05000269/LER-2001-002
| Docket Number | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv), System Actuation |
| 2692001002R00 - NRC Website | |
EVALUATION:
BACKGROUND
This report addresses a Reactor trip, which is reportable per 10CFR50.73(a)(2)(iv) as "any event or condition that resulted in a..automatic actuation of.. the Reactor protection System.
The Reactor Protection System (RPS) [EIIS:JC] is a safety related system which monitors parameters related to the safe operation of the plant. The RPS provides a two-of-four logic for tripping the reactor in response to unit/system conditions that require a unit trip.
One of the parameters monitored by RPS is the Main Turbine Trip. If the Main Turbine [EIIS:TA] should trip or both Main Feedwater (MFW) [EIIS:SJ] Pumps should trip, then the RPS will automatically trip the Reactor [EIIS:RCT].
The Emergency Feedwater (EFW) System [EIIS:BA] is designed to start automatically upon loss of MFW or low level in either Steam Generators S/G. The EFW system ensures sufficient feedwater supply to the S/G of each unit, in the event of loss of the Condensate/Main Feedwater System, to remove energy stored in the Reactor core [EIIS:AC] and primary coolant [EIIS:AB]. The EFW system consists of two motor driven pumps and one turbine driven pump.
The main electric generator [EIIS:TB] supplies power through an isolated phase bus to the unit step-up and unit auxiliary transformer.
The unit auxiliary transformer is connected to the bus between the generator disconnect switches and the unit step-up transformer. During normal operation, station auxiliary power is supplied from the main generator through the unit auxiliary transformer. During startup, shutdown, and after shutdown, station auxiliary power is supplied from the 230kV system through the startup transformer.
The original design for isolation and disconnecting the Main Generator consisted of disconnecting flex links housed in each Isolated Phase Bus. These flex links are very heavy and cumbersome to remove and install. This awkwardness created a significant personnel safety concern and damage to the flex links because of repeated handling.
This damage could result in poor connections and therefore hot spots.
The solution was to install a disconnect switch in each Isolated Phase Bus in lieu of the flex links. Modifications for the Isolated Phase Bus disconnect switches were done on Units 1 and 2 during their last refueling outages. Unit 3 modification was to be completed during the 2001 fall refueling.
Prior to this event Unit 1 was operating at 100% power with no safety systems or components out of service that would have contributed to this event.
EVENT DESCRIPTION
On September 12, 2001, at approximately 1813 hours0.021 days <br />0.504 hours <br />0.003 weeks <br />6.898465e-4 months <br />, Control Room Operators for Unit 1 observed an Isolated Phase Bus Ground flickering alarm (not locked in). Within approximately 15 seconds Unit 1 tripped from 1006 power due to problems with the Isolated Phase Bus disconnect switch. No abnormal events, testing or maintenance procedures were in progress immediately before or during the time this event occurred.
The Reactor Trip was due to a problem with the main generator disconnect switch on the Z phase. The event recorder indicated that the problem existed for more than 15 seconds prior to the trip. The electrical system protective relays monitor the Z phase for loss of power. The Z phase voltage transient prior to and during the trip actuated the under voltage relays in the non- essential 4kV switchgear. This resulted in the loss of Secondary Side Pumps (Hotwell Pumps, Condensate Pumps, etc.) causing the Main Feedwater Pumps to trip.
With the main feed pumps not being available to remove decay heat from the S/Gs, the Operators entered the Abnormal Procedure (AP) for loss of Main Feedwater. The Emergency Feedwater pumps auto started because of the loss of both MFW Pumps and maintained post trip S/G levels per design.
All plant systems responded normally to the plant trip with the exception of the Secondary Systems Pumps. The plant was maintained in Mode 3.
A post trip investigation found that the Z phase disconnect switch of the Isolated Phase Bus had failed because of apparent high temperature. Testing was done to determine if a fault in the electrical system caused the disconnect switch to fail or if the disconnect switch failure caused the fault. The results from the testing eliminated system faults as the cause of damage to the disconnect switch. The disconnect switch was inspected and found to be properly installed.
The Isolated Phase Bus disconnect switches were rebuilt and a modification to the disconnect switch enclosures was completed to allow for thermal imaging while at power.
The plant was placed back on line and power raised to 100%.
Engineering monitored temperatures at the disconnect switches and Isolated Phase Bus, while at 100%, using thermal imaging, local temperatures and computer points. This data indicated that temperatures were still high. Unit 1 power was decreased to approximately 92%. The cooling medium for the Isolated Phase Bus cooler was changed from Recirculating Cooling Water (RCW) to chilled water to help improve Isolated Phase Bus cooling air temperature. This temporary chilled water modification will be in effect until the next Unit 1 refueling (Spring 2002).
Concurrently, the Unit 2 Isolated Phase Bus, which had the same style disconnect switches, was exhibiting elevated temperatures.
Consequently Unit 2 was taken off line and the disconnect switches were replaced with a solid bus. This action was completed on September 30, 2001.
Unit 3 Isolated Phase Bus modification has been postponed until resolution of the disconnect switch problem.
CAUSAL FACTORS
The cause for the Isolated Phase Bus disconnect switch failure was high operating temperature. The high temperatures at the disconnect switch could have been caused by either: 1) Inadequate airflow or airflow distribution through the Isolated Phase Bus cooling ducts, or 2) Inadequate capacity of the disconnect switches.
The root cause for this event is still under investigation. Once the root cause is determined, Oconee will address the design of these disconnect switches and/or the Isolated Phase Bus cooling system to rectify the problem. If significant information becomes available when the root cause is completed then a supplement to this LER will be submitted.
CORRECTIVE ACTIONS
Immediate:
1. Operators took the appropriate actions to maintain the plant in Mode 3.
Subsequent:
1. The Unit 1 Isolated Phase bus disconnect switches were replaced with spare disconnect switches.
2. The cooling medium for the Isolated Phase Bus coolers was changed from Recirculating Cooling Water (RCW) to chilled water to help improve Isolated Phase Bus cooling air temperature. This temporary chilled water modification will be in effect until the next Unit 1 refueling (Spring 2002).
3. Unit 2 was taken off line and the disconnect switches were changed to solid bus pieces.
Planned:
1. The root cause process for the Isolated Phase Bus disconnect switches on all three Units will determine the permanent corrective actions. Problem Investigation Process (PIP) 0-01- 03367 will document the permanent solution.
There are no NRC Commitment items contained in this LER.
SAFETY ANALYSIS
The loss of Main Feedwater is an anticipated transient and is described in Section 10.4 of the Updated Final Safety Analysis Report (UFSAR).
Loss of MFW initiates a reactor trip and starts the EFW System to provide decay heat removal. In this event all the systems and equipment operated as designed to mitigate the consequences of the loss of MFW. During this event, the turbine driven EFW pump and both motor driven EFW pumps automatically started and properly controlled S/G levels to remove decay heat from the reactor coolant system (RCS). The unit was maintained at mode 3.
The motor driven EFW pumps are considered essential loads and are not tripped by undervoltage. Instead a power switching logic transfers the power from the faulted source to an alternate source. Therefore, the motor driven EFW pumps and all other safety pumps were not affected by the initiating event and were available to operate to mitigate the loss of MFW as described.
If the Unit 1 EFW pumps had not started, the Emergency Operating Procedures (EOP) and Abnormal Procedures (AP) direct operators to align EFW from one of the other two Oconee units. The EOP and AP also include the use of High Pressure Injection (HPI)[EIIS:BG] forced cooling and/or use of the Standby Shutdown Facility (SSF)[EIIS:NB] Auxiliary Service Water (ASW)[EIIS:BA] Pump. At the time of this event: 1) both trains of the HPI were available to provide feed and bleed decay and sensible heat removal, 2) Units 2 & 3 were at power and all three EFW pumps, for each unit, were available to provide EFW, and 3) The SSF and its ASW pump was available to provide heat removal via the S/Gs.
The core damage significance of this event has been evaluated by considering the availability of the EFW system, the SSF and the HPI system to provide S/G and feed and bleed cooling. The estimated increase in core damage probability associated with this event was determined to be insignificant.
Therefore, there was no actual impact on the health and safety of the public due to this event.
ADDITIONAL INFORMATION
There were no releases of radioactive materials, radiation exposures or personnel injuries associated with this event.
The trip of the Reactor constitutes a Maintenance Rule functional failure and is considered reportable under the Equipment Performance and Information Exchange (EPIX) program. The root cause is still under investigation but the NRC code is "B" because it is either the design of the disconnect switches and/or the Isolated Phase Bus cooling.
A review of reportable events indicated that no reactor trip events have occurred within the past two years due to the root cause identified in this event.