Abstract: A method and apparatus with which a sealed gas containing first container, which is disposed in a sealed inaccessible and not visually observable further container, can be detectably opened. A thermally inducible opening mechanism disposed in a gas flow or connecting path in the wall of the first container is actuated and the gas escaping through the connecting path is detected acoustically. Among other uses, the invention can be used, in particular, for containers having radioactive contents which can be manipulated, welded and leak checked only in hot cells. The method is particularly suitable for leak checking fuel element containers or molds containing highly radioactive vitrified waste since the closing member of the first container can be composed of a soft solder plug and this plug can be melted out by local heating.