Magnox is closing 12 sites accross the country which will lead to 1,600 job losses.
The closing sites include Berkeley, Gloucestershire and Bradwell and are to close by September next year.
Magnox said: “These proposed reductions arise from planned step downs in the work programme at a number of sites and the implementation of a more streamlined operating model for delivering decommissioning.
“We will seek wherever possible for these reductions to be through voluntary means and we will endeavour to re-train staff in roles where we are currently reliant on agency resources.”
The GMB union wants to discuss the job losses and “the lack of progress on nuclear new build”.
GMB National Secretary for Energy, Gary Smith said: “It was always envisaged that many of these workers would be moving over to the nuclear new build sites as the work on decommissioning and clean-up is finished.”
Magnox added: “We will work with Trade Unions to support our employees and ensure changes are managed fairly, with respect for the individuals involved and in a way which makes best use of the important skills the workforce have built up.”