Installations for mounting, controlling, and guiding control rods in and outside a vessel may be removed and passage through an upper internal part may thereby be avoided. An upper internal part is placed near the core of a reactor and laterally emerges on the outside of the vessel. All the transmission, control, and fluid systems pass through this internal part and emerge, by a mobile connection part, for each cluster of control rods, on an assembly head containing tanks of liquid neutron absorber, and injection circuits in injection channels placed between the fuel rods.