The company cannot confirm when the reactor will reopen, but emphasises the move is part of routine work.
A British Energy spokesman said: “There is a programme in place and we are carrying out full boiler inspections at Hunterston. We cannot say when it will reopen.”
The company had said in November it would shut the reactors at the plant in Scotland for checks after problems with the boilers kept the plant closed from late 2006 until spring 2007.
The company will inspect three more reactors at its Hunterston B and Hinkley Point nuclear power stations later this year.
Hunterston B has been running at just 60 to 70 percent of its capacity since being reopened last spring.