document: Unite said workers at the Atomic Weapons Establishment's (AWE) two Berkshire sites would strike for 12 days over a pensions dispute.
  AWE, which builds the UK's Trident submarine warheads, said it had put "resilient measures in place".
  It added that its pension scheme was "very competitive".
  The walk-out of around 700 workers at Aldermaston and Burghfield in June, July, August and September will bring the number of strike days since November to 30.
  Unite regional officer Bob Middleton said: "We know that our members' action has caused serious disruption to the production of the Trident warheads.
  "We also know that convoys to Scotland carrying the missiles have been limited."
  The dispute centres on Unite's claim that promises about pensions were broken after the scheme was moved to the private sector in the 1990s.
  An AWE spokesperson said: "We are actively seeking to re-engage with Unite to find a positive resolution to this dispute."
  The union Prospect agreed a pension settlement for its AWE members in February.
summary: A union claims strikes have resulted in "serious disruption" to the UK's nuclear weapons programme as it announced further industrial action.