[en] The take-apart flanges are provided with matching collars under which a sealing is compressed. The communication space between collars is connected to a test discharge pipe. The sealing rings should be made of soft iron, corrosion resistant to sodium, e.g., the BEHANIT type. It is advantageous to place the matching flange collars outside thermal insulation. Thus, under suitable temperature cond+tions and in case of leakage of the first collar, the alkali metal in the space between the collars solidifies and secures complete tightness of the joint. (B.S.)