The owners of the Wylfa nuclear power station on Anglesey, north Wales have confirmed that the plant has been shut down because of technical problems.
The plant has had serious problems in the past
BBC Wales has been told that a reactor was closed down after problems were detected during a routine maintenance check, which has to take place every two years.
A decision was then taken to close the plant's other reactor. Owners British Nuclear Fuels Limited have not said how long the plant is likely to remain closed, but it is believed it could be several weeks.
Tim Jones, head of media for BNFL Magnox, said the routine overhaul on reactor number one started a few weeks ago.
The latest problem is yet another blow to the future of the power station, which was opened in 1971.
Full scale electricity production was re-started two years ago after safety concerns forced a 15-month shutdown.
Those problems cost BNFL an estimated £200m.
The lastest difficulties are said to be unrelated to the technical problems suffered in 2001.
A statement from BNFL released on Friday said :
"Reactor one has been shut down for its statutory overhaul. Each reactor is shut down every two years and inspections are carried out of components inside and outside the reactor pressure vessel.
"Inspections inside the reactor vessel showed there were a few defects present in a small number of welds associated with brackets which support the boiler tubes in the reactor.
"A safety case had been made for return to service. During the preparation of the case, it was identified that further information is required about the condition of the supports.
"Because similar information is required on reactor two, in line with the station's safety culture and its conservative approach on any matters associated with reactor safety, the conservative decision was taken to shut down reactor two to obtain that information."
BNFL is in the process of preparing a case to bid for an extension to the plant's licence next year.
But critics, including leading environmental campaigners, say the latest problems demonstrate that the station should close now.