Almost a third of Sellafield nuclear workers are to stage a 24-hour strike in a dispute over pay.
The GMB union said its 3,000 members would walk out on 27 September in protest at an imposed pay rise of 1.5%. Sellafield has a workforce of 10,000.
Unite, which is currently balloting its 2,000 members at the plant, has described the pay offer as "paltry".
Sellafield Ltd said it was a publically funded organisation and had to deliver value for money to the taxpayer.
GMB official Chris Jukes said: "Despite repeated requests to try to get Sellafield management to come to the table to talk, it is clear that they seem intent on being intransigent and petulant as usual in their attitude to disputes."
In June, 88% of GMB members rejected the offer in a consultative ballot.
Mr Jukes added: "No-one likes to take strike action but GMB members have been given a pay cut, on top of a 0.25% increase last year, and their pay has failed to keep up with rising prices.
"Like everyone else, GMB members have bills to pay.
"Meanwhile, those at the top seem more interested in their big performance bonuses, as the gulf between them and those working on the shop floor at Sellafield gets ever wider.
"Clearly GMB members have signalled that they have had enough."