[en] The take-apart flanges for sealing tube joints filled with alkali metal are provided with matching collars with a conical sealing surface. Under that collar of the first flange that is placed first from the axis of the pipe or vessel, an overflow opening for the alkali metal entering there due to leakage is provided in the second flange while a discharge opening connected to a test discharge tube is provided under the collar of the first flange. It is advantageous that the conical matching collars of the flanges be placed outside thermal insulation. (B.S.)