[en] For refuelling the reactor vessel closure head is opened and the annulus between reactor vessel and biological shield respectively reactor cavity is filled with water containing boric acid. In this way an additional shield against neutron and gamma radiation is formed. For support and sealing of the annulus there is used a ring to be mounted on the site and consisting of equal-sized ring segments being firmly and tightly braced with each other. At its lower side there is fastened one each U-shaped sealing ledge sewing as contact surface on the pressure vessel and the biological shield. Thex meet closely and thus form two circular sealing rings. They consist of plastics on the basis of polymethane and get into close contact with the ring segments of ferritic steel. On the ring segments there may also be put a protective sheet of polyethylene. (DG)