Hundreds of workers at the Sellafield nuclear reprocessing plant are holding their second strike on Friday.
Both sides say safety will not be affected during the strikes
The action is being taken by members of the GMB and Amicus unions at the Cumbrian site in a long-running dispute over pay.
The action comes after unions claim the site's owner, British Nuclear Fuels Ltd (BNFL), has not met an agreement to close a £2,000 gap in shift pay between industrial workers and staff.
But on Thursday, BNFL said it was frustrated over suggestions it has not treated the unions fairly over the issue.
This is the second in a series of one-day strikes which are due to continue every Friday until 12 December.
Brian Strutton, national officer for the GMB, said: "We had hoped that last
week's action would have convinced BNFL to meet with us but our calls for
meetings have still been met with refusal.
"We have no choice but to escalate the action until BNFL agrees to talks."
Unions say BNFL made an agreement to harmonise the difference in shift pay between industrial workers and shift workers by April 2004.
But they say the company will not complete the process until April 2009.
But the company says it wants to put the record straight.
The director of Sellafield, Brian Watson, said: "We did a lot of work with our trade union colleagues over a long period of time and believed we had reached an agreed position with the trade unions.
"That position was not accepted by the Sellafield trade unions and that unfortunately has put the whole negotiating process that we went through in some doubt."
Both sides have said safety at the plant will not be affected by the strike action.
On Thursday, the Amicus union claimed the cost of closing the Sellafield nuclear reprocessing plant during strike action, is more than it would take to settle the dispute.