Construction workers involved in the decommissioning of the nuclear plant at Sellafield are taking part in a strike in a row over health and safety.
Members of Unite at the site in Cumbria are striking for two and a half hours until 8am as part of a campaign to have a full-time union convenor.
The strike follows a work-to-rule and overtime ban launched last week.
The construction workers, working for 15 sub-contractors, are pressing for the creation of a full-time convenor on the site which they maintain would improve industrial relations and help address health and safety concerns.
Unite regional secretary Mick Whitley said: "Sellafield is an important nuclear facility where health and safety and the welfare of workers should be paramount.
"Our experience shows that a full-time union convenor would help ensure a healthier, more productive and profitable workplace."
A Sellafield statement said: "We are disappointed that a dispute involving contractor companies on the Sellafield site and a section of their workforces has resulted in industrial action.
"Sellafield Ltd has no part in this dispute, which is entirely a matter for our those contracting companies and their workforces to resolve.
"During any industrial action, our overriding priority is the safety and security of the site and measures are in place to ensure this is maintained during this action.
"This is a dispute about the welfare of workers in the contractor community and not about the safety of the Sellafield site. Sellafield is a safe place to work, as proved by our track record, with 2014/15 producing one of our best ever set of safety statistics and eight gold awards from the Royal Society for the Prevention of Accidents."