05000266/LER-2012-004
| Point Beach Nuclear Plant | |
| Event date: | 08-14-2012 |
|---|---|
| Report date: | 10-03-2012 |
| 2662012004R00 - NRC Website | |
Description of the Event
On August 14, 2012, Point Beach Nuclear Plant (PBNP) Unit 1 was operating in a steady state condition at 100% power with no plant evolutions in progress. At approximately 2031 alarms were received in the control room indicating reduced PBNP Unit 1 turbine first stage pressure and generator megawatts, indicative of a reduction in turbine load. Control rods were responding as designed by inserting into the reactor to reduce reactor power. Based on these indications, 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 required.
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 main speed channel 'B' card in the electro-hydraulic EH [HCU] system.
Both 'A' and 'B' speed channel cards in the EH system were replaced with spares and calibrated. This was done as a conservative measure since only the 'B' 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 EH 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 Main speed channel circuit the speed channel 'B' card takes the pulse train from the speed channel 'A' card and converts it to an analog signal that is proportional to shaft speed. This signal is used as the supply to the speed controller circuit which is the circuit that drives the governor valves to maintain turbine speed.
The speed control circuit determines the speed error based on actual turbine speed versus demanded speed. This signal is then sent to the governor valve controller which uses it as an input to determine the governor valve control signal.
NextEra determined that the Main speed channel circuit channel 'B' card failed in a manner that caused the card to generate a high turbine speed signal. This caused the governor valve control circuit to close the governor valves, which caused the generator to shed load. These indications were the initiating event for the PBNP Unit 1 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 -22 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' main speed cards were replaced with spares and calibrated.
Similar Events
Failed Components
1C-039, Main Speed Channel B card, Motorola LM201AH Op Amp