Workers at the Atomic Weapons Establishment (AWE) will today launch the first of two strikes in a long-running dispute over pensions.
Unite said 600 of its members at AWE's two sites at Aldermaston and Burghfield in Berkshire will walk out for 48 hours, and again from January 30.
The union said workers felt "deeply betrayed" as promises made a quarter of a century ago guaranteeing their pensions, when they were transferred from the Ministry of Defence to the private sector, have been broken.
The union is protesting at plans to close the defined benefit scheme at the end of the month and replace it with a defined contribution one.
Unite regional officer Bob Middleton said: "It was in the House of Commons in the early 1990s that the then-Tory government made copper-bottomed promises to AWE workers regarding the future of their pensions, once they transferred to the private sector.
"It is quite clear that this pledge has been shattered and our members feel deeply betrayed. The hallmark of this dispute is a litany of broken promises.
"The most just course of action would be for the pension scheme to be taken back by the MoD.
"The four days of strike action are not being taken lightly. It is not a 'political' strike, but one taken reluctantly by our members who have no desire to see thousands of pounds wiped off their retirement incomes."
An AWE spokesman said: "AWE is committed to establishing future pension arrangements that are affordable for our staff and attractive to future employees.
"Over recent weeks, improvements have been made to the new scheme and additional enhanced benefits have been included.
"This is a very competitive scheme which has been benchmarked against external comparators and puts the revised AWE scheme in the top 25 percentile.
"Staff have been informed that from January 31 2017 the AWE defined benefit scheme will close and as of 1 February 2017 all members of the AWE Pension Scheme will be enrolled into the AWE Group Personal Pension scheme.
"The safety and security of AWE staff and the general public remains our top priority during periods of industrial action. We have undertaken detailed contingency planning and have resilient measures in place."
Picket lines will be mounted outside the sites.