Anti-nuclear protesters turned out for a demonstration at Berkshire's Atomic Weapons Establishment (AWE).
The gates were locked at one of the contractor's sites
The protest was being held against a new laser being developed at the centre in Aldermaston which will simulate the conditions of a nuclear reaction.
The "Block the Builders" campaigners blocked roads into the AWE site and four people were arrested.
Demonstrators also blockaded a site in Tadley belonging to one of the contractors working on the site.
The group has staged a number of protests since plans for the multi-million pound upgrade - named Project Orion - were announced.