A device, system and method permitting on-line explosives-based cleaning and deslagging of a fuel burning facility (31) such as a boiler, furnace, incinerator, or scrubber. A coolant, such as ordinary water, is delivered to the explosives (101) to prevent them from detonating due to the heat of the on-line facility. Thus, controlled, appropriately-timed detonation can be initiated as desired, and boiler scale and slag is removed without the need to shut down or cool down the facility. Alternative preferred embodiments include, but are not limited to: (1) using a non-liquid coolant, such as compressed air or other non-flammable gas, in place of the aforementioned liquid coolant; (2) using one or more highly-heat-resistant insulating materials (502, 504, 506) to insulate the explosive and detonator cap, in place of or in addition to the aforementioned liquid or gaseous coolants; and (3) preparing and using a highly-heat-resistant explosive device (101), in place of or in addition to the aforementioned liquid or gaseous coolants, and/or the aforementioned highly-heat-resistant insulating materials (502, 504, 506), in any desired combination.