05000301/LER-2012-001
| Point Beach Nuclear Plant | |
| Event date: | 06-27-2012 |
|---|---|
| Report date: | 08-26-2012 |
| 3012012001R00 - NRC Website | |
Description of the Event
On June 27, 2012, Point Beach Nuclear Plant (PBNP) Unit 2 was operating in a steady state condition at 100% power with no plant evolutions in progress. At approximately 2045 alarms were received in the control room indicating reduced Unit 2 turbine [TRB] first stage pressure and generator megawatts. This was indicative of a significant reduction in turbine load. Control rods responded as designed by inserting into the reactor to reduce reactor power. Operations noted that the turbine speed indication on the turbine EH control panel was reading high with the generator output breaker closed, indicating a control system failure.
Based on these indications, at 2046 the Shift Manager directed that the reactor be shutdown by manually actuating the reactor protection system. No automatic reactor protection setpoints were exceeded and an automatic shutdown was not actuated or needed. Plant response to this event was as expected.
A failure investigation process (FIP) was entered. Based on troubleshooting, NextEra determined that the loss of turbine load was due to a failure of the speed channel 'A' card in the EH system.
Speed channel cards in the EH system were replaced with spares and calibrated. This was done as a conservative measure since only the 'A' card had failed. The calibration was completed satisfactory and the EH system was subsequently returned-to-service.
This event is not reported as a safety system functional failure.
Analysis of the Event
Turbine speed is controlled by the turbine electro-hydraulic (EH) [HCU] system. The EH system has two independent speed sensing circuits, each comprised of a passive probe and two printed circuit cards.
The Main speed channel circuit is designed to regulate turbine speed. When the turbine is offline it inputs to the turbine speed controller, and is used for turbine roll up. When the turbine is connected to the grid, it is the input to the frequency correction circuitry.
The Auxiliary speed channel circuit is dedicated to overspeed protection. When an overspeed condition is sensed in excess of 103% of rated turbine speed, the Auxiliary Governor closes the governor valves and actuates other protective features.
In the Auxiliary speed channel circuit, turbine speed is sensed in the turbine pedestal and supplied to the speed channel 'A' card. This card converts the signal to a different signal type that is then sent on to the speed channel 'B' card.
The speed channel 'B' card uses the signal from the speed channel 'A' card and converts it to a signal that is proportional to shaft speed. This signal is used by the Auxiliary Governor circuit shaft speed display in the Control Room, to shut the turbine governor valves if the auxiliary speed channel senses a shaft speed of >103% of rated speed, and to actuate an alarm in the Control Room. Actuation of the Auxiliary Governor removes steam from the turbine. The generator loses load due to lack of motive force. The Turbine Stop valves are unaffected and stay open when the Auxiliary Governor is activated.
NextEra determined that the Auxiliary speed channel circuit channel 'A' card failed. The card failed high resulting in a high signal sent to the speed channel 'B' card which caused the speed channel 'B' card to generate a >2500 rpm signal. This was the cause of the Control Room turbine speed indication reading high. This also caused the Auxiliary Governor to close the governor valves, which caused the generator to shed load. These indications were the initiating event for the PBNP Unit 2 manual reactor trip.
Analysis of Safety Significance A manual reactor trip was initiated in response to the turbine governor valves closing and the loss of generator load while the reactor was still at power. No automatic reactor protection setpoints were exceeded, and the automatic reactor protection system was not actuated nor needed. Plant systems functioned as required following the trip.
The reactivity effects during this particular event had no impact on the safety of the core. The initial event resulted in reactor coolant system (RCS) temperature rising as steam demand was reduced by the closing governor valves. At this stage in core life, the moderator temperature coefficient is largely negative (approximately -25 pcm/deg F); thus, the rising temperature added significant negative reactivity. The control rods began to insert into the core to control the temperature increase as designed. The Shift Manager and operating crew responded appropriately by manually actuating the reactor protection system to shut down the reactor.
Corrective Actions
The 'A' and 'B' speed cards were replaced with spares and calibrated.
Similar Events None
Failed Components
NAND gate location A8B, Motorola HTL logic chip MC662L