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 Report dateSiteEvent description
05000271/LER-2014-00121 November 2014Vermont YankeeOn September 29, 2014, with reactor power operation at 96 percent power, the 'A' train Emergency Diesel Generator (EDG) did not complete the starting sequence during a scheduled Technical Specification monthly surveillance. The direct cause of the event was a hydraulic lock condition between opposing pistons on compression stroke in the #6 cylinder which prevented the EDG from turning. This condition resulted from an internal Jacket Coolant Water leak through a copper gasket that provides a sealing surface inside the cylinder. The copper gasket had been in service since installation during a cylinder liner replacement in 1992. There were no indications of a leak prior to its discovery, therefore the exact time when a hydraulic lock condition developed is unknown. The 'A' EDG was potentially inoperable since the completion of the last successful surveillance that demonstrated its capability to meet its safety function on August 25, 2014. Based on the ability of the 'A' EDG to be recovered within a few hours, the availability of the redundant 'B' train EDG, and the availability of normal and alternate off- site and onsite power sources capable of supplying power to engineered safety feature loads during and following design basis accidents, this event did not pose a threat to public health and safety.
05000271/LER-2013-00231 December 2013Vermont YankeeOn November 6, 2013, with the plant at 100% Reactor Power, Vermont Yankee Nuclear Power Station (VY) identified a conduit containing a loose screw type conduit seal plug and another conduit with a missing seal inside electrical manhole MH-S2 located outside the Administrative Building. This was discovered during a routine preventative maintenance surveillance of flood seals. On November 7, 2013, it was identified that the missing flood seal compromised the flood design controls for the Switchgear Rooms. The conditions were corrected by installing silicone elastomer seals in the two affected spare conduits, thus removing the potential flood paths. The causes of the incorrect and missing flood seals were due to not completing the appropriate corrective actions following a similar event reported in LER 2013-001-00, dated May 16, 2013. Plant procedure requires inspection of the Switchgear Rooms during a flood event and includes actions that would have mitigated any flooding; therefore, this event did not pose a threat to public health and safety.
05000271/LER-2011-00220 January 2012Vermont Yankee

On December 2, 2011, with the plant at 100 percent power, Vermont Yankee (VY) was modifying the tagging lineup on the "B" Emergency Diesel Generator (EDG) that was out of service for scheduled maintenance.

During the tagging evolution, an operator mistakenly entered the "A" EDG room and tripped the "A" EDG fuel rack making the "A" EDG inoperable. This resulted in both EDGs being inoperable requiring entry into a 24 hour limiting condition for operation. This event is reported in accordance with 10CFR50.73(a)(2)(v)(D) as an event or condition that could have prevented the fulfillment of a safety function since both EDGs were inoperable.

The investigation determined that this event was caused by a lack of adherence to procedures that provide administrative controls over tagging evolutions and direct the use of human performance tools to prevent occurrence of this type of an event. The condition was immediately identified by operations personnel due to alarms received in the main control room and the "A" EDG was returned to operable status in two minutes.

There were other sources of AC power available and therefore, this event did not pose a threat to public health and safety.

05000271/LER-2011-00114 April 2011Vermont Yankee

On February 16, 2011 with the plant at 100 percent power, Vermont Yankee was preparing to perform a scheduled quarterly surveillance on the High Pressure Coolant Injection (HPCI) system. During initial startup of the system, audible and visual indications of steam leakage were observed. The investigation determined that a flanged connection associated with steam trap ST-23-3 was the source of the leak. This steam trap is on a line that maintains the steam supply piping to the HPCI turbine free of accumulated water. The event was attributed to a maintenance activity that was performed on February 1, 2011 where the steam trap was disassembled to facilitate a piping weld repair. The cause of this event was a failure to follow procedures that resulted in incorrect gasket material being used as a replacement for spiral wound gasket material. This event was determined to be reportable per 10CFR50.73(a)(2)(v)(D) as an event or condition that could have prevented fulfillment of a safety function and under 10CFR50.73(a)(2)(i)(B) as a condition prohibited by Technical Specifications. The Automatic Depressurization System (ADS) serves as a backup to the HPCI system. If the HPCI System does not operate and one of the low pressure coolant injection pumps is available, the Nuclear System is depressurized using ADS to permit the Low Pressure Coolant Injection (LPCI) and Core Spray (CS) systems to operate to protect the fuel barrier. This event did potentially affect the ability of HPCI to perform its safety function from February 1, 2011 through February 19, 2011 when the system was returned to service.

During that time period, ADS and either LCPI or CS were available to perform the required safety functions.

Therefore, this event did not pose a threat to public health and safety.