Abstract: A tube-plugging assembly for sealing a leaking or damaged tube of a heat exchanger, either provisionally for later repair or permanently, comprises a plug, and an explosive charge and buffer subassembly disposed in the central portion of the plug. The charge and buffer subassembly is placed within the plug so as to leave an unoccupied space between the closed end of the plug and the end of the subassembly. The embodiment for provisional plugging comprises a plug having two to six grooves around its outer circumference for containing four to five annular rings of a gold-nickel alloy, and also comprises a plurality of female threads on the inside of the plug near the closed end. The embodiment for permanent plugging comprises a plug having two to six annular prominences around its outer circumference. The buffer is of a ductile polyethylene resin that transfers the explosive power to expand the plug and also absorbs any explosion debris. A connecting cord connects the explosive charge to a remote detonator.