British Nuclear Fuels has shut two units at its Wylfa nuclear power station on Anglesey as part of planned maintenance.
No2 reactor at Wylfa is shut for maintenance
The company said No2 reactor was shut down last Friday, with No1 reactor still in production.
A BNFL spokesman declined to say when the units would
resume production, due to commercial confidentiality
The two Wylfa reactors have a combined capacity of 980 megawatts.
Last summer, the plant was shut down for several weeks after problems were found in a reactor.
They were uncovered at the ageing station during a routine maintenance.
Opened in 1971, the station, which at full capacity produces enough daily electricity for two cities the size of Manchester and Liverpool, is due to stop generating permanently in 2010.
The station also underwent a 15-month shutdown in 2001.