Firefighters at the Sellafield nuclear site have launched the first of two 12-hour strikes in a dispute over pay and conditions.
Members of the GMB union walked out at 6am, with a similar stoppage planned for Thursday.
The union accused Sellafield management of failing to keep promises made in July to settle the dispute.
GMB official Chris Jukes said: "Management have broken their promises to the firefighters and have failed to make a fair offer that would put an end to this dispute
"Firefighters have remained professional throughout this process, which started in 2012, undertaking all the relevant training courses required, many done on their rest days.
"Sellafield Ltd has spun a message of greed with 2,500 senior personnel earning eye-watering bonuses which are an obscenity compared to what firefighters were promised."
A Sellafield spokesman said: "We made an offer to the firefighters which included an additional allowance for carrying out extra work.
"We agreed to work with the GMB and the firefighters to assess the value of that allowance. We remain open to further discussions.
"Our focus is on putting in place arrangements to ensure the site remains safe during any industrial action."
A separate dispute over pay saw thousands of GMB members go on strike for 24 hours last month.