More than 1,000 workers at the Sellafield nuclear site in Cumbria have walked out in a dispute over pay.
Members of the Unite union are taking the action after management was accused of "going cold" on attempts to reach a pay deal.
The union has demanded talks over an imposed 1.5% wage increase.
A spokesman for Sellafield Ltd said the company was working with Cumbria Police and Cumbria County Council to try to minimise disruption.
The union's regional officer, Graham Williams, warned that further industrial action could be taken in the run-up to Christmas.
'Safety and security'
He said the firm's imposed wage increase was "completely unacceptable".
A Sellafield spokesman said: "As always, the safety and security of the Sellafield site, our workforce and the community are our priorities during this industrial action.
"However, strikes by their nature are designed to cause disruption and people should expect traffic problems while pickets are in place.
"We have taken all steps available to us to minimise disruption including opening all access gates and advising staff on steps they can take to help."
About 3,000 members of the GMB union staged a 24-hour strike in September as part of the same pay dispute.
Sellafield employs about 10,000 people.