05000368/LER-2013-002
| Arkansas Nuclear One - Unit 2 | |
| Event date: | 02-04-2013 |
|---|---|
| Report date: | 04-04-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 3682013002R00 - NRC Website | |
A. Plant Status At the time that the Control Room Emergency Chiller 2VE-1A breaker tripped, Arkansas Nuclear One (ANO) Unit-1 and ANO Unit-2 were both at approximately 100% power. The redundant Control Room Emergency Chiller 2VE-1B was available and operable.
B. Event Description
On February 4, 2013, at approximately 1255 CST, Control Room Emergency Chiller [VI][CDU] 2VE-1A breaker tripped shortly after being started. 2VE-1A is one of two emergency chillers normally powered from the ANO Unit-2 electrical distribution system, and designed to maintain the common control room environment during accident conditions for both ANO Unit-1 and ANO Unit-2. The subject 2VE-1A breaker (2B-52D5)[EC][BKR], is a Siemens 480 volt, 100 amp molded case circuit breaker that was installed by contract electricians on November 15, 2012.
After breaker installation, the 2VE-1A chiller was started for post maintenance testing (PMT) which included the monthly Technical Specification (TS) surveillance (the TS surveillance acceptance criterion requires that the chiller must be run for greater than one hour). The chiller monthly TS surveillance was subsequently performed in December 2012 and January 2013, with satisfactory results. The 2VE-1A breaker then tripped after being started for TS surveillance testing on February 4, 2013.
An Apparent Cause Evaluation determined that on November 15, 2012, the compression style lug or "wire grip" that tightens the "C" phase load side wire to the breaker stab had been incorrectly installed in front of the breaker stab instead of over the stab, resulting in a loose connection between the lug and the breaker stab. Although three successful starts and surveillance runs were achieved over the course of three months with the incorrect configuration, the electrical connection degraded to the point that the breaker tripped on February 4, 2013, when starting 2VE-1A.
C. Event Cause The apparent causes of the condition were determined to be a human performance error due to the inadequate use of human performance tools, and an inadequate molded case circuit breaker testing procedure, which did not provide a step to perform a visual inspection of the wire grip style lug after installation.
Interviews with the electricians identified that it is not uncommon for the wire grips to come out of the breaker during transportation or installation, and it is a common practice to check the lugs for proper installation after installing a new breaker.
aware of the ability to improperly install the lugs, but they thought that by pulling on the wire after installation, they could verify that the lugs were installed properly. Both electricians and a supervisor pulled on the wire to verify installation. The Apparent Cause Evaluation stated that pulling the wire in this case would not have identified the deficiency and that only a visual inspection could have identified the incorrect lug installation. Previous training was provided to the contract electricians on the replacement of the circuit breakers and the ability to incorrectly install the wire grip style lug was discussed during the training. Since both individuals attended the training and were aware of the possibility of incorrectly installing the lug, neither lack of knowledge nor insufficient training is considered a cause that contributed to this event. The HPER stated that the physical location of the breaker required the electricians to lay down on the floor to see the load side breaker lugs after the breaker was installed, potentially contributing to the error. The HPER did not identify any time or situational pressures associated with this work activity.
D. Corrective Actions
After repairs to the 2VE-1A breaker, a surveillance was satisfactorily completed on February 6, 2013. Other initial corrective actions included a human performance error review, and an additional requirement for visual verification of the wire grip style lugs was added to the remaining breaker replacement work orders. A work request was generated to inspect previously replaced breakers identified in the Apparent Cause Evaluation extent of condition.
Other actions being tracked in the corrective action process are to revise the molded case circuit breaker testing procedure to include a step to visually verify the lugs are installed correctly, and to review the results of the Apparent Cause Evaluation with electricians.
E. Safety Significance Evaluation 2VE-1A and 2VE-1B are part of the the Control Room Emergency Air Conditioning System (CREACS) that provides temperature control for the common ANO Unit-1 and ANO Unit-2 control room when the control room is isolated. The emergency chiller units are manually started from the ANO Unit-2 Control Room. The design basis of the CREACS is to maintain control room temperature for 30 days of continuous occupancy. The CREACS components are arranged in redundant, safety related trains.
ensure a habitable environment and equipment operability. The operability of the control room emergency ventilation and air conditioning system ensures that the ambient air temperature does not exceed the allowable temperature for continuous duty rating for the equipment and instrumentation cooled by this system, and that the control room will remain habitable during and following all credible accident conditions.
Probabilistic Risk Assessment (PRA): Engineering confirmed through a review of the ANO Unit-2 fault tree logic that the CREACS is not included in the ANO Unit-2 PRA model. The CREACS is not included in the ANO Unit-1 PRA Model based on documented industry recommendations (Reference WCAP-16679-P November 2006 Accident Sequence Phenomena Considerations). The reference states that control room heatup analyses are typically not available to predict the control room transient environment for all events. Typical industry heatup calculations for control rooms show that temperatures do not approach the limiting conditions for equipment operation (120 degrees F) in the first 12 to 16 hours1.851852e-4 days <br />0.00444 hours <br />2.645503e-5 weeks <br />6.088e-6 months <br /> after the loss of control room cooling. Analyses also show that simply opening doors and allowing air circulation into and out of the control room can significantly reduce these temperatures. The reference states that most of the manual operator actions modeled in industry PRA for the applicable accident sequences are taken within the first 6 to 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> of the event. Therefore it is concluded that these temperatures should not impact control room habitability or human reliability for manual operator actions modeled in the PRA.
During the time that 2VE-1A is being considered inoperable from November 15, 2012 to February 6, 2013, the redundant chiller 2VE-1B was considered inoperable during a period from November 26, 2012, to December 3, 2012 (approximately 7 days), due to its emergency power supply (Emergency Diesel Generator 2K-4B) [EK] being out of service for overhaul. During this time, the Alternate AC Diesel Generator (Station Blackout Diesel Generator) [EK] was available to power the 2VE-1B if it had been required for control room temperature control concurrent with a loss of normal offsite power event. Additionally, during this seven day period, the normal power supply was available for the redundant 2VE-1B except for a brief period on November 29, 2012 (approximately 2.2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />), while the 2VE-1B chiller unit was out of service planned maintenance.
During the time period from November 15, 2012, to February 6, 2013, the emergency control room chillers were not required to be in service for accident mitigation, therefore, there was no actual nuclear or radiological consequence as a result of the event described in this report.
Reporting Guidance) that the inappropriate breaker configuration was introduced at that time.
Although the 2VE-1A Chiller surveillance was performed three times during the subsequent three months after the breaker replacement, the incorrect breaker configuration introduced uncertainty regarding the ability of 2VE-1A being able to run for a 30 day mission time as required in accident conditions.
10CFR 50.73(a)(2)(v): Any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to:
(D) Mitigate the consequences of an accident During the time that 2VE-1A is being considered inoperable, the redundant chiller 2VE-1B was also inoperable for a period of time, which constitutes a potential prevented safety function for both ANO Unit-1 and ANO Unit-2.
G. Additional Information
10CFR 50.73(b)(5) states that this report shall contain reference to "any previous similar events at the same plant that are known to the licensee." NUREG-1022 reporting guidance states that term "previous occurrences" should include previous events or conditions that involved the same underlying concern or reason as this event, such as the same root cause, failure, or sequence of events.
A review of the ANO corrective action program and Licensee Event Reports for the previous three years revealed no relevant similar events.
Energy Industry Identification System (EllS) codes and component codes are identified in the text of this report as p0q.