Abstract: A plug for sealing the internally threaded circular cylindrical stud holes in the flange of a nuclear reactor vessel during refueling comprises a circular cylindrical body of dense wear resistant polymer, a circular reaction plate axially spaced from one end face of the plug, an annular grommet of elastomer resistant to weak acid held between the reaction plate and the body having an exterior conical surface. A tool extends into the body and applies a torque for turning threads into the internal threads of the stud hole thus drawing the grommet into the stud hole such that it deforms against the interior of the stud hole and the surface of the flange adjacent the stud hole.