About 1,400 contractors have gone on immediate strike at Sellafield.
Unite union has confirmed workers have taken strike action today.
Workers at the nuclear plant walked out this morning following a mass meeting of workers, held at 7am.
Although details of staff concerns have not been made public, they centre around the changing rooms in the welfare facility.
An investigation has now been launched.
A Sellafield Limited spokesman said: "We are aware of an industrial relations issue involving contractors at the Sellafield site, and we are closely monitoring further developments.
"We have taken steps to ensure that safety, security and operations at the site will be unaffected."
No more information has been released at this time and other unions have been approached for comment.
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