ML19309H136

LER 80-027/99X-0:on 800220,during Reactor Trip from 100% Power,Abnormal Noise in C Steam Generator Detected by Installed Loose Parts Monitoring Sys.Cause Unknown. Westinghouse Personnel Called in to Conduct Investigation

ML19309H136
Person / Time
Site:	North Anna
Issue date:	05/01/1980
From:	Cartwright W VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.)
To:	NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II)
Shared Package
ML19309H135	List: ML19309H134 ML19309H136
References
LER-80-027-99X, LER-80-27-99X, NUDOCS 8005080493
Download: ML19309H136 (4)

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S 64 68 DOCKET NUMSER 6e 69 EVENT OATI AEPCMT DATE EVENT DESCRIPTION AND PROBA8LE CONSEQUENCES h i o l ::1 pn February 20, 1980, durino a reactor trip from 100% power, an abnonnal noise in l I

"C" steam generator was detected by the installed loose parts monitoring system.

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g subsequent testing revealed that more than one loose part may be present and that g f i

g ,the impacts may be originating from inside the Reactor Coolant System. Since the g ioisg ginit was maintained in a hot standby condition while the investigation of the l

[o}7; l loose parts indications was being performed, the health and safety of the general l

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Virginia Electric and Power Company North Anna Power Station, Unit #1

Attachment:

Page 1 of 3 Docket No. 50-338 Report No. LER 80-27/99X-0 Description of Event Coincident with a reactor trip on February 20, 1980, an alarm sounded indicating a noise in "C" steam generator and upper reactor vessel. Impact noise could also be heard on the loose parts monitoring speaker. The "C" reactor coolant pump was shut down and the noise disappeared. The noise resumed when the pump was restarted and then disappeared when full flow was reached. Subsequent testing revealed metal impacts from as many as four separate distinct sources possibly originating from inside the Reactor Coolant System.

Probable Consequences of Occurrence Analysis of noise data obtained by Westinghouse concluded that the reactor is presently in a safe and operable condition as long as the indicated loose parts do not migrate and affect internal plant equipment or interfere with control rod movement. Because the unit remained in a hot standby condition throughout the duration of this analysis, the public health and safety were not affected. There are no generic impli-cations associated with this event.

Cause of Event Although the metal impact noises can be attributed to loose parts, the exact cause and origin of these indications is not known at this time.

Immediate Corrective Action '

Westinghouse specialists were called in to obtain noise data and analyze the results. The objectives of this investigation was to determine if actual loose parts exist and if so the number of objects present, their approximate size, and if possible, their origin. To collect the necessary data, special signal conditioners were substituted for the installed Rockwell signal conditioners in the Loose Parts Monitoring System. The in-containment Rockwell equipment was used without modification.

With special test equipment available, data was recorded on a 14 channel FM recorder, allowing for a frequency of 0-20 KHZ. Signals from the tape recorder then allow simultaneous recording of several channels on a visicorder chart, where time differences of the various impacts of a loosa object (s) from different locations may be determined. Additionally, after noting impacts from ty visicorder charts, or by directly scanning the magnetic tapes, the imr e.s may be examined very closely in the time

, and frequency domain by us- . a spectrum analyzer.

Conclusions of th .nalysis were as follows:

1. There are no adications of parts in the bottom of the reactor vess , therefore the part is considered to be on I

the Tg side.

2. Upper vessel detector signals are approximately the same when the ro/s are latched and the RCC's raised approximately one foot, nen unlatched with the drive shafts free from i

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Attachment:

Page 2 of 3  !

l l the CRDM. Reactor vessel impact signals are independent of steam generator signals and have the same character as those attributed to impacting of the in-core detector pressure thimble against the guideways.

3. Steam generator detector signals were obtained from each steam generator during reactor coolant pump transients; however, no impact indications were noticed during steady 1 full flow operation of the pumps. These impacts could I not be detected by listening by ear to a metallic rod that was in contact with the channel head.

Since testing showed that plant instrumentation and rods were operating normally and because no activity was reported in the steam generators, the plant was determined safe for operation provided the indicated loose parts in the steam generator channel head do not migrate so as to affect internal plant equipment. Rod drop tests were satisfac-torily performed which confirmed free rod movement. Attempts to link the noises with loose parts in the plant secondary side were partially successful in that changes in steam and feed flow tended to produce noticeable effects on detected metal impact frequencies.

Actions recommended by Westinghouse have been conducted on a con-tinuing basis to ensure that the indicated loose parts do not migrate and that the plant remains in a safe and operable condition. From the three sets of nuclear noise data taken, it has been observed that the signals in the 5 to 10 hertz frequency range have increased 33%; however, there is no indication of any structural degradation. The latest analysis of observed metal impacts has concluded the following:

1. The impacts are originating from the steam generator secondary side and are being transmitted to the sensors (located on outside of steam generator) by indirect mode.

The upper reactor head noises differ from the steam generator soundings in amplitude only and are suspected to be the result of noise carryover.

2. The object weight is believed to be a 1/4 pound or less and as a result no safety concerns exist at this time.

3. Westinghouse now believes that there is at least one small object in the secondary side of "B" steam generator with the possibility of more in steam generators "A" and "C".

Scheduled Corrective Action During the next outage of sufficient duration, the secondary side of "B" steam generator will be opened and inspected for loose parts.

Due to the conclusions recently deduced from the ongoing evaluations, the following changes will be made to the original eleven Westinghouse recommended actions. These changes will not be implemented until NRC Region II concurrence is obtained.

1. Because the loose part(s) is no longer considered to be in the Reactor Coolant System, present primary side surveil-lance frequencies will be reduced to their normal required periodicity.

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Attachment:

Page 3 of 3

2. Operation in reverse flow, due to one or more reactor coolant pumps tripping, will no longer require analysis to determine if loose part indications have migrated.

3. Investigations will be performed for secondary side transients instead of primary side transients as previously recommended.

With the preceeding changes incorporated, all Westinghouse recommen-dations will continue to be performed on a regular basis to assist in the attempt to identify and resolve the loose parts problem.

Actions Taken to Prevent Recurrence Actions to be taken will be determined af ter further evaluation and identification of the indicated loose part.