ML20087B913

Ro:On 710314,after Reactor Trip & Subsequent Turbine Trip, Lubricating Oil Flow Was Lost to Turbine & Generator Bearings.Caused by Decrease in Battery a Voltage

ML20087B913
Person / Time
Site:	Robinson
Issue date:	04/07/1971
From:	Utley E CAROLINA POWER & LIGHT CO.
To:	Morris P US ATOMIC ENERGY COMMISSION (AEC)
Shared Package
ML20087B830	List: ML20087B913
References
NUDOCS 8403090395
Download: ML20087B913 (5)

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Carolina Power & Light Company /:,9' Raleigh, North Carolina 27602

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Dr. Peter A. Morris Division of Reactor Licensing U. S. Atomic Energy Commission Washington, D. C. 20545 FACILITY LICENSE NO. DPR-23 DOCKET NO. 50-261

Dear Dr. Morris:

On March 14, 1971 at approximately 2:55 P.M. an incident occurred at the II. B. Robinson Unit No. 2 that led to the failure of the turbine. After a reactor trip and subsequent turbine trip, lubricating oil flow was lost to the turbine and generater bearings.

The rotor came to rest in the abnormally short time of approximately 17 minutes and seized in several failed bearings.

Assessment of the damage indicated that all eight turbine and generator bearings had suffered some damage. Bearings 4 and 5, between the two low pressure turbines, failed to the extent that molten babbit flowed through the bearings. As a result, the No. I low pressure turbine rotoi- dropped sufficiently to cause rubbing between the rotor and the shell. Damage in the turbine was limited to failure of some steam seal strips and cracking of three shroud segments on the No. 6 wheel nearest No. 4 bearing. The most serious damage to the No. 1 rotor was surface hardening of the rotor material 4 on the oil seal adjacent to the No. 4 bearing. The rotor was removed to the Westinghouse turbine shop.in Charlotte, North Carolina,_for a complete inspection. The "hard spot" was removed by machining.

The No.1 low pressure rotor was removed from the shell and inspected thoroughly at the plant. Magna-flux readings were taken on all critical areas. No damage was indicated on the No. 2 rotor or shell and that unit is being re-assembled. i During the reactor shutdown, seal flow was interrupted to two reactor coolant pumps for approximately one minute. Subsequent investigation indicated.the shaft for "A" reactor coolant pump may have been distorted by uneven heating.- This pump has been disassembled and a new shaft is being installed. The seal on "C" reactor coolant pump will be disassembled for inspection before the unit is returned to vice.

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l Dr. Peter A. Morris - 2 - Ahril 7,1971 SEQUENCE OF EVENTS:

The unit was on the line carrying a load of 615 MWe as directed by the system load dispatcher. All plant conditions were normal. The operating staff was conducting the weekly routine checkoff of plant equip-ment.

8:30 A.M. Auxiliary Operator checked battery room and noted all indications normal in process of completing the shift Auxiliary Operator check-off list.

10:30 A.M. Auxiliary Operator started DC emergency oil pump, fed from "A" battery bus, for routine two (2) hour test run. This~ pump _was not stooped as planned atJ12:30.. EL,-

because the. Auxiliary Operator became involved in other routine duties. All conditions remained normal until shortly before the reactor trip. One momentary alarm was received on "B" battery charger trouble annunciator. _

The alarm cicared immediately and no further troubic ~

was experienced on "B". battery or battery charger.

2:49 P.M. The plant computer failed. Subsequent investigation indicated the failure was due to low DC voltage fed from battery bus "A". ,

2:50 P.M. The Control Operator observed reactor . trip breaker "A" indication light was.out and the Shift Foreman was notified.

The_ bulb was changed with no s'uccess. Several other lights on'RTGB were observed to be out. The Shift Foreman'and Control Operator suspected instrument bus (AC) trouble.

I & C technician assistance was summoned:and Shift Foreman .

proceeded to. check out MCC-5 and MCC-6 and all instrument busses for failure.

2:5.1 P.M.. . Received reactor coolant pumps thermal barrier cooling water low flow alarm. -Outlet. isolation valve 735 (air /

. operated) closed on low DC control voltage. RTGB valve-f position indication also lost. .

2

55 P.M.- Received reactor - trip due to low voltage on' DC trip coils;

.on react'or trip breakers. Reactor trip initiated turbine trip. ;No battery charger alarms were received.

l' 2:56 P.M.~ Received generator lockout (one minuteLtime delay) due to-OCB. closed.and stop. valves closed.. A portion of the control -

. room lights were lost. 4KV~ bus 4 switched 'to start-up -

transformer. properly leaving 4KV bus 3 and '4 on start-up.

transformer. 4KV busses 1 and 2 were lost-because DC con-trol voltage on "A" battery was;too low to'close breaker s (_,,

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52/12 and put these busses on the start-up transformer.

"A" diesel started properly due to loss of E-1 bus voltage.

Diesel supply breaker 17B to E-1 bus did not close because of low DC control voltage from "A" battery.

The Shift Foreman and several other individuals while checking on 4KV voltage found no DC control voltage on several breakers.

The batteries were checked immediately and "A" battery was found with 60 volts and "0" amps. All breakers on "A" "A" battery battery bus were closed. The AC supply from MCC-5 to the charger was lost when 4KV busses 1 and 2 were lost.

It should be noted that the battery chargers are rated at 300 amps with a current limiting device set at approximately 375 amps. It has been reported that the normal DC load on the "A" bus is approximately 150 amps. Assuming a 350 amp load imposed by the DC cmcrgency oil pump, the load on the i

DC bus could have been as high as 500 amps. This would have ,

imposed a minimum discharge rate on the "A" battery of at 1 cast 125 and possibly as much as 200 amps. A 200 amp ~

discharge rate will lower battery voltage to 105V DC in approximately four hours assuming a fully charged battery initially.

'3:00 P.M. The "B" charging pump was lost when E-1 voltage was lost.

The "C" charging pump was started bnmediately to re-establish seals and charging flow. The seal leak-off temperature on reactor coolant pumps "A" and "C" rose sharply as indicated.

1. A RCP Recorder Point No.1 - Seal leakoff 300 F.

Recorder Point No. 2 - Upper thrust 300 F.

Recorder Point No. 7 - Pump bearing is 280 F. ' q dB RCP Recorder Point No.15 - Pump bearing is 230 F.

1. C RCP Recorder Point No. 17 - Seal leak'off 300 F.

Recorder Point No. 23 - Pump bearing 230 F.

"A" and "C" pumps tripped when 4KV b'usses 1 and 2 were lost.

The seal flow on "A" and "C" pumps decreased to zero. "B" pump continued to run with normal temperature and seal flow.

Charging flow was lost for less than one minute.

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The volume control tank had no level indicated. Operator switched to. refueling water storage tank for supply to re-establish volume control tank for supply to re-establish volume control tank level. Switched back to normal lineup after 30 inches indicated in volume control tank.

A safety injection signal was activated due to instrumentation power supply failure. Pressurizer icvel remained above 20*/.

and pressure above 2000 psig. All safety injection equipment except that from E-1 bus operated satisfactorily.

Operator observed turning gear oil pump and emergency DC oil pump lights we: e out.

3:12 P.M.- The turbine rolled to a stop approximately 17 minutes after the turbine trip. Condenser vacuum was maintained.. The AC turbine gear oil pump was not operable due to loss of MCC-5 power supply; i.e. , loss of 4KV bus 1.and inability of diesel , ,

breaker 17B to close. The DC emergency oil pump was not operable due to low voltage on "A" battery. ,

3:20 P.M. The battery bus. tie.was closed and "A" battery voltage immediately increased to 125 volts. Breaker 15B closed re-establishing voltage on E-1 from "A" diesel. The Control Operator closed breaker 52/12 and picked up 4KV busses 1 and 2 from the start-up transformer. The Control Operator're-established power to 480V bus 1 via station service trans '

former "A". The' turning gear ' oil pump started when E-1 power was're-established.- The generator oil lif t pump . started and the turning gear engaged attempting to roll the turbine. The' turbine would not turn and the turning--gear motor began smoking >

heavily. -The turning gear motor was manually tripped. . Unsuc-cessful attempts were made to roll the turbine manually.and with an air drive motor. . Oil was pouring from.the No. 5 turbine bearing. All attempts to roll the turbine were unsuc-

.cessful.

Condens'er vacuum and steam seals-vere maintained on'the unit' and steam dump continued to the' condenser.

3 Restoration of DC power provided f6r opening.of ' valve 735.-

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l to restore reactor coolant pump thermal barrier flow.'.

i 3525P.M. - Restored normal letdown from reactor. coolant system? -

[, - Approximately five -(5) minutes after restoring power to nErmal,

- ' MCC-6 ~ tripped. Operator was unable to close breaker, j The'-

> control fuses were-replaced and the breaker then closed properly.. "

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3:30 P.M. Turning gear oil and lift pump were secured due to oil leakage from No. 5 turbine bearing.

With the reactor in a safe condition, assessment of the tur-bine damage commenced.

The incident was determined to have been initiated by the decrease l in "A" battery voltage to the point where components supplied by the "A" i DC bus could not function as designed. Several factors contributed to the loss of voltage on "A" DC bus and the failure to detect this decrease in .

voltage.

1. The DC emergency oil pump was operating from the "A" -- t DC bus. This resulted in an unanticipated discharge of the "A" battery. Due to personnel error, this pump l was left operating and not shutdown after the scheduled i two hour test, j l

Operating procedures and operator training are being reviewed to insure that unsatisfactory conditions *

. are corrected.

2 No alarm is provided to warn the Control Operator of low DC bus voltage. The installation cif suitable alarms '

is being investigated.

3. The AC oil pump is currently fed off of MCC-5. With the loss of E-1 voltage, MCC-5 is without a power supply.

'Ihe possibility of changing the AC oil pump power supply to MCC-6 is being investigated in order to provide a (

completely redundant back-up lubricating oil supply to the I turbine.

It is estimated that the unit will be returned to service by May 15, .n, 1971.

Yours ver trul ,

- f f E. ,E.~Ut1ey

-Manap,er Generation & System Op'erations NBB:dds cc:. Mr. J. A. Jones G 9

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