05000498/LER-2003-002
| South Texas Unit 1 | |
| Event date: | 0-1-2003 |
|---|---|
| Report date: | 09-30-2003 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv), System Actuation |
| 4982003002R01 - NRC Website | |
DESCRIPTION OF EVENT
At 0149, on March 1, 2003, Unit 1 was operating at 100% power when alarms annunciated on all three Unit 1 turbine-driven Main Feedwater Pumps for low seal water differential pressure. Over the subsequent 90 seconds, the "Hotwell Standpipe Level Hi / Low", "Flash Tanks 11, 12 and 13 Level Hi / Low", "Gland Steam Condenser Vacuum Low", and the "Low Pressure Heater Drip Pump 12 and 13 Trip" alarms also annunciated. Condensate pumps 11 and 13 were running at the time.
The control room staff observed the following indications:
- Decreasing deaerator water level;
- Mini-flow recirculating valves for the running Condensate Pumps cycling open;
- Higher than normal condensate header pressure; and
- No condensate flow.
The control room staff took manual control of deaerator level to ensure that the normal Level Control Valve was receiving a "full open" signal. The normal Level Control Valve was confirmed to be fully open.
At 0154, with deaerator level at 37% and continuing to decrease, the reactor was manually tripped. All equipment operated as designed following the reactor trip except pressurizer heater group 1B which failed to energize in automatic, but did energize manually. Based on indications and the short response time available, operator response was appropriately conservative and in accordance with station expectations.
At approximately 0205, the control room was notified that bypass valve CD-MOV-0132 was closed. This valve enables the Condensate Polishing Demineratizer System to be bypassed in the event of high differential pressure. The control room alarm for high condensate polishing system differential pressure did not annunciate at any time during the event. The control room staff directed that valve CD-MOV-0132 be opened manually. With CD-MOV-0132 open, flow was restored to the deaerator.
Event investigation found that Power Supply #1 in the Condensate Polishing Control Panel had failed, disabling Pressure Differential Indicating Transmitter (PDIT) 5701. The function of this pressure transmitter is to provide an input function to two alarms for system differential pressure, as well as an "open" permissive for bypass valve CD-MOV-0132. The alarm card circuit is powered from Power Supply #2, which remained energized throughout this event. The card is designed to provide both alarm and valve actuation in the event of a loss of power. However, the pressure transmitter input function to these alarm cards fails low when its power supply (Power Supply #1) is de-energized. Although the differential pressure had exceeded the alarm card setpoint (105 psid for valve actuation), there was no signal for CD-MOV-0132 to open.
Power Supply #1 also supplies the Condensate Polishing ManuaVAuto Control Stations. On loss of power (Power Supply #1) to the Manual / Auto Control Stations for each CP Service Vessel Outlet Valve, no signal is provided to enable pneumatic output to the valve positioner. This resulted in the outlet valves being closed by the positioner, cutting off flow to the deaerator.
Event Significance There were no adverse safety or radiological consequences associated with this event. No equipment damage occurred as a result of this event. This event did not affect the operability of any safety-related equipment, and all safety-related equipment performed as designed. This event is reportable pursuant to 10CFR50.73(a)(2)(iv) because it was resolved by manual actuation of the reactor protection system.
This event was not risk significant for nuclear safety. The Conditional Core Damage Probability for this event is 1.9E-07. This probability value demonstrates that this event was not risk significant, and the value South Texas Unit 1 05000 498 YEAR I SEQUENTIAL NUMBER
SECIJENTIAL
2. DOCKET 1 05000 498 2003 02 � 01 substantially below the 1E-06 threshold used in the Nuclear Regulatory Commission Significance Determination Process.
Causes of the Event The original design was not fault tolerant for individual power supply failures. The design description states, "The service inlet and outlet valves are provided with lock-up devices. These valves will remain in the position held at the time of an electric failure? The "electric failure" portion of the design statement refers to a loss of 120 Volts AC (VAC) to the entire distribution panel for the Condensate Polishing Control Panels, and not for an individual 24 VDC Power Supply failure. The solenoid that actuates the lock-up devices receives its actuation signal from a relay powered from Power Supply #2. The lock-up devices will not perform their function of holding the CP Service Vessel Inlet and Outlet Valves In place if Power Supply #1 fails. This Is addressed by Corrective Actions 1, 5, 6, and 7.
Failure analysis of Power Supply #1 determined that the voltage regulator card in the power supply failed due to age-related degradation. This is addressed by Corrective Actions 2, 3, and 4.
Corrective Actions
1. Revise plant procedures to add steps to check for condensate polishing system differential pressure and open the system bypass valve if required. This action has been completed.
2. Replace Power Supply #1 in Unit 1 with a new power supply. This action has been completed.
After replacement, an alarm input was simulated that resulted in an alarm in the Control Room, on ICS and at the CP Watch Station. In addition, the new Power Supply #1 was momentarily de- energized and no alarms were received in either the Control Room or the CP Watch Station. These results match the expected response.
3. Review Master Equipment Database for other applications of similar power supplies. This action has been completed.
4. Replace Power Supply #1 in Unit 2 with a new power supply. This action has been completed.
5. Provide a lock-up feature for the Unit 1 CP Service Vessel Outlet Valves that will lock the CP Service Vessel outlet valves in place in the event of a Power Supply #1 failure to ensure against loss of condensate flow. This action has been completed.
6. Provide a lock-up feature for the Unit 2 CP Service Vessel Outlet Valves that will lock the CP Service Vessel outlet valves in place in the event of a Power Supply #1 failure to ensure against loss of condensate flow. This action has been completed.
and Unit 2 to prevent recurrence.
This action is expected to be completed in November, 2003.
Generic Implications The Master Equipment Database was reviewed (Corrective Action 3) to identify other applications of these power supplies. There are presently four such power supplies in place at the South Texas Project. Their locations are as follows:
- Make-up Demineralizer Building Panel ZLP140 (9Q210ZLP140)
- Plant Yard Water System Control Panel ZLP139 (90210ZLP139).
There are no significant plant effects from loss of the power supplies in the Make-Up Demineralizer and Yard Water Systems. The Unit 1 and Unit 2 Condensate Polishing Control Panel applications are addressed under Corrective Actions 2 and 4.
Additional Information
There have been no previous plant trips at the South Texas Project caused by loss of flow through the CP System and failure of the Condensate Polishing System Bypass MOV to open.
A review of industry operating experience found no other failures of this model power supply unit.