A fire broke out at the atomic weapons base which maintains warheads for the UK's Trident nuclear deterrent.
The fire, inside a concrete bunker at the Atomic Weapons Establishment (AWE) in Aldermaston, Berkshire, started at about 2100 BST on Tuesday.
A male worker was injured in the blaze, which broke out within a non-nuclear explosives area.
A 600m (650yd) cordon was put in place as a precaution and residents in nearby Red Lane were evacuated from homes.
An AWE spokesman said the Ministry of Defence and AWE's regulators had been informed and fire crews were still at the scene, although flames had been put out.
He added there were no "radiological implications" as a result of the fire. He said an investigation had started.
A spokesman for the Health and Safety Executive (HSE) said it had been notified and would make initial inquiries before a decision was taken to start an investigation.
The AWE provides and maintains the warheads for the submarine-launched nuclear deterrent.
The injured worker was treated on site by first aiders and then sent home, the spokesman added.
Crews from AWE's fire and rescue service were called to the scene and were supported by up to 35 local firefighters.
Some workers and residents were put up in hotels in Aldermaston and Basingstoke overnight.
Resident Peter Bradshaw, who has lived on Red Lane for more than 50 years, said he was told there were some explosives burning in a building.
"It's the first time it has happened externally," he said.
"They are quite well organised in there.
"Obviously in an establishment of that nature one expects to have difficulties because half the time it's an unknown science."
Road closures were in place all morning as a precaution, affecting traffic between the towns of Aldermaston and Padworth Common, but reopened about 1100 BST.
The Nuclear Information Service (NIS) has called for an independent inquiry into the fire.
Peter Burt, director of the independent Reading-based group which says it aims to foster debate on nuclear disarmament, said: "This incident shows that accidents can and do happen.
"There is no room for complacency and last night's accident is a reminder that AWE poses considerable risks to local communities.
"The results of the inquiry must be made public so that local people can see that lessons have been learnt and that their safety is AWE's number one priority"