A steam generator has a bundle of U-shaped tubes arranged in spaced rows and columns with a pair of spaced rows of elongated primary antivibration bars installed between U-shaped portions of the tubes in the spaced columns and a pair of spaced retainer members disposed about the exterior of U-shaped portions of the tubes with opposite ends of the primary bars being rigidly attached to the respective retainer members. An antivibration apparatus is provided for substantially closing gaps between the U-shaped tube portions in the columns and the primary bars. The apparatus includes a row of elongated supplementary antivibration bars disposed between the rows of the primary bars. The supplementary bars extend between the U-shaped portions of the tubes through the space between, at the most, every other one of the spaced columns thereof. A support member extends across the outside of the U-shaped portions of the tubes and the supplementary bars are rigidly attached at their outer ends to the support member such that they are unsupported at their inner ends and thereby extend in cantilevered fashion between tubes. Each supplementary bar is capable of expansion in a transverse dimension extending between the tubes so as to cause deformation laterally outward away from the respective supplementary bars of longitudinal spans of the tubes extending between the spaced row of primary bars for substantially closing any gaps existing between the tubes and primary bars.