[en] The invention relates to a sealing device in a graphite-moderated light water-cooled nuclear reactor. The reactor comprises a plurality of fuel channels and each fuel channel at its upper part encloses a plug. A gap is provided between said fuel channel and said plug and sealing surfaces are arranged correspondingly on the inner part of the fuel channel and the outer part of the plug, respectively. The sealing device is arranged in the gap between said sealing surfaces, and is characterized in that it comprises one or more annular bodies arranged in the axial direction one after the other and making direct contact with each other. Each body is plastically deformable for transforming the force in the axial direction obtained via the plug into a sealing pressure in the radial direction whereby it achieves a sealing effect of the sealing device. Further, the sealing device is characterized in that at least that part of it, which faces the sealing surfaces, is monolithic. (author)