitor is not known at this time, in response to the February 3rd CBI events, the A Train radiation monitor was inspected to identify problems which could have precipitated the spurious high radiation signals. After troubleshooting the

• radiation tuomtor, the radiation detector was replaced as the most likely contributor to the spurious high radiation signals. The basis for the detector replacement was historical concerm wnh detector

- performance in an outside ambient erwitonment - although the detector had been m semce for only 7 months.

For the February 16th event, it was concluded that a detector malfunction, although potentially a contributing factor, was not the must likely cause for the intermittent (spurious) hipb radiation signals.

Because of the signal profile, cable shieldmp method and possibly ino.fficient isolation t'ft he detector from the ventilation duct work are being reviewed as factors contrit u ing to the intermutent high  :

1. radiation signals. The results of the ongoing root cause analysis in,e tigation sW Se submitted as an update report. Factois to be considered in the root cause analys include C = M Bulldmg radiation monitor malfunctions that have occurred on June 20,1990, thru July 1. !(rys, as described in Licensee Event Report (LER)90-024, and on September 4.1990, as described in LER 90-028, 111. Analysic of Event l

This event is reportable under 10CFR50,73(a)(2)(iv), as an event or condition that resuhed in automatic actuation of an Engineered Saiety Feature. Immediate notifications were made in accordance with 10CFR50.72(b)(2)(ii).

There were no significant safety considerations since an actual high radiation condition did not exist.

l Throughout the events, radiation levels in the control room did not increase and the unaffected train indicated only background radiation. The Control Buildmg ventilation inlet radiation monitors process radiation counts from beta scintillation detectors, and provide indication and alarms one alarm for l radiation exceeding the high radiation alert setpoint, and one for radiation exceeding the high radiation l alarm setpoint. A CBI signal is mitiated by either monitor when radiation exceeds the high radiation alarm setpoint. Detector malfunctions occur in the fail safe condition because failures result in the generation of a CBI signal, The control room remained in filtered recirculation until the moperable monitor was returned to semce at 1933 hours0.0224 days <br />0.537 hours <br />0.0032 weeks <br />7.355065e-4 months <br /> on February L 1991. In response to the February 16.

1991 event, the control room remains in filtered recirculauon until the monitor is restored to operable condition.

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v. . w w . w t w s.1 w . .v.,w or rma 5 AOld? Y N AM[ (iJ $ MW.f 7 NWbf b (2 F ', F L N. 96 e t 0 ef* I' A E8E 3 yg n eno<m. mvem eu etw ma Millstone Nuclear Power Stauon Un't 3 o j f. l o j 0 l o l 4 l 2 j .* , v ] . , O j 0l3 0l0 0l 3 OF 0l3 n c a, - . . .: ... - - ,.. . - .n . - a n n ,, n lY. Corteeth e ANirm in response to the February 3,1941 esent, the A Train detectoi was replaced with an idenocal new beta scintillation detector. In re3porne to the February 16. 1941 nent modificanons to the cable shielding and nolauon of the detector from the sentilanon duct woth are planned to imprme the performance of the momtor. Since the investigauon to date has not resobed the specific root cause, the resuhs of the ongoing mvesuganon (as well as any addiuonai correctne acuon) will be sobrmited as an update report by August 30, 1991.

V. Addnional Inf ormatWth LER 66-011 reported spurious CBI signals generated b) Electro-Magnetic interference (Ehll) m the A Train and B Train radiauon monitors. As part of the corrective bcuon for LLR F6-Oll, software noise filters, which prevent CBI signals due to Eh11 mduced i. diauon spikes, were tratalled m the Control Buikhng inlet ventilation monitors, in order to filter out near-instanoneous events The CBI events dacussed in thn LER occurred over penods of mmutes. Therefore the software none filter could not have prevented the CBI signals, and the events dacussed in LER $6-011 are conudered unrelated to the radiauon monitor mailunction dncussed in thn LER.

LER 00-024 reported a similar CBI signal ptneration due to the malfunction of the A Train Control building inlet ventilation radiation monitor. The detector had been in sernce for greater than f years at the ume of the event. In response to the LER 90-024 esent, the Preventatne hiaintenance Program was modihed to replace these detectors at 3 year mtervals.

LErt 90-028 reported a previous similar CBI signal generation due to the malfuncuon of the B Train con *rol t .ilding inlet ventilanon radiation monitor. The detector had been in sernce for 16 months at the ume of the event. This event, once considered an isolated event, is now considered related to the subject events, and shall be factored into the root cause analysn.

A computer sort of the NPRDS data bar provided no addiuonal information pertinent to the subject event.

The (KDB Beta detector) radiation mon tors discussed ate hiodel 450-860-L001, manufactured by Kaman Science Corporation.

Ell 5 Codes Svtanu ll - Radiation Momtonny System.

Comoonems MON - Momtor i

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