Patent Number: 053012127
Section: summary

FIELD OF THE INVENTION The invention relates to a process and an apparatus for dismantling an irradiated component of a nuclear reactor, particularly a vessel of a nuclear reactor cooled by pressurized water. BACKGROUND OF THE INVENTION Water-cooled nuclear reactors, particularly pressurized-water nuclear reactors, comprise a vessel which is intended for containing the core of the nuclear reactor and which is connected to the reactor cooling circuit in which the cooling water circulates. The wall of the reactor vessel which is in contact with the cooling fluid and which is exposed to the radiation emitted by the reactor core can be activated and contaminated after the reactor has been in operation for some time. In the case of nuclear power stations which have reached the end of their life and which require a complete shutdown, the solution adopted in the past has been to leave these power stations in their existing state and to allow the activity of the constituent materials of their components to decrease, in order subsequently to dismantle them under more satisfactory conditions than at the time of the shutdown, without the need to employ complex, remotely controlled equipment. The number of power stations put out of industrial operation will increase appreciably in the future, and it is therefore necessary to consider dismantling these power stations in order to restore the site where they are installed to its original state. The dismantling of the conventional part of the power station presents no particular problem, but, in contrast, the dismantling of the part of the power station constituting the actual nuclear reactor poses problems which are difficult to solve in view of the radioactive emissions of the constituent materials of the reactor components. In particular, the vessel of water-cooled nuclear reactors, which contains the fuel assemblies and which is in contact with the cooling water of the reactor during its operation, is activated and contaminated where reactors which have reached the end of their life are concerned. As regards pressurized-water nuclear reactors in operation at the present time, the reactor vessel takes the form of a body of generally cylindrical shape closed by domed bottoms, of large size and having a considerable wall thickness. The vessel, which has a very high mass, is arranged within a vessel well made in a concrete structure which also delimits one or more pools located above the upper level of the vessel. The vessel which contains not only the fuel assemblies but also various internal structures, is connected by means of connection pieces to pipelines of the primary circuit of the reactor. The core assemblies and some components of the internal structures can be dismantled and removed from the vessel, in order to ensure their disposal and, if appropriate, their elimination at the time when the reactor is put out of operation. Some components of the highly activated internal structures of the reactor, such as the shroud of the core, may need to be kept inside the vessel so as to be cut under water (radiological protection). Their dismantling has to be carried out within the vessel and during the operations of dismantling the vessel itself. To date, no process and apparatus is known which enables the vessel of a pressurized-water nuclear reactor to be dismantled under very good safety conditions without the risk of radioactive contamination in the work zone, while at the same time using machining and handling means of relatively simple design in order to carry out the fragmentary disposal and elimination of the material of the vessel. SUMMARY OF THE INVENTION The object of the invention is, therefore, to provide a process for dismantling an irradiated component of a nuclear reactor, comprising at least one wall of tubular shape arranged with its axis in the vertical direction and fastened inside a well made in a concrete structure, this process making it possible under very good safety conditions and in a simple way to carry out the fragmentation of the wall of the component and the disposal and elimination of the fragments obtained. To achieve this object: the connecting elements between the concrete structure and the component are destroyed, the component is displaced some distance in the vertical direction along its axis on the inside of the well and in successive steps, the wall of the component is cut over a height corresponding substantially to the vertical displacement distance, so as to obtain blocks of the irradiated material of the wall, at the upper level of the well of the concrete structure after each displacement of the component, the cut blocks are disposed of for the purpose of effecting their elimination or storage, and the cutting of the component is carried out in successive steps separated by a vertical displacement. Advantageously, and in order to increase the safety of the process, to carry out the displacement of the component in the vertical direction in successive steps: first means for raising the component are placed under a lower part of the component and so as to bear on a stationary support resting on the concrete structure of the reactor, in the vicinity of the bottom of the vessel well, the component is lifted by a push of the first raising means on the lower part of the component, a first modular supporting element is introduced between the lower part of the component and the stationary support on which the modular element comes to bear, the first raising means are actuated oppositely to the lifting direction, in order to bring the lower part of the component to bear on the first modular element, and for each of the subsequent successive displacement steps of the component: a unit lift of the component over a specific vertical distance is executed by second raising means bearing on the support and in engagement with a modular supporting element interposed between the component and the stationary support and resting on the stationary support before the unit lift of the component, a modular supporting element, the height of which is smaller than the vertical distance of unit lift of the component, is introduced between the modular element with which the second raising means interact and the stationary support, and the second raising means are actuated oppositely to the lifting direction, in order to bring the component to bear on the support by means of the superposed modular elements.