The 24-hour industrial action got underway at 6.01am today, 27 September and is scheduled to end at 5.59am on Thursday, 28 September.
GMB said 3,000 of its members are downing tools after management imposed a below-inflation pay offer of 1.5%, with RPI inflation currently sitting at 3.9%.
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Unite the Union is also balloting its 2,000 members over taking action, with the result due on Monday, 01 October.
Chris Jukes, GMB Senior Organiser said: "GMB members have not sought dispute and they do not want to strike but Sellafield Limited has created this.
"Sellafield imposed a below inflation 1.5% rise for 2017/18, causing the dispute, because - by their own admission - they think that only staff grades matter.
"GMB members just want a fair pay rise, an end to the upstairs downstairs culture and a leadership that can deliver vision, growth and jobs for Sellafield and the surrounding areas."
(LM)
