Device for operating a cluster of neutron absorber elements in a pressurized water nuclear reactor

In a spectral shift pressurized water reactor comprising:

• (a) a vessel,
• (b) a core comprising a plurality of nuclear fuel assemblies, each containing a plurality of first vertical guide tubes and a plurality of second vertical guide tubes,
• (c) a plurality of sets of control rods for controlling the power of the reactor having control rods movable within and along respective ones of the first guide tubes between a predetermined lower position and a predetermined upper position,
• (d) a plurality of clusters of spectral shift elements for changing the neutron energy spectrum of the reactor, each of said cluster being associated to a set of control rods, having elements movable within and along respective ones of the second guide tubes of the corresponding assembly of said set of control rods and being arranged to be maintained in one of two fixed positions including a low position of maximum insertion of the elements in the core and a high position of complete withdrawal of the elements from said core,
• operating means for operating said set of control rods and said clusters of spectral shift elements comprising:
• first operating means for operating sets of control rods up and down comprising a plurality of first vertical control shafts movable vertically between a second predetermined lower position and a second predetermined upper position and arranged to go further when forced to a higher overtopped position situated above said second predetermined upper position, each of said first vertical control shafts having a cylindrical tubular shape having a bottom end connected respectively to a corresponding set of control rods and having over the greatest portion of its length a part of a first internal diameter and over a small portion of its length a part of a second internal diameter, said second internal diameter being substantially smaller than said first internal diameter and being situated near the bottom end of said first shaft,
• second operating means for operating clusters of spectral shift elements comprising a plurality of second vertical control shafts each respectively mounted coaxially inside the corresponding first shaft of the associated set of control rods, respectively connected to each cluster and movable in the internal cylindrical space of corresponding first shaft, each of said second shafts having a widened cylindrical part along a small portion of its length, said small portion constituting a piston situated near the upper part of the said second shaft and coming into position around the internal part of substantially smaller diameter of the corresponding first shaft when said first shaft has raised to is higher overtopped position,
• said piston comprising at its periphery at least one solid joint, radially expandable to a certain extent under a predetermined pressure and which diameter in rest condition is substantially smaller than the first internal diameter of the first control shaft and substantially similar to the second internal diameter of said first control shaft and when sollicited by pressure is similar to said first internal diameter,
• said operating means further comprising pressure reduction means arranged to be connected to the internal cylindrical space of each of the first control shafts when said first shaft has raised to its higher overtopped position,
• and capable of moving in the high position of complete withdrawal of the elements of the cluster the corresponding second control shaft connected to said cluster of spectral shift elements under the effect of a predetermined pressure reduction created in the internal space of the first control shaft above the piston.