05000269/LER-2010-002
| Docket Number | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2692010002R01 - NRC Website | |
BACKGROUND
The Unit 1 vibration sensor system senses motor / pump shaft vibration and motor frame vibration on the four reactor coolant [AB] pumps [P] (RCP). RCP 1A1 and 1A2 vibration sensor systems share a common power source which includes redundant power supplies [JX]. RCPs 1B1 and 1B2 vibration sensor systems share a separate common power source which also includes redundant power supplies. Both power supplies are contained in the control module. A three position switch in each control module can be placed in Power Supply No.1 position (up), Power Supply No. 2 position (down), or the Auto position (center). When set in the Auto position, Power Supply No.1 and No. 2 are auctioneered whereby Power Supply No.1 is the active supply and Power Supply No.
2 is the backup. However, at the time of the event there were no procedural controls to ensure that the power supply selector switch was properly positioned. Additionally, no Preventive Maintenance (PM) activities were established to ensure that the power supply was functioning properly.
There are three pumps per unit in the High Pressure Injection (HPI) [BG] System. Normally one pump (either A or B) is operating to supply both normal charging to the Reactor Coolant System (RCS) and seal injection to the Reactor Coolant Pumps. A second pump (A or B) is capable of starting manually via control room switch or automatically via a non-safety low RCP seal injection flow signal. The C HPI pump along with the other two HPI pumps will start in response to an Engineered Safeguards [JE] signal.
This event is reportable per 10CFR 50.73(a)(2)(iv)(A) because a valid Reactor Protective System (RPS) [JC] actuation, including a reactor [RCT] trip, occurred.
Prior to this event Oconee Nuclear Station (ONS) Unit 1 was operating in Mode 1 at 100% power with no significant activities in progress (e.g., testing, surveillance, maintenance, etc.).
At the time of the trip no safety systems or components were out of service that would have contributed to this event.
EVENT DESCRIPTION
On August 7, 2010, at 1130 hours0.0131 days <br />0.314 hours <br />0.00187 weeks <br />4.29965e-4 months <br />, Unit 1 was at 100% steady state power and no work was in progress associated with RCPs when 1SA-9-D2 (RCP Vibration High) statalarm [IB] activated. The Unit 1 control room operator referred to the applicable alarm response guide and dispatched an operator to the RCP status computer. The operator reported that RCP status computer indicated 1A1 RCP vibration indication spiking.
At 1211 hours0.014 days <br />0.336 hours <br />0.002 weeks <br />4.607855e-4 months <br /> the Operations Shift Manager directed Unit 1 Control Room Operators to enter the appropriate Abnormal Procedure (AP) because of the high vibration indication on 1A1 RCP. In addition to the indication on 1A1 RCP, control room operators entered the AP at 1300 hrs due to indicated high vibration on the 1A2 RCP.
At 1435 hours0.0166 days <br />0.399 hours <br />0.00237 weeks <br />5.460175e-4 months <br /> control room operators began reducing Unit 1 power per procedure because of the increasing vibration trend. Station engineering began troubleshooting the cause of the statalarm. At 1451 hours0.0168 days <br />0.403 hours <br />0.0024 weeks <br />5.521055e-4 months <br />, the reactor was at approximately 17% power, and the vibration readings of pumps 1A1 and 1A2 reached the procedural limits requiring an immediate reactor trip. At this point control room operators manually tripped Unit 1 and secured the 1A1 and 1A2 RCPs.
The plant response during down power was normal and the post-trip response was normal. No actuation or actuation demands occurred related to emergency feedwater [BA] or engineered safeguards (i.e., Emergency Core Cooling, Containment Isolation [JM], Containment Spray/Cooling, and Emergency Power). A second idle HPI pump automatically started due to a non-safety signal for low RCP seal injection flow. The second HPI pump operated for a short period (per procedure) to maintain RCP seal flow. This is a routine action to compensate for post-trip RCS temperature and volume changes.
Troubleshooting results revealed that the power selector switch was in the down position, indicating that Power Supply No. 2 was the active power supply. Power Supply No. 2 was found to be failed. This power supply was feeding the 1A1 and 1A2 RCPs vibration monitoring system.
CAUSAL FACTORS
The root cause for this event was a power supply that failed in a manner that the vibration indications appeared to be valid. As a result Operations, initiated a unit shutdown and eventually tripped the reactor based on the invalid RCP emergency high vibration levels. Past operating experience was that the power supply would fail in a manner that was easily diagnosed. For example in June 2010, a similar power supply failed on ONS Unit 2. In the Unit 2 case, the vibration points on the Operating Aid Computer (OAC) status indicated "Bad" then "Good" such that operators were able to quickly identify that the indications were invalid.
CORRECTIVE ACTIONS
Immediate:
1. RCPs vibration sensor system power supply selector switches were set to "Auto" on Units 1, 2, and 3.
2. The control module containing the failed power supply was replaced on Unit 1.
There were no subsequent / interim Corrective Actions; however, the planned Corrective Actions to prevent reoccurrence are listed below. The below planned corrective actions are all completed.
1. Revise the applicable Alarm Response Guides (ARGs) to address actions to validate RCP high vibration alarms.
2. Replace the degraded power supply on Unit 2.
3. Establish a Preventive Maintenance (PM) program in accordance with industry best practices that accomplishes the following:
a) Defines a replacement frequency for the power supply.
b) Establishes a periodic PM to ensure the power supply is functioning properly.
SAFETY ANALYSIS
The risk significance of this event has been evaluated. This event was an uncomplicated reactor manual scram with no impacts on any safety systems. All systems responded normally following the trip, no safety limits were challenged and the Unit was placed in Mode 3 without complications.
Consequently this event was not considered safety significant.
No safety limits were challenged and shutdown was as expected for a manual trip scenario. Based on analysis this event has a low impact on core damage risk.
ADDITIONAL INFORMATION
It was determined by the root cause extent of condition review that the condition described in this LER was applicable to all three Oconee units. The necessary corrective actions were performed in the immediate and planned corrective actions to resolve this problem. However, on June 15, 2010 ONS Unit 2 2A1 and 2A2 RCP's had experienced a similar RCP vibration instrument failure. Troubleshooting determined that the cause for the sporadic RCP vibration indications was a faulty power supply as well. However this particular event did not result in a unit transient.
Energy Industry Identification System (EllS) codes are identified in the text within brackets [ ].
There were no releases of radioactive materials, radiation exposures or personnel injuries associated with this event.
This event has been reported to the Equipment Performance and Information Exchange (EPIX) program.
The failed component was a model 500-IM22 vibration monitor control module manufactured by Indikon Company Inc.