Patent Number: 047088435
Section: summary

The invention relates to a control unit for a nuclear reactor used for the production of electricity or for naval propulsion, including a plurality of sealed vessels in communication with the inside of the tank of the reactor, each enclosing a mechanism for moving a cluster of neutron-absorbing material in the core of the reactor. BACKGROUND OF THE INVENTION For controlling nuclear reactors, in particular pressurized water nuclear reactors, clusters of materials highly absorbent to neutrons are used, these being moved vertically in the core of the reactor between the fuel elements, so as to adjust the power supplied by the reactor according to the power program required. On the other hand, these clusters of absorbent material also serve for producing the emergency shut-off of the reactor, when all the units are caused to fall into maximum insertion position of the core of the reactor. To achieve the movement or the falling back of these clusters of absorbent material extended upwards by an operating rod of great length, displacement mechanisms cooperating with the operating rod are used, and these are arranged inside sealed vessels communicating with the inside of the tank of the reactor within which the core is located. These sealed vessels must permit a displacement of the operating rod corresponding to a movement of the absorbent unit between its positions of maximum and minimum insertion. The amplitude of these movements corresponds substantially to the height of the fuel assemblies, i.e., in the case of currently constructed pressurized water nuclear reactors, about 4.20 m. The displacement mechanisms, for example pawls, are driven by a driving device which is generally arranged at the lower part of the sealed vessels which extend the tank of the reactor upwards from the cover of this tank. Consequently, the height of the sealed vessels above the pawl mechanism, cooperating with the operating rod, including notches also distributed over the length of the rod, must be at least equal to the height of the fuel assemblies. The sealed vessels of very great height must be held at their upper part by means of an anti-earthquake device constituted by a plate itself held in position by means of tie-rods arranged on the walls of the pool of the reactor. It is extremely important, in fact, to limit stresses and distortions in the mechanisms in case of earthquakes to permit the emergency shut-down of the reactor by falling back of the clusters of absorbent material in the case where the reactor undergoes seismic shocks. This plate holding the upper part of the sealed vessels also plays the role of anti-missile plate since it is designed to stop the sealed vessels in the case when the latter would be ejected, so as to prevent any deterioration of the adjacent equipment. It is necessary on the other hand to create, at the level of the drive devices of the mechanisms, ventilation preventing too considerable a rise in temperature under the effect of the primary fluid filling the tank of the reactor and the sealed vessels, and to cool the drive devices to enable them to operate under good conditions. Finally, it is also necessary to heat-insulate the cover of the tank from the outer medium, in order to avoid any loss which could lower the yield of the boiler and oblige the conditioning device, for the building which encloses the reactor, to be reinforced. The control assemblies such as described above have drawbacks due to the fact that their considerable height above the cover of the tank substantially increases the height and bulk of the latter, and that the presence of anti-earthquake devices fixed to the walls of the pool of the reactor complicates operations of opening and closing the cover of the tank. The presence of the anti-earthquake plate above the motors and at the upper part of the sealed vessels is moreover troublesome when it becomes necessary to change a faulty motor or a part of the mechanism situated in the sealed vessel, which operation must be carried out in the presence of ionizing radiation. On the other hand, it is necessary to use a complex and bulky heat extraction device at the level of the drive devices at the lower part of the vessels. Another drawback is that only the cover of the tank is heat-insulated, so that there is produced a considerable heat loss at the level of the sealed vessels. SUMMARY OF THE INVENTION It is an object of the invention to provide a control unit for a nuclear reactor including a plurality of sealed vessels, communicating with the inside of the tank of the reactor, extending this tank above its cover in the vertical direction, and each enclosing a displacement mechanism for a cluster of neutron absorbent material into the core of the reactor, driven by a drive device, this control unit being of reduced height above the cover of the tank and enabling effective protection of the sealed vessels and of the mechanisms in the case of earthquake shocks, easier dismounting and remounting of the cover of the tank, good ventilation of the drive devices for the mechanisms without the use of a complex ventilation device, effective heat-insulation of the group of sealed vessels, as well as easier access to the motors and to the mechanism placed in the sealed vessel to carry out maintenance and possible repair operations. To this end, the drive devices are positioned at the upper part of the sealed vessels, within vertical aeration ducts, and the control unit includes a supporting and insulating device for the sealed vessels constituted by a strong vertical structure fast with the cover of the tank, occupying the whole height of the sealed vessels up to a level below the level of the drive device, a horizontal plate fixed to the upper part of the vertical structure having openings for the passage of the sealed vessels, and an envelope thermally insulating the sealed vessels from the external medium to a level below the level of the drive device. In order that the invention may be more clearly understood, an embodiment of a control unit according to the invention will now be described with reference to the accompanying drawings, purely by way of illustrative example, in comparison with a control unit according to the prior art.