A device comprises a high-explosive charge with an internal zone or wall of metallic fuel. The explosive is initiated simultaneously on opposite sides of the zone so that implosion pressure waves act from both sides on the zone. The explosive on one or both sides of the zone contains or contacts further metallic fuel so that the implosion is maintained to permit ignition of the fuel in the zone. The zone pref. defines cylindrical inner and annular outer areas of the explosive charge, and the further fuel is provided as powder dispersed in the explosive and/or an otuer casing of the fuel. The fuel is pref. boron, beryllium, lithium, magnesium, titanium and/or zirconium, partic. boron or boron alloy, opt. with up to 20% inorganic nitrate as oxidant. There is pref. a low-density foamed aluminium outer casing.