ML20090D635

Updated Ro:On 711231,unit Tripped Offline as Result of Surveillance Testing.Sensor Support Accidentally Bumped Causing High Level Trip Sensor to Operate Resulting in Turbine Trip.Operator Given Review of Sys Response

ML20090D635
Person / Time
Site:	Nine Mile Point
Issue date:	04/19/1972
From:	Burt P NIAGARA MOHAWK POWER CORP.
To:	Skovholt D US ATOMIC ENERGY COMMISSION (AEC)
References
NUDOCS 8303020205
Download: ML20090D635 (4)

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- NIAGARA MOH AWK POWER CORPORATION l HlAGARA MOHAWK Nine tiile Point Nuclear Station Unit #1 Post Office Box 32 Lycoming, New York 13093 April 19, 1972

. Ftr. Donald J. Skovholt Assistant Director for Reactor Operations -

Division of Reactor Licensing on qq]~/ $N, United States Atomic Energy Commission ..,

Washington, D. C. 20545 &

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Re: Provisional Operating License: DPR-17 . ,f

Docket No.: 50-220 N @, @

In our Ictter of January 20, 1972- to Dr. Peter h! orris, we described a prob-t lem with the reactor feedwater system of Nine Flile Point Nuc1 car Station, Unit #1. Although we felt at the time that the problem was understood and proper remedial action initiated, the investigation was continued. These findings did substantiate those set forth in the January 20th letter and we are now able to present a final description of the incident.

On December 31,1971 at 10:08 am, the Nine !!ile Point Nuclear Station, Unit #1 tripped off line as the result of surveillance testing.

i Initial Conditions Steady state operation i

!dfl llWth - 1752 Reactor pressure - 1015 psi

llWe - 601 (gross) Steam flow - 6.8 X 106 lbs. per hr.

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Introduction I! Routine surveillance testing of the reactor protection high/ low water level

') fy sensors was being conducted at the time of the trip. The sensor support was accidently bumped causing each high level trip sensor to operate

& resultiiig liFa~ turbine trip. A reactor scram resulted from the turbine 8 8 anticipatory trip signal because the load was greater than 45%.

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Mr. Donald J. Skovholt April 19, 1972

. ' Introduction (cont'd)

Following the reactor scram, the reactor water level decreased rapidly due to void collapse. The feedwater control system responded by overfeeding, as it should, when in the automatic mode. The feedwater system was left in the automatic mode for approximately 20 seconds after the scram, and then switched to the manual mode, because the feedwater flow to the reactor was high in the operator's opinion. Manual action was too slow and excessive feedwater flow continued to the reactor. Feedwater flow was reduced to ::ero at approximately 2 minutes after water overflowed into the main steam lines.

Several operations of the electromagnetic relief valves occurred for approx-inately 17 minutes after which reactor level was brought under control. The emergency condenser was placed in service to control reactor pressure after the water icvel was brought under control.

Sequence of Events 10:08:02 an A turbine trip occurred fr7m an erroneous high reactor water IcVel signal caused by bumping the sensors 10:08:02 Reactor scram from turbine anticipatory 10:08:20 (approx) Shaft feedwater pump in manual control 10:08:27 (approx) #12 motor pump in manual control 10:08:30 (approx) Main steam isolation valve closed 10:08:33 #11 Motor pump in manual control 10:09:30 Reactor level +3 feet above normal 10:10 Reactor pressure 1117 psi 10:10:11 Relief valve 121 open, @ ' I3 I .

10:10:13 Relief valve 121 closed @ 7- O5 10:10:56 Relief valves 111, 112, 122 open eV I #1-10:10:59 Relief valve 112 closed fB T iD b 10:11:00 Relief valves 111, 122 closed 10:12 (approx) Feedwater flow to ::ero 10:20 Level under contr,1 J

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Mr. Donald J. Skovholt April 19, 1972

. Analysis of Data A turbine trip occurred at 10:08:02 from an erroneous high reactor water level signal caused by bumping the sensors. The sensors were bumped while surveillance testing was being conducted on the sensors, ceWwy*-~% CW6#f h wftqdde&Q*

A turbine trip causes a reactor scram from the turbine anticipatory trip if turbine load is greater than 45*.. All control system followed the expected transient response characteristic for the first 18 seconds following the scram.

There were three feedwater pumps running in the automatic mode before the 6

trip. Two motor driven pumps were cach delivering about 1.5 X 10 lbs/hr.

and the shaft pump was delivering about 5.2 X 106 lbs/hr. -

Reactor level response after c scram results in a 3 ft drop in level due to steam void collapse. Die feedwater responds with a large increase in flow. Total flow 20 seconds after the trip was approximately 8.2 X 10 6 lbs/hr.

At this time, the shaft pamp was placed in the manual mode and 7 seconds later, a motor pump was placed in manual mode. The second motor pump was placed in manual approximately 30 seconds after the scram. The feedwater controls were placed in manual because the operator observed the high flow, which in his judgement required some action. Analysis of data shows that the flow was reducing before the shaft pump was switched to manual, and one of the motor driven pumps flow had reduced to :ero before being switched to manual. The total feedwater flow was reduced to zero at approximately 4 minutes after the trip. The first relief valve opened 2 minutes after the trip and stayed opened for 4 seconds. Three more valves opened for (3-4) seconds, h'ater overflowed into the main steam lines at about the time the first relief valve operated. Feedwater level was brought under control at approximately 12 minutes after the trip.

Cause of the Reactor High tlater Level Investigation of the feeduater system has shown that the control response is adequate to handle the transient after a scram. The decision by an operator to pine" 'he system in manual is a judgement decision based on the interpretation of the instrumentation he is observing. Once he has made the decision and goes to the manual mode; he must be extremely dexterous as level varies so rapidly for the first few minutes following the scram that it becomes almost humanly impossibic to differentiate the variables and perform the correct manipulations in the required interval. At this time, IcVel was near the +3 feet level, and flow was greater than 6 X 106 lbs/hr.

Flow was reduced to 2 X 106 lbs/hr. at 2 minutes after the scram. Data indi-cates that overflow of water into the steam lines occurred about 2 minutes after the scram. Some feedwater flow continued for the next 2 minutes before being reduced to zero, m

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April 19, 1972

, . Mr. Donald J. Skovholt Conclusion "11tc turbine trip and reactor scram occurred as a result of an accidental bump to 1cvel sensors during surveillance testing.

"Ilte feedwater response in the automatic mode was normal for the transient conditions that existqd.

Placing the feedwater system in manual when fast response is required may cause a level problem if the operator does not pay close attention to the system during the transient.

Corrective Action A review of expected system response has been given to the operators as part of the continuous educational program. This would help the operator in making the right decision during future trips.

Very truly yours, kM P. Alli, ster Burt General Superintendent, Nucicar Generation i

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',, NIA0 MOHAWK POWER CORPORATIO N BA G A R A J.- MOHAWK Nine t~ tile Point Nuclear Station Post Office Box 32 Lycoming, New York 13093 January 20, 1972 Dr. Peter A. ?! orris, Director #

f Division of Reactor Licensing g ,',

United States Atomic Energy Commission h'ashington, D. C. 20545 y ,(' ,, ,' ( Q 3 5 1C h

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Dear Dr. Florris:

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,- -T Re: Provisional Operating License: DPR-17 '- W

Docket No.: 50-220 The second stage of reheat was removed from service at Nine blile Point Nuclear Station Unit #1 on January 18, 1972. A crack had developed in the drain line from the coil of reheater

1. 112 This heating coil is supplied with primary steam, and condensate fron the coil drains by gravity to a receiving tank, hhen the crack was located, the second stage reheater was secured. The part having the defect was an 8" schedule 80 seamless weld " tee" ASTBI A-234 Grade h'PB.

A crack developed at the edge of weld deposit metal and extended about an inch through the heat affected zone in a direction longitudinal to the major axis of the tee.

This system will remain out of service until the turbine is next removed from service when a better appraisal of the cause of the diffic::lty can be obtained and proper repairs affected.

l3 The next scheduled outage is to commence April 2, 1972.

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Very truly yours, a /. , ,/ , q C

.) , bb abr'. h '1b P. Allister Burt C0 Station Superintendent

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