(Removes reference in 1st paragraph to emergency classification. Clarifies NRC position on backup electrical power supply in paragraph seven)
June 24 (Reuters) - The U.S. Nuclear Regulatory Commission (NRC) said on Tuesday that after a series of investigations, it has declared a 2013 incident at an Arkansas nuclear power plant to be of substantial safety significance.
An industrial accident at Entergy Corp's Arkansas Nuclear One plant in Russellville killed one worker and injured eight others on March 31 last year, when a generator fell as it was being moved out of the turbine building.
The accident was initially classified as an unusual event, the lowest of four emergency classifications, because the incident caused a small explosion inside electrical cabinets.
The incident caused unit 2 of the plant to automatically shut down when a reactor coolant pump tripped due to vibrations caused by the heavy component hitting the turbine building floor, damaging electrical cables and equipment, leading to a loss of off-site power.
Unit 1 at the plant was in a refueling outage at the time, with all of the fuel still in the reactor vessel, safely cooled.
Emergency diesel generators were relied upon for six days to supply power to cooling systems.
The NRC said the falling turbine component damaged electrical cables and equipment needed to route alternate power to key plant systems at both units, which increased the risk to the plant. However, in meetings with Entergy officials, operators identified other approaches they could have used to supply electrical power to the plant's safety-related systems, an NRC spokesman said.
"The NRC determined that the lifting assembly collapse resulted from the plant's failure to adequately review the assembly design and ensure an appropriate load test in accordance with its procedures or approved standards," the NRC statement said.
NRC held a regulatory conference with Entergy officials on May 1 and after considering information provided by Entergy, determined that 'substantial safety significance' was appropriate to characterize the risk significance of the event for both Unit 1 and 2.
"The NRC will determine the appropriate level of agency oversight and notify Entergy officials of the decision in a separate letter," the statement said.
(Reporting by Anupam Chatterjee in Bangalore; Editing by Marguerita Choy in New York and Eileen O'Grady in Houston)