05000369/LER-2008-003
| Docket Number | |
| Event date: | |
|---|---|
| Report date: | |
| 3692008003R00 - NRC Website | |
Event Description:
On October 31, Unit 1 was in Mode 2 and performing Zero Power 2008, Physics Testing when operators received a "Rod Control Urgent Failure" annunciator. Abnormal procedure 14 was entered and subsequently re-entered when control rod K-2 dropped to'the fully inserted position. . The condition was terminated when the operators manually. opened the Unit 1 Reactor Trip Breakers per operating procedure and completed subsequent emergency response procedure actions. This event is considered to be of no significance with respect to the health and safety of the public.
Event Cause:
A root cause was completed following the'event and it was determined a CRDM power cable head connector failed causing the K-2 control rod to drop to. the fully inserted'position. ' It was concluded the CRDM power cable head connector design was inadequate for the application.
Corrective Actions:
The Unit 1 CRDM connectors were removed and the cables were spliced and tested prior to returning Unit 1 to service.
BACKGROUND
The following information is provided to assist readers in understanding the event described in this LER. Applicable Energy Industry Identification [EIIS] system and component codes are enclosed within brackets. McGuire unique system and component identifiers are contained within parentheses.
Rod Control System [JD](IRE):
The Rod Control System provides for reactor power modulation by manual or automatic control of full length control rod banks' in a pre-selected sequence and for manual operation of individual banks. Alarms are
- provided to alert the operator in the event of a control rod deviation exceeding a preset limit.
Reactor Protection System [JC](IPE):
The Reactor Protection.System automatically keeps the reactor operating within a safe region by shutting down the reactor whenever the limits of the region are approached. The safe operating region is defined by several considerations such as mechanical/hydraulic limitations on equipment, and heat transfer phenomena. Therefore, the Reactor Protection System monitors process variables which are directly related to equipment mechanical limitations, such as
- pressure, pressurizer water.
level and also on variables which directly affect the heat transfer capability of the reactor. Still other parameters utilized in the Reactor Protection System .are calculated from various process variables.
Whenever a direct process or calculated variable exceeds a setpoint the reactor is shut down in order to protect against' either gross damage to fuel cladding or loss of system integrity which could lead to release of radioactive fission products into the Containment.
The various reactor trip circuits automatically open the reactor trip breakers whenever a condition monitored by the Reactor Protection System reaches a preset or calculated level.
Station operators may elect to manually actuate the reactor trip switchgear (manual reactor trip) using either of two control board switches. One switch actuates the train A trip breaker; the other switch
- actuates the train B trip breaker. Operating either manual trip switch removes the voltage from the under-voltage trip coil,e energizes the shunt trip coil, and trips the reactor.
EVENT DESCRIPTION
On October 31, 2008, Unit 1 was in Mode 2 (Startup). Control Rods were being inserted while performing Zero Power Physics Testing when Control Bank B Group 2 stopped moving and the operators received the "Rod Control Urgent Failure" annunciator alarm. The operators entered into and executed the control room annunciator response procedure which then directed the operators to enter the abnormal procedure for "Rod Control Malfunction" (AP-14). The purpose of AP-14 is to provide operators with the proper response in the event of a rod control malfunction. The AP provides guidance for operators to assess plant conditions and identify appropriate steps for Dropped or Misaligned Control Rods, Failure to Move Control Rods on Demand or Continuous Control Rod Movement.
Subsequently, control rod K-2 dropped to the fully inserted position, operators re-entered AP-14 and rods could not be moved using the rod control system.
Enclosure 1 "Response to Dropped or Misaligned. Rod" of AP-14 directed operators to shutdown to Mode 3 per the normal operating procedure.
The operators manually opened the Unit 1 Reactor Trip Breakers in accordance with the normal operating procedure steps. At the time the reactor protection system was manually activated the reactor core was sub-critical. Manual actuation of the Reactor Protection System to mitigate a condition when the reactor is subcritical is 8 hour9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> reportable per 10 CFR50.72 (b) (3) (iv) (A) followed up with a written report within 60 days per 10 CFR50.73, (a) (2) (iv) (A).
CAUSAL FACTORS
A root cause was completed following the event and it was determined the Control Rod Drive Mechanism (CRDM) power cable head connector failed causing the K-2 control rod to drop to the fully inserted position. It was determined the CRDM power cable head connector design was inadequate for the application.
Connector causal factors supported by metallurgical analysis are:
- Aging due to radiation, temperature, and vibration causing particle migration.
- Faulty barrier internal to connector.
The Root Cause investigation revealed conductive material was on the upper surface of the rubber grommet (an insulator within the CRDM connector assembly). The non-routine act of disconnecting and reconnecting the CRDM cable from the CRDM head connection. can generate conductive material from within the CRDM head connector. The design and orientation of the CRDM head connector allowed for collection of conductive material around the base of the pins at the grommet interface. Degradation of the grommet material caused a loss of compression over time, allowing a horizontal pathway for collected conductive material to migrate across the grommet face, providing conditions for arcing and eventually providing a pin-to-pin conductive path. This condition is also applicable to Unit 2 (when'the CRDM cable is disconnected/reconnected to the CRDM head connection) and a planned corrective action will address the condition.
CORRECTIVE ACTIONS
Immediate:
1. Operations personnel responded to the K-2 rod drop in accordance with normal and abnormal station procedures. Following the manual opening of the reactor trip breakers operators entered and executed the emergency procedure for reactor trip.
Subsequent:
1. Following troubleshooting, all Unit 1 CRDM head connectors (53) were removed and spliced.
2. A Nuclear Network message was issued December 11, 2008 to inform the industry of the failure mechanism associated with PYLE STAR- LINE Connectors.
Planned:
1. Eliminate or replace the Unit 2 CRDM Pyle National connectors at the reactor head.
The Planned corrective action is voluntary 'and does not constitute a commitment. It may be modified as operating experience, judgment and evaluations dictate and will be documented in Duke's corrective action process.
SAFETY ANALYSIS
Duke Energy used a risk-informed approach to determine the risk significance associated with the reactor trip of October 31, 2008.
The Conditional Core Damage Probability (CCDP) and the Conditional Large Early Release Probability of this event was evaluated by considering the following:
- A reactor trip initiating event
- Actual plant configuration and maintenance activities at the time of the trip The CCDP associated with this event was evaluated to be less than 1.0E- 7. The Conditional Large Early Release Probability (CLERP) associated with this event was evaluated to be less than 1.0E-8.
This event is considered to be of no significance to the health and safety of the public.
ADDITIONAL INFORMATION
A review of McGuire's corrective action database was performed and it was determined that this is NOT a recurring or similar event.