[en] It is shown how the mechanical problems concerning the tightness of main sodium pump penetrations through the deck of a pool type reactor have been solved in joint SNECMA - HISPANO-SUIZA/CIRNA studies. Extensive general studies have been performed and a comprehensive dossier of different possible solutions compiled. Whereever appropriate, information has been extrapolated from PHENIX experience. In the solution finally adopted, an elastic torus shaped ring, which forms a ball-and-socket type joint is used. A thin shell structure connects the bottom to the reactor internals in such a way as to permit both vertical and rotational displacements. Studies and calculation have been completed with the realization of a 1/5 scale model of the upper torus ring. Displacement, and angular and axial stiffness measurements have confirmed the validity of the hypotheses. (Auth.)