EDF Energy, owned by French utility EDF, took the reactor offline for planned refuelling and maintenance work on April 16, initially scheduled to end on May 29.

"Our anticipated return to service has been updated to 30th August following additional work required on some components identified during the shutdown," a spokeswoman said on Tuesday.

"We have a team on site working on this whilst the other outage work continues to plan," she added.

(Reporting by Nora Buli in Oslo; editing by Jason Neely)