05000316/LER-2004-001
| Donald C. Cook Nuclear Plant | |
| Event date: | 03-29-2004 |
|---|---|
| Report date: | 05-26-2004 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 40622 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation, 10 CFR 50.72(b)(3)(iv)(A), System Actuation |
| 3162004001R00 - NRC Website | |
� �2004 001 00 17.TEXT (If more space is required, use additional copies of NRC Form (366,4) Conditions Prior to Event Unit 1= 80% power Unit 2 =100% power
Description of Event
On March 29, 2004, at 1404 hours0.0163 days <br />0.39 hours <br />0.00232 weeks <br />5.34222e-4 months <br />, Unit 2 received an automatic actuation of the Reactor Protection System (RPS) [JD]. The RPS actuation occurred while an operator was attempting to rack out the train A Reactor Trip Bypass Breaker (RTBB) [BKR] during restoration from Solid State Protection System (SSPS) [JG] testing. At the time of the trip, Unit 2 was at 100% power with systems aligned for steady state operation.
On March 29, 2004, at approximately 1200 hours0.0139 days <br />0.333 hours <br />0.00198 weeks <br />4.566e-4 months <br />, preparations were made for conduct of SSPS testing. The train A RTBB was in the closed position; however; the plant process computer did not indicate closed.
SSPS testing was interrupted to investigate and correct train A RTBB closure indication.
On March 29, 2004, at approximately 1400 hours0.0162 days <br />0.389 hours <br />0.00231 weeks <br />5.327e-4 months <br />, an operator was dispatched to remove the control power fuses and rack out the train A RTBB. The operator located what he thought was the racking bar alignment pin on the left side of the breaker cubicle. With the left side of the bar inserted into the cubicle near the left pin, the operator turned his attention to the right side of the cubicle in an effort to align the right side of the bar with the alignment pin on the right side. While doing this, he moved the bar to the right, contacted an energized component and shorted one phase of the rod control motor-generator set 260 VAC output to ground. This de-energized the stationary gripper coils for multiple control rods, causing them to drop into the core, which in turn resulted in an automatic RPS actuation on Power Range Nuclear Instrument Hi- Negative rate. The Auxiliary Feedwater (AFW) System [BA] automatically started as designed and functioned correctly.
In accordance with 10 CFR 50.72(b)(2)(iv)(B) (EN#40622), the reactor shutdown was reported as a valid actuation of the reactor protection system. In accordance with 10 CFR 50.72(b)(3)(iv)(A), the automatic start of the AFW System was reported as a valid actuation of an Engineered Safety Feature System. This LER is being submitted in accordance with 10 CFR 50.73(a)(2)(iv)(A).
Cause of Event
The root cause of the trip was human error and inadequate task management. The operator incorrectly inserted the racking bar while manipulating the RTBB for testing.
Analysis of Event
A review of this event found that it was bounded by the existing accident analysis associated with unplanned reactor trips with the main condenser available. The change in risk with respect to core damage and large early release frequency due to inadvertently tripping the control rods, and subsequent plant trip, have been qualitatively assessed and judged no different than any other unplanned reactor trips with the main condenser available. This assessment is based on the following considerations:
_ 17. TEXT (If more space is required, use additional copies of NRC Form (366A)
- The automatic plant trip, due to Power Range Nuclear Instrument Hi-Negative rate, functioned properly.
Automatic post-trip features also functioned dependably, with the operators taking procedurally-directed actions to control reactor coolant system T-avg following the trip. The plant and operators responded in an appropriate and timely manner, resulting in a safe and stable plant configuration.
- The inadvertent cause of the trip, due to shorting out the control rod drive power supply, does not contribute to the increased likelihood of any initiators other than transients that result in or from a reactor trip.
- Neither the human error which faulted the control rod drive power supply, or the subsequent unit trip, degraded any system used to prevent core damage, assure containment integrity, or maintain defense-in depth and safety margins.
This event was significant with respect to industrial safety, as the incident involved unintended contact with an energized electrical circuit. This was mitigated by the use of proper personal protective equipment.
Because safety systems operated as designed and no significant difficulties were experienced during the trip recovery and plant stabilization, the event had no actual nuclear safety significance, but it had potential nuclear safety significance in that the transient stressed plant components and challenged safety systems.
Corrective Actions
The following corrective actions were completed:
- The affected breaker cubicle and breaker mechanism were examined for damage. Minor electrical arc traces and a bent auxiliary contact arm were found. The bent contact arm was identified as the source of the failed plant process computer indication of breaker position. (Condition Report (CR) 04089033).
- The DB-50 breaker was replaced. (CR 04089033) The following interim actions were initiated:
- Operations management implemented a program of direct management oversight of control room activities including monitoring of pre-job briefs, communication, peer checking quality, and confirmation that task level supervision is provided for "medium" or higher risk level tasks.
(CR 04089034)
- Operators are prohibited from racking DB-50 breakers until their proficiency to properly perform the task has been verified. (CR 04089034-02) � 17. TEXT (If more space is required, use additional copies of NRC Form (366A) The following actions are planned to prevent recurrence:
- Procedures will be revised to require that racking of DB-50 breakers receive a peer check by another operator who is also qualified for this operation. The due date for this action is July 30, 2004.
(CR 04089034-03)
- An operator aid will be posted at the breakers in both units providing a photographic representation of the breaker, with components important to the racking activity labeled. The due date for this action is June 28, 2004. (CR 04089034-08)
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