ML20059D130

Intervenor Exhibit I-MFP-149,consisting of LER 1-91-006-00, Re Docket 50-275,dtd 910425

ML20059D130
Person / Time
Site:	Diablo Canyon
Issue date:	08/21/1993
From:	Shiffer J PACIFIC GAS & ELECTRIC CO.
To:
References
OLA-2-I-MFP-149, NUDOCS 9401070026
Download: ML20059D130 (6)

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! PG&E Letter No. DCL-91-099

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U.S. Nuclear Regulatory Cosnission Me "M '* I

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ATTN: Document Control Desk

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l Re: Docket No. 50-275, OL-DPR-80 Diablo Canyon Unit 1 Licensee Event Report 1-91-006-00 Actuation of Containment Ventilation Isolation Due to a Spurious High Radiation Alars Resulting from Radio Frequency Energy Generated by a Faulted Motor Gentlemen:

Pursuant to 10 CFR 50.73(a)(2)(iv), PG&E is submitting the enclosed i Licensee Event Report (LER) concerning an actuation of containment- .

ventilation isolation on March 26, 1991, due to a spurious high i radiation alarm resulting from radio frequency energy generated by a faulted motor arcing to ground. l This event hn in no way affected the health and safety of the public.. I incerel )

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e UCENSEE EVENT REPORT (LER) 169462 1

tKKETT MaGIS (I) Pads f (3 Y F AC]L.!TY thafE (a) 0l5l0l0l0l2l7l5 1 5 l DIABLO CANYON UNIT 1 i situ (.) ACTUATION OF CONTAINMENT VENTILATION ISOLATION DUE TO A SPURIOUS HIGH RADIATION ALARM RESULTING FR0 RADIO FREQUENCY ENERGY GENERATED BY A FAULTED MOTOR tytut tra sasets at atmost mit (n etnia Fac Littts invoLvio (a) it (s) son gav va va stwt at arv one asse mv va cocati saseta (s) 0 5 0 0 0 03 26 91 91 - 0l0l6 -

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l On March 26, 1991, at 0015 PST, with Unit 1 in Mode 5 (Cold Shutdown) at 0 percent power, a containment ventilation isolation (CVI) actuation occurred. This event constitutes an Engineered Safety Features actuation. The actuation occurred due to a spurious high l radiation alarm from containment air particulate monitor RM-11. A l four-hour, non-emergency report was made to the NRC in accordance with 10 CFR 50.73(b)(2)(ii) on March 26, 1991, at 0216 PST.

The root cause for the spurious high radiation alarm and resulting CVI was radio frequency interference (RFI) produced when the RM-11 sample pump seized. After the sample pump seized, the pump motor faulted and arced over to a bus ground. The arcing produced sufficient RFI to induce the spurious high radiation alarm signal. Immediate corrective actions were to verify that the high radiation alarm was spurious and reset the CVI logic; the RM-11 sample pump and motor were also replaced.

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1 LICENSEE EVENT REPORT (LER) TEXT CONTINUATION 169462 l

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1. Plant Conditions Unit I was in Mode 5 (Cold Shutdown) at 0 percent power for refueling.

I II. Descriotie of Event A. Event:

On March 26,1991, at 0015 PST, during perfomance of Surveillance

Test Procedure (STP) V-3T1, " Exercising Containment Atmosphere Sampling Valves," an attempt was made to start the sample pump (P) for containment air particulate Radiation Monitor RM-11 (MON)(IL). The design class 11 sample pump seized and locked the rotor for the sample pump motor (MO). After several seconds,_ the motor faulted and arced l

over to a bus ground'(GBU). The arcing produced sufficient radio j freque-cy interference (RFI) to induce a_ spurious high radiation alarn:

i (RA) . rom RM-II. The high radiation alarm caused a containment ventilation (JM) isolation (CVI) actuation. This event constitutes an Engineered Safety Features (ESF) actuation.

Operators noted that the RM-11 count rate was unstable for. a few  ;

seconds preceding the CVI. Following the CVI, the RM-Il count rate returned to normal background level. Other radiation monitors remained normal during this event. On March 26, 1991, at 0016 PST, operators returned the containment ventilation system to normal operating mode.

A four-hour, non-emergency report ..s made to the NRC in accordance with 10 CFR 50.72(b)(2)(ii) on March 26, 1991, at 0216 PST.

The RM-Il faulted sample pump and motor were replaced.

l l B. Inoperable Structures, Components, or Systems that Contributed to the l

Event:

None.

l C. Dates and Approximate Times for Major Occurrences:

1. March 26, 1991, at 0015 PST: Event / discovery date. RM-11 ,

high radiation alarm initiates  ;

a CVI. l

2. March 26, 1991, at 0016 PST: Operators confirm RM-ll high  !

l radiation alarm was spurious I and reset the containment ventilation system to normal mode.

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UCENSEE EVENT REPORT (LER) TEX 1 GUNilNUAllVN A03040 4 y 4

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3. March 26, 1991, at 0216 PST: A four-hour, non-emergency report was made to the NRC in accordance with 10 CFR-50.72(b)(2)(ii).

D. Other Systems or Secondary functions Affected:

None.

.1 E. Method of Discovery: ,

The event was imediately apparent to plant operators due to alarms and indications in the control room.

F. Operator Actions:

The operators determined that the CVI was due to a spurious high radiation alarm because the other radiation monitors remained normal and the low flow alarm for RM-Il failed to clear. Additionally, a bus ground at the sample pump motor was detected. Since'no abnormal radiation levels existed, the operators reset the CVI logic and restored the containment ventilation system to its normal mode of I operation. g G. Safety System Responses:

All containment isolation valves closed per design.

III. Cause of the Event Imediate Cause:

The imediate cause of the CVI was a high radiation alarm from RM-11.

B. Root Cause:

l The root cause of the spurious RM-11 high radiation alarm and resulting CVI was an RFI-induced signal caused by-the faulted sample i pump motor arcing to the bus ground after the sample pump seized.

C. Contributory Cause(s):

1. The RM-Il radiation monitor pump and motor combination were operating in an elevated temperature environment. There have been past failures of sample pumps and motors but they did not result in a CVI or occur on less than a six month interval.

Preventive maintenance has been performed at six month intervals, which was more conservative than vendor recomendation and which 5335S/0085K

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0l0 4l"l:5 Ytat 07; provided acceptable service. The motor currently has no thermal overload protection.

2. The radiation monitoring system has demonstrated'a sensitivity.to electronic noisa since the Units have been operating.. The power circuit for the motor is in close proximity to the electronic leads for the RM-11 monitor. A number of-events have resulted 1 from this sensitivity as listed in Section.VI. An upgrade  !

program for the radiation monitoring system is underway. -)

i IV. Analysis of the Event A. Safety Analysis:- i The spurious high radiation' alarm signal resulted in a CVI. This. ,

actuation is conservative, regardless.cf plant conditions. Plant-

-equipment functioned as designed. Had there actually been a high:

airborne particulate condition, the. ventilation systems would have been ready to perform their: accident prevention functions.

Consequently, the CVI did not adversely affect the health and safety i of the public.

V. Corrective Actions -

i A. Imediate Corrective Actions:

1

1. Operations verified that the high radiation alarm was spurious ,

and reset the CVI logic.

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! 2. The RM-Il sample pump and motor were replaced.  !

B. Corrective Actions to Prevent Recurrence: 2

1. A design change will be implemented to provide thermal' overload protective circuitry for the sample pump motors to prevent locked- l rotor current from continuing for a duration ~ sufficient to short J

the motor's insulation.  ;

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2. As discussed in PG&E letter DCL-89-254, dated October 2,1989, the radiation monitoring system is being upgraded to reduce sensitivity to noise and to remove the capability of design Class l

' II equipment to initiate CVIs.- The CVI: function will be assigned-to two new monitoring channels.: New radiation monitors will be added directly to the containment purge exhaust line and will be safety-related and seismically qu'alified. No pumps or motors-will be directly associated with the new monitors.

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VI. Additional Information A. Failed Components:

1. Conde Milking Machine Co. pump, model no. DA06CM00.

2. Reliance Electric Co. motor, model no. P18G431G-XM.'

8. Previous Similar LERs:

1. LER 2-88-005, " Containment Ventilation Isolation Initiation Due to Electronic Noise Caused by Nechanical Wear.on the Check Source Latch," reported a CVI caused by noise generated from a motor that was cycling because a check source latch failed to' latch properly. The check source drive motor assembly was replaced, and since failure of the motor assembly was not a recurring .

problem, no further corrective actions were deemed necessary.

The co'..ctive actions taken for this LER could not have prevented the event for LER 1-91-006 since they did not address RM-ll maintenance.

2. LER l-89-001, " Containment Ventilation Isolation: Initiation Due.

To Personnel Error," reported a CVI caused by a technician shorting a lead with a screwdriver which caused a voltage .

transient. The corrective actions taken for this-LER could not  :

have prevented the event for LER 1-91-006 since they did not l address RM-11 maintenance.

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3. LER 1-90-019, " Actuation Of Containment Ventilation Isolation Due To Personnel Error," reported a.CVI caused'by a worker shorting terminals with a pair of pliers which caused a voltage transient. l The corrective actions taken for this LER could not have prevented the event for LER 1-91-006 since they did not address RM-11 maintenance.

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,r LERs 1-89-001 and I-90-019 describe spurious CVIs caused by voltage transients during maintenance operations. LERs 2-88-005 and I-91-006 describe spurious CVIs caused by'RFI that was~ induced by an arcing control device and a failed motor, respectively.- The measures taken to lengthen the integration time delay.as described in DCL-89-254 have-been particularly effective in reducing ~ the incidence of spurious CVIs caused by short bursts of RFI. Events caused by. sustained RFI, such-as LERs 2-88-005.and I-91-006, are not as preventable with the current radiation monitoring system configuration. '

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