ML17238A712

Report of Safety Valve Operation Following a Feedwater Transient - Dresden Unit 3

ML17238A712
Person / Time
Site:	Dresden
Issue date:	12/17/1971
From:	Brian Lee Commonwealth Edison Co
To:	Morris P A US Atomic Energy Commission (AEC)
References
Download: ML17238A712 (10)

Text

..... * . ,_, ",. << 131 . . Address Reply to, 1 \.

POST OFFICE BOX 767 *CHICAGO, ILLINOIS 60690 Dr. Peter A. Morris, Director Division of Reactor Licensing U.S. Atomic Energy Conunission Washington, D.C. 20545 December 17, 1971

Subject:

Report of Safety Valve Operation following a Feedwater Transient

-Dresden Unit 3 (DPR-25)

Dear Dr. Morris:

This is to report a condition in whi.ch a condensate pump tripped, which caused a reactor water level transient.

This water level transient resulted in filling the main steam line with water, opening of a safety valve, and pressurization of the drywell.

The following information pertaining to thi,s occurrence is submitted pending completion of the investigation which is currently in progress.

S\immary A 1413 on December 8, 1971, as a result of a condensate*

l:)ooster pump trip on Dresden Unit 3, the reactor feed pumps tripped.

Tripping of the feed pumps resulted in a reactor water level transient.

This eventually resulted in filling the main steam lines with water, opening of a safety valve for approximately minutes and pressurization of the drywell.

Pressurization of the drywell.resulted in a high drywell signal which initiated starting of emergency

diesels, low pressure core cooling pumps, and HPCI. During the transient, drywell pressure reached a maximum of 20 psig. The maximum and minimum reactor pressures were 1.050 psig and 795 psig, respectively, and the reactor water level reached a minimum of -20 inches and a maximum of +130 inches. With water level at -20 inches, there is more than 9 feet of water above the fuel. A detailed sequence of events is attached.

All safety systems functioned

• a*s designed.

No significant radioactivity was released to the environment as a result of the incident.

During post incident

recovery, both the primary system and the primary containment were maintained in a "bottled up" condition until analysis of reactor water and containment atmosphere could be made. '**

e Commonwealth Edison Comp. .or. Peter A. Morris December 17, 1971 Damage Assessment

,. A preliminary inspection in the drywell following the incident revealed damage to the following equipment: 1 -The rupture discs on all the safety valves showed cracks. This may not be related to the incident since this condition has been encountered previously on normal shutdowns. 2 -The _3A electromatic valve was damaged by :the steam jet from the "F" safety va,_lve.

One steam discharge rams horn on the "F" safety valve was directed towards the electromatic valve. The cover on the solenoid assembly of this valve was blown off. The holding coil portion of the solenoid assembly was found open. Wiring to a position indicating limit switch was also damaged rendering the position indication circuit inoperative. 3 -Miscellaneous thermal insulation was damaged and requires 4 -The top coat of paint on the containment wall over an area about 3' x 3' was removed by the steam jet from the "F" safety valve impinging on the surface of the containment. 5 -Sections of ventilating duct in the vicinity of the steam jet were dislodged and require repair. 6 -The LPRM cables were found damaged.

This c&Tube must be replaced in total. '* 7 -The SRM/IRM caJ!>lbes were tested and found to be good. Following the Dresden Unit 2 June 5 incident, this c&b.1:e was replaced on Dresden Units 2 and 3 and Quad-Cities Units 1 and 2 with cable having a higher temperature (302°F 10 hour1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> rating) rating. 8 -One containment cooling fan motor was found to have a ground caused by moisture in the containment.

This motor will require drying out. The other six cooling fan motors were found to be. in good condition.

Preliminary conclusions The following conclusions have been reached the Dresden Unit 3 incident of December 8, 1971:

e Commonwealth Edison Comp. .or. Peter 17, 1971 1 -There were no radiological consequences since no significant release to the environment resulted from the incident. 2 -No compromise of the health and safety of the public resulted from the incident. 3 -All operations during the incident and post incident recovery pepiod were within Technical Specifications. 4 -All safety systems functioned as designed including High Pressure Coolant Injection (HPCI) , Low Pressure Coolant Injection (LPCI), Core Spray, Main Stearn and Containment Isolations, Standby Gas Treatment System, Pressure Suppression System, and Standby Diesel Generators. 5 -Feedwater control system performance during the transient was deficient, in that, the control system locked out on low air pressure, probably during rapid valve movement.

Previous experience has demonstrated the inability of the feedwater control system to automatically control level below the high water level trip point for main steam valves during a system transient.

This was the primary reason for the need to take operator.*

action. 6 -Operator response was in accordance with operating procedures the incident and post incident recovery with two exceptions.

The operator did not reset the feedwater regulating valve lockout condition when it occurred, and he did not trip the feed .pump when the water level reached +60 inches. Had he done so, the incident may have been prevented.

It is important

• to place these actions in proper perspective, and it should be emphasized that he did take a number of steps to control feedwater input to the vessel. The operator actions were: (a) He reduced the master controller set point to minimize the error signal between the actual level and set point level. This response was previously established on shift by General Electric during the startup program to compensate for the known overshoot which has been experienced ing scrams. While not specifically called for by the station operating procedure.600-ANI;'it

.is consistent with the intent of the procedure to keep the level on scale.

• Commonwealth Edison Com ply Peter A. Morris -4 December 17, 1971 (b) He closed the minimum flow feedwater valve. (c) He reduced the manual output control potentiometer on the "manual-auto" controller to zero and transferred them to manual to terminate feedwater input. 7 -The 11F" safety valve lifted at approximately 1020 psig reactor pressure.

The safety valve set point is 1240 psig. The lifting of this valve was probably caused by some mechanism resulting from the effects of feedwater flooding the main steam line. The pressurization of the drywell could probably have been avoided if this valve had not lifted. Corrective Actions The following corrective actions will be accomplishe!i prior to startup: 1 -Safety evaluations.

(a) . Effects on fuel (b) Vessel internals (c) Performance of suppression pool (d) Effects of pressure, temperature and steam impingement on *primary containment (e) Differential temperature on vessel 2 -LPRM repair 3 -Thermal insulation repair 4 -Replace 11F11 .safety valve with tested valve 5 -Check operability of 3A electromatic valve 6 Investigate reorientation of safety valve discharge

• Commonwealth Edison Comply Dr. Peter A. Morris December 17, 1971 7 -Check calibration of feedwater control system and verify water regulating, valve response to control signal. 8 -Increase torque setting on feedwater regulator isolation, valve so that it will.close urider pressure and-seat properly.' 9 -Modify and emphasize procedure for handling water level transients.

Leave control system in automatic, reduce feedwater controller set point and trip feed pump at 45". 10 -Evaluate the need for non-destructive inspection of the main steam lines. 11 -Check calibration of head-to-flange temperature indicator.

12 -Test all electrical penetrations and'main steam line bellows.

13 -Test all motors in drywell.

14 -Repair damaged paint on drywell wall. 15 -Perform test on all equipment exposed to drywell incident environment.

16 -Check system for freedom of movement during startup.

17 -Hydrostatic test of 1000 psig. Additionally, changes the feedwater control system to improve its operation are under consideration.

When our investigation is complete, we will file a final report with you. cc: Mr. Boyce H. Grier, Director Region III Compliance

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