ML20084R046

Ro:On 691108,reactor Scrammed on Momentary Upscale Trips from Intermediate Range Monitoring Channels.Flux Spike Caused by Increase in Feedwater Flow.Some Rods Not Fully Inserted

ML20084R046
Person / Time
Site:	Nine Mile Point
Issue date:	11/18/1969
From:	Burt P NIAGARA MOHAWK POWER CORP.
To:	Morris P US ATOMIC ENERGY COMMISSION (AEC)
Shared Package
ML20084R049	List: ML20084R046
References
98813, NUDOCS 8306140776
Download: ML20084R046 (2)

Text

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.s'o - n o NI AG ARA MOH AWK POWER CORPORATION NIAGARA MOHAWK Nine Mile Point Nuc1 car Station P. O. Box 32

's /s Lycoming, New York 13093 /'( $ k November 18, 1969 jfjy' [ ,],['/h<[

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.g,,/y f I Attention: Dr. Peter A. Morris, Director Gentlemen:

hhile in the process of heating the Nine Mile Point Nucicar Station reactor to oper-ating pressure and temperature prior to supplying steam for turbine testing, on November 8, 1969, the reactor scrammed on momentary upscale trips from intermediate range monitoring channels. The flux spike had occurred due to an increase in feed-water flow. Examination of the rod display disclosed that ten of the 129 rods had  !

i missed being driven to their fully inserted position by one noteb, and one had missed by two notches. 'lhe redctor was made sub-critical. These control rods were then  ;

individually driven to full insertion by operator action.

Prior to the complete insertion of all rods, the I101 returned below high trip setting; and the scram trip was reset, stopping the scram action. The operator contends that he did not operate the scram reset button. No occurrence of an unsafe condition existed as the reactor was shut down as was called for by the high IRM trip; and if .)

the trip condition had persisted, no manual or spurious reset could have prevented full insertion of all control rods lnvestigation by the Station Operations Review Committee disclosed inadequate coordi-

/jnation of the relay which interrupts power to the four sets of scram solenoid pilot valves.

g{, P ' relays and'this relay is normally energized through a series arrangement of sensor trip an auxiliary seal-in contact. The main contacts of the GE type CR-105 j F [ relay which interrupts power to the scram solenoid pilot valves open in

[g#' jailli-seconds . llowever, the auxiliary " scal-in" does not open in less than 20 milli-

,fysecondsafterthecoildeenergizes.

cl Sensor trip relays operate much faster than 20 v>

y /guilli-seconds which can cause the CR-105 relay to drop out and pick up before the

p)* / " seal-in" (actually prevents coil to pick up unless reset by pushbutton) contact opens 8 producing an auto reset, f

Y 'Ib Relay circuits were revised to prevent the possibility of future auto resets. An l' existing relay type Gli ilFA whose coil is parallel with the CR-105 relay and had spare contacts available was utilized. This type relay has contacts which open in approx-inately 10 milli-seconds thus producing good coordination with the main contacts of l the CR-105 relay. The contacts were added in series with the seal-in contacts of i the CR-105 relay. This arrangement insures that the trip will not reset automatically l on fast sensor trips because the sensor trip relays are no faster than the added contacts of the type llFA relay.

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United States Atomic linergy Commission Page 2 Novenber 18, 1969 l

l Investigatio'n also disclosed that it was possible to be manually resetting a " half- ' '

scram" on one channel and imposing a reset on the other channel (there was only onc ,

reset button for two channels). Should the second channel be in the process of i initiating a full scram, the operator would by this action reset the scram trip.

If the reactor protection system trip was only momentary in nature and no longer present, the scram solenoids would reset arresting the rod travel in mid-travel causing a strong pressure surge in the index tubes of the drives. To avoid this possibility, reset buttons were installed in each of the two separate ilPS channels .

instead of one button for two. J Presently design investigations are underway to install timers in the reset circuitry to prevent op,crator reset action before the control rods are fully inserted on scran ,

action. l Very truly yours, e ,

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P. Allister llurt Station Superintendent mjs ec: fir. M.11. Prat t Mr. F. J. Schneider Mr. J. N. I! wart , Chairman, Sit G A lloard Mr. M. liildreth, A.li.C. Compliance ShYY O'

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