The Nuclear Regulatory Commission (NRC) issued two “yellow” findings on June 24 to Arkansas Nuclear One—the 1,823-MW dual-unit nuclear power plant located near Russellville that is operated by Entergy Corp.—in connection with a heavy equipment handling incident at the facility last year.
On March 31, 2013, a 525-ton generator stator fell when a temporary lifting assembly collapsed killing one person and injuring eight. The component was being moved out of the plant’s turbine building as part of a maintenance activity to replace the component.
The stator fell on the Unit 1 turbine deck, causing extensive damage to the area, and subsequently fell over 30 feet into the train bay. Unit 2, which had been operating at full power prior to the incident, tripped due to high vibration readings on a reactor coolant pump. Unit 2 lost offsite power to one vital bus while Unit 1 lost all offsite power for six days. The dropped stator ruptured a common fire main header in the train bay causing flooding in Unit 1 and water damage to the electrical switchgear for Unit 2. The station blackout diesel generator electrical supply cables to both units were also pulled out of the electrical switchgear making the equipment unavailable to either unit.
The NRC evaluates regulatory performance at commercial nuclear plants using a color-coded process that classifies inspection findings as green, white, yellow, or red in order of increasing safety significance. The yellow findings indicate “substantial safety significance” that may require additional regulatory oversight.
Extensive reviews of the event were completed during Entergy’s root cause evaluation. The ultimate cause for the stator drop was determined to be failure to follow a quality-related procedure, in that, the overhead temporary hoisting assembly was not properly designed; the associated calculation was not reviewed; and the assembly was not load tested as required.
Corrective actions included: repairing the damaged Unit 1 turbine structure, fire main system, and electrical systems for both units; modifying procedures related to handling of heavy loads; training staff on the revised requirements for handling heavy loads; and providing additional oversight for the subsequent Unit 1 replacement stator lift. However, the NRC noted that Entergy did not address oversight of the contractors involved with the original stator lift in its root cause analysis, which the NRC believes also contributed to the event.
—Aaron Larson, associate editor (@AaronL_Power, @POWERmagazine)