A method for sealing an opening extending radially from an outer circumferential surface to an inner circumferential surface of a tubular object in a nuclear power plant includes inserting a stopper from outside of the outer circumferential surface through the opening into the tubular object; and actuating a fastener from the outside of the circumferential surface to force the stopper radially outward to seal the opening. A mechanical seal assembly for plugging an opening in a tubular object by contacting an inner circumferential surface of the tubular object includes a stopper configured for insertion into an interior of the tubular object for plugging the opening. The stopper includes a surface configured for matching the inner circumferential surface of the tubular object. The mechanical seal assembly also includes a fastener passing through a hole in the stopper such that the fastener is actuatable from outside of the tubular object to force the surface of the stopper against the inner circumferential surface of the tubular object.