05000370/LER-2011-002
| Mcguire Nuclear Station, Unit 2 | |
| Event date: | 03-30-2011 |
|---|---|
| Report date: | 10-26-2011 |
| 3702011002R00 - NRC Website | |
Event Description:
On March 30, 2011, Unit 2 was in Mode 5, performing control rod movement testing at the end of a refueling outage. A control rod system malfunction resulted in a control rod misalignment. The condition was terminated when the operators manually opened the Unit 2 Reactor Trip Breakers as prescribed by the test procedure. As part of subsequent troubleshooting, the identical control rod misalignment recurred, and the operators again manually opened the Unit 2 Reactor Trip Breakers. This event is considered to be of no significance with respect to the health and safety of the public.
Event Cause:
A cause evaluation determined that the control rod drive mechanism head cable for the L13 control rod, which was replaced during the refueling outage, was incorrectly wired by the vendor, and subsequent testing failed to detect the issue.
Corrective Actions:
The affected cable was replaced with a correctly wired cable.
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. 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, energizes the shunt trip coil, and trips the reactor.
EVENT DESCRIPTION
Prior to the Unit 2 refueling outage in the spring of 2011 (2E0C20), five spare control rod drive mechanism (CRDM) head cables and plugs [CBL] were procured from a vendor (Amphenol). Engineering and Supply Chain personnel visually inspected the cables, plugs and data sheets for acceptability and placed the cables in stock in late-2010. A Work Order was subsequently generated to test the spare cables and ensure the plugs and receptacles would mate-up correctly. An optional continuity check was not performed at that time and the cables were returned to stock.
In 2E0C20, control rod L13 CRDM cable had unacceptable resistance readings (due to pin failure) and was required to be replaced. A spare CRDM cable was removed from the warehouse to be installed on control rod L13. At the same time, since the connection had been disturbed, the CRDM receptacle had to be replaced and this required a butt splice to the CRDM coil stack wiring. The spare CRDM cable was mated and installed with the new receptacle. A series of tests were performed to ensure that the cable's physical properties were acceptable and to ensure that the cables were wired correctly. A polarity test, which is a continuity test that detects crossed wiring and checks pin to pin connections, was included, but failed to detect the problem.
During performance of procedure PT/2/A/4600/001, "Rod Cluster Control Assembly (RCCA) Movement Test," on 3/30/2011 at 1245, with the plant in Cold Shutdown (MODE 5), control rod L13 (Shutdown Bank C) did not move out as expected when the bank was withdrawn. When the bank was reinserted, rod L13 moved out. The reactor trip breakers were opened by Operations per procedure PT/2/A/4600/001, to allow troubleshooting. The rod control cabinets were inspected for physical damage and none was identified. Test equipment was connected to the rod control cabinets, the reactor trip breakers were closed and the RCCA movement test was repeated for shutdown bank C. The results were the same, control rod L13 (Shutdown Bank C) did not move out as expected when the bank was withdrawn, and when the bank was reinserted, rod L13 moved out. Engineering reviewed the waveforms and determined the moveable and stationary wires were reversed. The cable wires were later confirmed by Maintenance Technicians to be rolled (stationary and movable wires were reversed at one end of the cable). A continuity check was performed pin to pin on all four remaining spare cables. Three of the cables were wired incorrectly; only one remaining cable was correct. This cable was checked out of the warehouse and installed for L13. Tests were repeated for the L13 cable and Engineering verified the data obtained was satisfactory. The RCCA movement test was then repeated with success.
This event was originally reported to the NRC per 10 CFR 50.72(b)(3)(iv)(A) as an eight hour report. The report was subsequently withdrawn because the manual actuation of the Reactor Trip Breakers was performed at the discretion of the licensee as part of a pre-planned evolution during a period of reactor startup testing in which there was no possibility of consequences. However, a non-cited violation was issued related to this determination (see Reference). Accordingly, this report is being submitted in accordance with 10 CFR 50.73 (a) (2) (iv) (A), any event or condition that resulted in manual or automatic actuation of any of the systems listed in 10 CFR 50.73 (a)(2)(iv)(B).
CAUSAL FACTORS
The apparent cause is an incorrectly wired and fabricated CRDM head cable received from the vendor. The contributing causes involve the failure to identify the wiring issues, and include the lack of testing of the CRDM head cable prior to installation, inadequate testing after installation, and omission of clear acceptance criteria in the procedures.
CORRECTIVE ACTIONS
Immediate Corrective Actions:
1. Replaced cable L13 with correctly wired cable from Amphenol.
2. Removed all inventory in warehouse of Amphenol CRDM cables.
3. Removed Amphenol from the Approved vendors list.
Subsequent Corrective Actions:
1. Obtained new CRDM cables from a new manufacturer, which had successfully undergone factory acceptance testing to ensure quality. Successfully performed site acceptance testing upon receipt.
SAFETY ANALYSIS
The RCCA movement test was performed in mode 5. Control rod L13 did not move out as expected when the bank was withdrawn. When the bank was reinserted, rod L13 moved out. In this condition, rod L13 is considered capable of being tripped, and hence operable; so it had no affect on shutdown margin. There was approximately 2600 ppm of boron concentration in the reactor coolant system and therefore there was sufficient boron to cover both shutdown margin (1730 ppm) and the required mode verification (1882 ppm) to be able to withdrawal rods in Mode 5. Therefore, this event had no impact on nuclear safety.
ADDITIONAL INFORMATION
To determine if this event is recurring, a search of the McGuire Nuclear Station Problem Identification Process (PIP) database was conducted for a time period covering five years prior to the event. No operating experience related to control rod movement failure as a result of an incorrectly wired CRDM cable was identified. A similar event involving an incorrectly wired CRDM cable (control rod H14) occurred on Unit 1 in November 2008. However, the incorrectly wired cable was identified and corrected prior to experiencing any control rod movement failure.
Therefore, this event is not considered recurring.
The cable assemblies are not safety-related; hence, these assemblies are not basic components and 10 CFR Part 21 reporting is not necessary.