The company was quoted by BBC News as saying that the move was "unplanned but standard procedure and there was no danger to the public.”
The closure will not affect power supplies in the area, the company added, noting that the unit will start operation in the coming week.
One of the two reactors of the Heysham 1 NPP was taken off the grid in August 2013, after smoke was seen coming from it, according to the news agency.
Both Heysham 1 reactors are of the advanced gas-cooled reactor (AGR) type, and commenced operations in 1983 and 1984, respectively.
