The blast happened at about 11:00 BST at a centre for disarming weaponry, run by defence contractor QinetiQ, in Blackgate Road, Shoeburyness.
About 10kg (22lbs) of explosives were involved, Essex Fire Service said.
No-one was hurt, although four weapons testers had to be rescued from a bunker. The fire service remains on standby and a full investigation is under way, QinetiQ said.
Five fire crews were called to the incident at the site near Southend.
A fire service spokesman said: “Crews will remain standing by at this incident until specialist scientific teams, who have been called in to deal with the technical aspects of this incident, give them the all clear.
“Four weapons testers… have been brought to safety by MoD officers in an armoured personnel carrier.”
A spokesman for QinetiQ said: “No-one was hurt in the incident at our environmental test centre, and we are conducting a full investigation into what happened.
“The emergency services were called as part of standard procedures, their assistance was not required and they were subsequently stood down.”