The blocking device includes sets of antivibration bars (25, 25') interposed between layers of tubes (24) of the bundle of the steam generator, in the upper part of the bundle, termed the tube bend region. The antivibration bars (25, 25') include parts (25b, 25'b) which project with respect to the tube bend region, and the blocking device includes a linkage of the end parts of the antivibration bars, which include aligned openings. The linkage includes a screw (21) whose head (21a) bears on a first antivibration bar (25) and whose threaded rod (21b) is screwed into a spacer (22). The spacer (22) is fixed either in a second spacer or in a tapped fastening element (23), by a threaded part (22b). The threaded parts (21b) of the screw and (22b) of the spacer pass through the antivibration bars (25, 25') through aligned openings.

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