ML19317E920
| ML19317E920 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 09/30/1974 |
| From: | DUKE POWER CO. |
| To: | |
| Shared Package | |
| ML19317E917 | List: |
| References | |
| RO-287-74-01, RO-287-74-1, NUDOCS 8001070721 | |
| Download: ML19317E920 (3) | |
Text
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DUKE POWER COMPANY OCONEE UNIT 3 Report No.:
UE-287/74-1 Report Date:
September 30, 1974 Event Date: August 18, 1974 Facility: Oconee Unit 3, Seneca, South Carolina Identification of Event:
Reactor Coolant Pump 3B2 Reverse Rotation Conditions Prior to Event:
Reactor Shut Down, Coolant Temperature 220 F, Pressure 480 psi Description of Event:
On August 17, 1974, the 3B2 Reactor Coolant Pump for Oconee Unit 3 was stopped after completion of venting the Reactor Coolant System. A short time later, it was noted that the zero motor speed indicator showed the pump was not at zero speed and the AC oil lift pump had been energized.
Since the reactor coolant pumps have devices which prevent reverse rotation, it was assuraed that a malfunction of the zero motor speed switch had occurred, and hence, the AC oil lif t pump had been energized.
At 0930 on August 18, 1974, it was datermined by vibration and observation of the motor shaft that 3B2 Reactor Csolant Pump was rotating in the reverse direction at approximately 680 revolutions per minute.
The AC oil lift pump was started and low pressure service water was supplied to the oil cooler. All other reactor coolant pumps were stopped in order to stop 3B2 Reactor Coolant Pump.
Tests performed by running various pump combinations could not reproduce the reverse rotation in 3B2 Reactor Coolant Pump.
Designation of Apparent Cause of Event:
The reverse rotation device for this particular reactor coolant pump consists of eleven pawls mounted in the flywheel of the pump.
One of the pawls is shown in Figure 1.
The pawls are supported by cutouts in the flywheel and retainer plates along the periphery of the flywheel. When the flywheel is rotating in the correct direction, the pawls become horizontal due to centrifugal action.
As the flywheel slows, the pawls become vertical and drop into a rachet plate to prevent reverse rotation.
In RCP 3B2, the pawls contacted the retainer plate after the pump was started. The friction between the pawls and the retainer plates prevented the pawls from becoming vertical and preventing reverse rotation when the pump slowed.
Corrective Action:
Maintenance testing performed by using Prussian bluing determined that the pawls on 3B2 Reactor Coolant Pump were contacting the retainer plate on the l
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w periphery of the flywheel. The retainer plates were modified by milling the area of contact, as shown in Figure 1, to ensure freedom of motion of the pawls. Subsequent testing has demonstrated that the pawls drop at 50 to 60 rpm, as designed.
There are 12 reactor coolant pump motors of two different designs at the Oconee Nuclear Station.
Only two motors, 3B1 and 3B2, are of the earlier design and both have been modified to prevent interference by the retainer plate.
The remainder of the pumps at Oconee are of the later design.
The Unit 2 and 3 pumps have been satisfactorily tested. The Unit 1 pumps will be tested during the pending refueling outage.
A written standing order has been issued to Control Room operators to check for reverse rotation each time a reactor coolant pu=p is stopped.
Indications which will be monitored are:
pump speed, =ero speed indication, and pump vibration.
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