According to Essex Fire Service, around 22lb (10kg) of explosives were involved in the blast, which occurred at a weapons testing centre at MoD Shoeburyness, near Southend.
The weapons testers were freed by MoD officers in an armoured personnel carrier, but there were no injuries.
Five fire crews attended the incident, which happened at around 11am today.
A spokesman for QinetiQ, the private defence contractor that runs the Blackgate Road munitions centre, said:
"No-one was hurt in the incident at our environmental test centre and we are conducting a full investigation into what happened as is part of our standard safety procedures."