Abstract: A burster for rupturing a casing containing an active explosive or propelt material of an ordnance device to prevent cook-off has an inert and thermally expansible material in an enclosure defined by a wall fixed to the casing so that thermal expansion of the inert material exerts on the wall a force to rupture the casing and vent the active material. The wall may have a stress riser groove to promote rupture at a predetermined location on the casing, and the volume of the inert material at ambient temperature may be less than the volume of the enclosure so that rupturing force is not exerted until a selected venting temperature is approached.