ML20084F860

Ro:On 710720,control Rod Partially Inserted Following Spurious Reactor Scram Signal,Resulting in Turbine Trip, Producing Average Power Range Monitor Trip & Reactor Scram. Unit Adequately Protected

ML20084F860
Person / Time
Site:	Dresden
Issue date:	09/08/1971
From:	Hoyt H COMMONWEALTH EDISON CO.
To:	Morris P US ATOMIC ENERGY COMMISSION (AEC)
Shared Package
ML20084F863	List: ML20084F860
References
4037, NUDOCS 8304210298
Download: ML20084F860 (9)

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Commomvealth Edison $

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ILLINOf9 Address Rep.'s to POST OFFICE SCX 767

• C H 8 C A G O. ILilNOss 60690 Dresden Nuclear Power Station R. R. #1 Ed Morris, Illinois 60450

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September 8, 1971 k

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. 19 Dr. Peter A. Morris, Director Y

Division of Reactor Licensing Y

U. S. Atomic Energy Co nmission

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Washington, D. C.

20545

SUBJECT:

LICENSE DPR-19, DRESDEN NUCLEAR POWER STATION UNIT 2:

REPORT OF PARTIAL CONTROL ROD IN3ERTION FOLLOWINC SPURIOUS REACTOR SCRAM SIGNAL.

Dear Dr. Morris:

This letter is to report a condition that occurred during surveillance testing of the reactor protection system. The condition caused a partial control rod insertion following a spurious reactor scram signal. This partial control insertion ultimately resulted in a turbine trip, which produced an Average Power Range Monitor trip and a reactor scram.

A.

Seqt*ance of Events The reactor una in the run mode and operating at 406 MWe on July 20, 1971.

Surveillance testing of the main steam line radiation monitor was in progress with tests en the "A", "B",

and "C" monitors satisfactorily completed. A high radiation signal was then simulated to the "D" monitor and resulted in a trip of Reactor Protection System (RPS) Channel B, as expected.

Following normal surveillance procedures the operator began to reset the trip, f irs t turning the reset switch to the group 263 position. He then began to turn the switch to the group 164 position but, before he reached that po.ition, a spurious trip occurred and cicared on the "C" steam line radihtion monitor in RPS Channel A.

Because the Channel B group 2&3 scram solenoids had already been reset, only the eighty-eight (88) rods associated with scram solenoid groups 164 began inserting. The operatore reset motion then completed to the group 164 position. At this point the group 164 rods ceased their insert motion after moving approximate!; six (6) notches from their original positions. This rod insertion caused a drop in electrical load from 406 MWe to 320 M4e.

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COPY SENT REGION e~ / W.. 9304210298 710909 PDR ADOCK 05000237 S PDR _J

Dr. Peter A. Morr September 8, 1971 The turbine control valves closed down to maintain turbine-throttle pressure at 920 psig. A reactor pressure spike, resulting from the rapid, partial closure of the turbine control valves, caused steam voids in the reactor to collapse resulting in a reactor water level decrease of approxi-mately 20 inches. The feedwater system went into the " runout" mode to regain reactor water level. Reactor water level then increased rapidly and tripped the turbine on high level. The reactor pressure spike (960 psig) caused by the rapid closure of the turbine stop valves resulted in void collapse and APRM Hi Hi Flux flow based scram. B. Reactor Protection Svstem Logic Description + The RPS is designed with two independent logic Channels A and B. Each of these channels is divided into two sensor operated and one manual operated subchannels as shown in Figure 1. Subchannels A-1, A-2, B-1, and B-2 are equippea with a sensor, a sensor relay, a scram relay and pilot scram valve solenoids (two for each reactor control rod hydralic control unit). Two subchannels (A-3 and B-3) have a manual scram push button in place of the sensor and sensor relay. The control rod.s are divided into four nearly equal groups, identified as Groups 1, 2, 3 and 4. Group 1 and 4 control rods are scrammed by Channel "A" relays 108A, 108E and 109A and Channel "B" relays 1088, 108F and 109B. Group 2 and 3 control rods are scrammed by Channel "A" relays 108C, 10EG and 109C and Channel "B" relays 108D, 108H and 109D. (See Figure 1 & 5). A scram requires that a Channel "A" and a Channel "B" scram relay must be de-energized causing the two pilot scram valve solenoids on the control rod hydralic control unit to de-energize. A minimum of four scram relays, two in each channel, must de-energize to achieve a scram i.e.: a Group 1 and 4 plus a Group 2 and 3 scram relay in Channel "A" coincident with a Group 1 and 4 plus a Group 2 and 3 relay in Channel "B". Resetting the scram relays is accomplished through the reset relay circuit shown in Figure 2. The operator uses a singic reset switch as shown in Figure 3 for the reset circuit. When the reset switch is in the center position, all switch contacts are open. When the switch is operated to the right, scram relays for control rod Groups 2 and 3 in 4 i Channels "A" and "B" are reset (re -energized. ) When the switch is operated to the left, scram relays for control rod groups 1 and 4 in Channel A and ) B are reset. A reset circuit is provided and is so arranged that following a scram signal f rom Channel A & B there is a 10 second time delay before the scram relays can be reset. The time delay is provided to allow time for the control rods to move their full travel before the scram relays can be reset. This prevents stopping the control rods in an intermediate position in the core for a scram.

, _ _, = Dr. Peter A. Morr September 8, 1971 ~ A half scram reset is permitted without time delay because a half scram does not produce control rod movement. i Relays 114A, B, C and D cre the reset relays. The 114 relays are energized through the reset switch following a scram. When the 114 relays are energized their contacts close and energize the scram relay thereby returning the scram relays to their normal energized position. When the reset switch is released, the 114 relays de-energize (see figure 3) but the scram relays remain energized through their own seal-in contacts (see figure 2). The 114 relays are energized through the reset switch (303 device) and either the 125 relay contacts or the 122 relay contacts (see figure 3). Energizing through the 125 relay contacts permits immediate resetting (as permitted for a half scram). Energizing through the 122 relay contacts results in a ten second time delay before resetting the scram relay (as re-quired for a full scram.) i The 125 A and B relays (see figure 4) are not energized for a half scram but they are energized for a full scram. When energized (full scram) the 125 relay contacts energize the 122A and B time delay relays. When the 122 relay contacts close af ter a ten second tise delay the reset relays (114) are energfand to reset the scram relays. i C. Analysis of Events During surveillance testing, a Main Steam Line Radiation 4:onitor scusor was operated on Channel "B" which de-energized scram relays 108F, l Groups 1 and 4, and 108H, Groups 2 and 3 (See Figure 1 subchannel B-2). The Channel "A" relays remained energized and, therefore, there was no movement of the control rods. This is a half scram condition. The test was completed and the operator was restoring the RPS to normal by resetting i the scram relays. Relay 108H uas reset (re-energized) by operating the i reset switch in Figure 3 to the right to reset the Groups 2 and 3 scram relays and re-energize their scrnm solenoids. The operator was in the process of moving the reset saicch to the left reset position when a Main Steam Line Radiation Monitor on Channel "A" generated a spurious trip. This spurious trip de-energized scram relays 108 E and 10BG in Channel "A" (See Figure 1 subchannel A-2). Scram relay 108F in Channel "B" had'not yet been reset and, therefore, the pilot scram valve solenoids de-energized and initiated insertion of Groups 1 and 4 control rods into the core (scram) because their A & B scram solenoids were de-energized. The Groups 2 and 3 control rods did not scram since only a Channel "A" Groups 2 and 3 scram relay (10BG) and ecram solenoids were de-energized. The channel "B" Groups 2 and 3 scram telay (108H) tripped during surveil-lance testing, had aircady been reset. At this time relays 108E, 108F, and 10BG vere de-energized. Relay 125B was energized and time delay relay 122B was timing out. Relay 125A could not be energized because relay 108H had been re-energized. (See figures 3 & 4.) r i I t _1

Dr,. Peter A. Morris / 3sptember 8, 1971 The operator then completed his reset action by turning the switch to the group 1 & 4 position. This successfully energized reset-relay 114A and re-energized scram relay 108E in Chaunci "A" and the pilot scram valve solenoids in Channel "A" groups 1 & 4. This left the solenoids of Channel "A" groups 2 & 3 and Chanel "B" groups 1 & 4 de-energized; no rod group had both Channel "A" and "B" solenoids de-energizsd. (See figure 5). This reset occurred before the control rods were fully inserted and resulted in stopping the rod movement at an intermediate position in the core. Stopping the rods at an intermediate position in the core resulted in dropping about 86 M4e of load. The load rejection resulted in a pressure transient which in turn caused a high flux scram. All control rods scrs=ned normally and shutdown the reactor. D. Conclusions A review of the RPS circuitry by Consonwealth Edison and General Electric indicated that: 1. The circuit functioned as designed and no component failures i occurred. 2. The aborted ceram described herein could only occur with a partially recet trip en one channel coincident with a short duration trip on the second channel occurring precisely as the operator is in the process of completing the reset of the trip on the first channel. 3. A trip on both channels would override the reset function and go to completion even with the reset switch held in a reset position. A test was successfully performed on Dresden Unit #2 on July 23, 1971 to verify this, j 4. The unit is safe to operate and the RPS provides the necessary l protection to safely shutdown the reactor, l The Station Review Board and Nucicar Review Board have each reviewed the sequence of events and analysis and concur with the above conclusions. { j lr ll l 7 j /.!ll/f.yx H. K. Hoyt l Superintendent l HKH:do )

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