Device for operating a cluster of neutron absorber elements in a pressurized water nuclear reactor, comprising two coaxial control shafts (2, 3), the central shaft (3) being rigidly joined to the cluster of absorber elements. A widened part (22) of this shaft (3) forms a piston (22) which is fitted with segments (35). A pressure difference on either side of a segment (35) permits the clusters of absorber elements to be lifted by acting on the piston (22). The segment (35) is radially expandable and the leaktight sealing of the segment is initiated by entry of the piston (22) into the reduced diameter zone (10) of the shaft (2). The invention applies, in particular, to nuclear reactors with spectral shift employing clusters of fertile elements.
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