A Balfour Beatty worker died following a serious incident at the Atomic Weapons Establishment Aldermaston site in Reading on 6 July.
The Office for Nuclear Regulation (ONR) confirmed the death in a statement and said staff on site were working with the relevant authorities, including the Thames Valley Police.
An ONR spokesperson said: “As this is a live investigation, we will not be commenting further so as not to prejudice those inquiries.
“The incident did not involve any nuclear materials and there are no radiological consequences to the public.”
A spokesperson for Balfour Beatty said: “Our deepest sympathies are with our colleague’s family, friends and co-workers at this time.
“Works have since stopped as we support the authorities with their investigation. As these are ongoing, it would be inappropriate to comment further.”