05000369/LER-2010-003

LER-2010-003, Dropped Control Rods Resulting in Completion of a Technical Specification Required Shutdown and Actuation of the Reactor Protection System and Auxiliary Feedwater System.

Docket Number
Event date:	06-12-2010
Report date:	08-11-2010
3692010003R00 - NRC Website

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 Nuclear Station unique system and component identifiers are contained within parentheses.

Rod Control System [JD](IRE):

The IRE 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. Each Unit has four Control Banks and five Shutdown Banks and each of these Banks are divided into Groups. For each Group, Group Step Counters indicate the demand position of a Group in "steps". The system provides a means to trip the Reactor and place it in a shutdown MODE by inserting a large amount of negative reactivity.

The IRE System takes input from the Reactor Control System (while in automatic) or the Reactor Operator (while in manual) to position the full length control rods to the desired position in the core. The major components necessary to convert an input signal to actual rod motion are: the Logic Cabinet, the Power Cabinets, and the CRDMs. The Logic Cabinet generates signals for speed and direction based on input information from the Reactor Control System or the Reactor Operator.

The five Power Cabinets receive signals from the Logic Cabinet and utilize regulation cards [JC], phase control cards [JC], and firing cards [JC] to generate the appropriate currents to the CRDMs for holding or moving the rods served by their respective Power Cabinet. Opening the Reactor Trip Breakers will disrupt power to the Power Cabinets and the CRDMs, allowing the rods to fall into the core.

Technical Specification (TS) 3.1.4 - "Rod Group Alignment Limits" specifies that all Shutdown Bank rods and Control Bank rods shall be operable in MODES 1 and 2, with all individual indicated rod positions within 12 steps of their Group Step Counter demand position. TS 3.1.4 Condition B provides the Required Actions and associated Completion Times when one rod is not within the above alignment. None of the Required Actions of Condition B require the affected Unit be shutdown.

With more than one rod not within the above alignment limit, TS 3.1.4, Condition D, Required Action D.2 requires the affected Unit be in MODE 3 within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and plant procedures direct Operators to manually trip the Reactor.

Reactor Protection System [JC](I,RE):

The IPE System automatically keeps the Reactor operating within a safe region by shutting down the Reactor whenever the limits of the region are approached. Whenever a direct process or calculated variable monitored by the IPE System exceeds a set point, the IPE System automatically actuates to initiate a Reactor trip 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. Using either of two Control Board switches, Reactor Operators may elect to manually actuate the IPE System to open the Reactor Trip Breakers and initiate a Reactor trip.

Auxiliary Feedwater System [BA](CA):

The CA System provides an emergency feedwater supply to the Steam Generators [SG](SG) if the respective Unit's Condensate and Feedwater System [SJ](CF) is not available to maintain SG water inventory. This ensures the capability to transfer fission product decay heat and other residual heat loads from the Reactor Coolant System [AB](NC) during both normal operation and accident conditions. Each Unit's CA system contains an "A" and "B" Train Motor Driven Pump [P] (MDCAP) and a "C" Train Turbine Driven Pump [N(TDCAP). These pumps will automatically actuate upon receipt of a signal satisfying the logic for automatic start of the respective pump or each pump can be manually actuated from the Control Room.

EVENT DESCRIPTION

On June 10, 2010, Unit 1 was at 100% power with all Shutdown Bank rods and Control Bank rods operable and within the alignment limit specified in TS 3.1.4. At approximately 15:54 hours, Unit 1 Shutdown Bank "B" Rod N7 dropped fully into the Reactor core which represented a condition where this rod was not within the alignment limit of TS 3.1.4. Subsequent to the dropped Rod N7, the Required Actions of TS 3.1.4 Condition B were implemented within the Required Action times and Operations began a load reduction on Unit 1 as directed by procedure AP/1/A/5500/14 - "Rod Control Malfunction".

On June 12, 2010 at approximately 06:44 hours, Unit 1 Shutdown Bank "B" Rod J13 dropped fully into the Reactor core. At the time Rod J13 dropped, the Shutdown Bank "B" Rod N7 was in progress, with Rod N7 still dropped into the Reactor core. With two rods dropped into the Reactor core, this represented a condition where more than one rod was not within the alignment limit of TS be placed in MODE 3 within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />. As directed by procedure AP/1/A/5500/14, Operators initiated a Unit 1 Reactor trip at approximately 06:44 hours by manually actuating the IPE System. Upon trip of the Reactor, Unit 1 entered MODE 3 which represents a shutdown condition.

As expected, following the Unit 1 Reactor trip, the "1A" CF Pump went to rollback hold and SG levels dropped. At approximately 06:48 hours, the 1A and 1B MDCA Pumps were manually actuated in response to decreasing SG levels. These pumps operated as designed to maintain Unit 1 SG inventories.

Manual actuation of the IPE and CA Systems is reportable pursuant to the requirements of 10 CFR 50.73 (a)(2)(iv)(A) - "System Actuation". As per 10 CFR 50:73 (a)(2)(i)(A), The Unit 1 Reactor trip is being reported as a completion of a shutdown required by plant Technical Specifications.

CAUSAL FACTORS

Trouble shooting and testing determined that a solder joint on a regulation card in the 1BD IRE System Power Cabinet experienced an intermittent connection. On June 10, 2010 and June 12, 2010, this intermittent connection caused a drop in the current supply to the CRDMs Shutdown Bank "B" Rod N7 and Rod J13. These current drops were not of sufficient duration to initiate any rod control alarms. On June 10, 2010, current dropped to a level which allowed the CRDM gripper for Shutdown Bank "B" Rod N7 to drop this rod into the core. Similarly, on June 12, 2010, current dropped to a level which allowed the CRDM gripper for Shutdown Bank "B" Rod J13 to drop this rod into the core. On both June 10, 2010 and June 12, 2010, the currents to the CRDMs for all other rods served by the 1BD IRE System Power Cabinet were maintained at values sufficient to hold these rods.

The intermittent connection on the regulation card in the 1BD IRE System Power Cabinet is attributed to a degraded solder joint. The most probable cause of the degraded solder joint is cracking due to vibration experienced during normal plant operation. Further cause investigation associated with this event is in progress. If the results of this ensuing evaluation identify any information that might materially affect the understanding of this event, the cause, or the corrective actions, Duke Energy will submit a supplement to this LER.

Note, periodic preventative maintenance (PM) is performed on the cards in the IRE System Power Cabinets to identify and repair as needed any degraded solder joints. In 2008, during the last performance of this PM on the 1BD Power Cabinet, no' solder joint degradation was identified on the regulation card that experienced the degraded solder joint. Also, McGuire Nuclear Station has not experienced any previous occurrences of similar rod events caused by degraded solder joints on cards in the IRE System Power Cabinets.

CORRECTIVE ACTIONS

Immediate:

1. Initial investigation identified that the dropped N7 and J13 rods could possibly be attributed to a degraded 1BD IRE System Power Cabinet regulation card, phase control card, or firing card associated with the rods. All three cards were replaced and quarantined.

2. The quarantined 1BD IRE System Power Cabinet regulation card, phase control card, and firing card were tested onsite. No issues were identified.

Subsequent:

1. The quarantined 1BD IRE System Power Cabinet regulation card, phase control card, and firing card were sent to the vendor for failure analysis and testing, which identified the items discussed in the Causal Factors section of this LER.

Planned:

1. Additional PM strategies will be evaluated for identifying degraded solder joints on cards in the IRE System Power Cabinets.

2. Evaluate replacement of selected cards in the IRE System Power Cabinets with an upgraded card.

SAFETY ANALYSIS

Duke Energy used 'a risk-informed approach to determine the risk significance associated with the Unit 1 Reactor trip experienced on June 12, 2010.

The Conditional Core Damage Probability (CCDP) and the Conditional Large Early Release Probability (CLERP) of this event were evaluated by considering the following:

• A Reactor trip initiating event.

• Actual plant configuration, equipment unavailability, and maintenance activities at the time of the Unit 1 Reactor trip.

The CCDP associated with this event was determined to be less than 1.0E­ 06. The CLERP associated with this event was determined to be less than 1.0E-7.

This event is considered to be of no significance to the health and safety of the public.

ADDITIONAL INFORMATION

To determine if ,a recurring or similar event exists, a search of the McGuire Problem Investigation Process (PIP) database was conducted for a time period covering 5 years prior to the date of this event. Based on Duke's definition of a recurring event, similar significant event with the same cause code, no recurring events were identified.