Patent Number: 048266485
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Art The invention relates to neutron absorbing bars for liquid cooled nuclear reactors of the type having a cluster of vertical parallel neutron absorbing rods fixed to arms of a spider having a central pommel connectable to a vertical moving mechanism and a damping device in the pommel. It is particularly suitable for use in pressurized water cooled and moderated reactors. 2. Prior Art Neutron absorbing bars for nuclear reactors include rods which contain a neutron poison for controlling the reactivity in the core of the reactor. They are inserted into the core to a variable degree. To cause an emergency shutdown of the reactor, all control bars are simultaneously lowered into the core by dropping them so that they enter the core under the action of their own weight. To damp the shock when the pommel abuts against the upper core plate of the reactor or against the upper end piece of a respective fuel assembly, the provision of a shock damper has already been proposed. A control bar described in European Patent No. 159 509 has a damping device consisting of a cylinder formed in the pommel and slidably receiving a piston urged downwardly by resilient means contained in the cylinder. From the moment when the piston abuts the upper core plate, continued downward movement is opposed by the compression of the resilient means and by the pressure loss undergone by the liquid which flows out of the cylinder between the wall thereof and the piston. Such a damping device has, however, only a limited effect: the damping effect due to the pressure loss does not change substantially during movement of the piston and only the increasing stiffness of the spring provides progresivity. Furthermore, the device in the pommel interferes with the flow of cooling liquid. Such shortcomings could be accepted for bars whose rods contain a neutron poison in coherent form and which do not require substantial cooling. It is no longer acceptable when the bars contain other compounds of limited resistance, and particularly when the rods of the bar contain a material used for varying the energy spectrum of the neutrons in the core. This material often consists of fertile material pellets (depleted uranium oxide, and/or thorium oxide for example) which do not withstand shocks. SUMMARY OF THE INVENTION It is an object of the invention to provide a neutron absorbing bar having a damping device reducing the shock impressed to the bar when it engages the core plate and allowing satisfactory cooling, particularly of the rods, under all operating conditions. It is to be kept in mind that, contrary to the rods containing a poison having parasitic absorption, the rods containing fertile material must be cooled by a flow of cooling liquid. To this end, there is provided a bar of the above defined type wherein the cylinder and piston are formed so that the leak cross-sectional area offered to the liquid driven out of the cylinder by the piston decreases gradually during penetration of the latter from its position of maximum extension, and the piston has a hydromechanical damper for damping the impact when the bar initially contacts the upper core plate or fuel assembly for reducing the speed of the piston.