Patent Number: 054328284
Section: summary

The invention relates to a method of replacing an adaptor for penetrating the vessel head of a nuclear reactor cooled by pressurized water. Pressurized-water nuclear reactors generally include a vessel enclosing the core of the reactor which is submerged in pressurized water for cooling the reactor. The reactor vessel of cylindrical general shape includes a head of hemispherical shape which can be attached to its upper portion. The head is pierced with openings at each of which is fixed a tubular penetration part constituting an adaptor providing the passage and the actuation mechanism for moving a follower of a rod for controlling the reactivity of the core or a penetration passage for a means of measurement inside the core, such as a thermocouple column assembly. The tubular part is generally shrink-fitted inside the opening of the head and is fixed in a sealed manner by means of a weld, deposited in a bevel machined on the concave inner surface of the vessel head, around the penetration opening for the adaptor. To the end portion of each of the adaptors is fixed a mechanism for actuating the movement of a rod for controlling the reactivity of the core or a means for supporting a thermocouple column assembly. Inside each of the adaptors for penetrating the vessel head providing the passage of a follower of a reactivity control rod is fixed, in a position coaxial in relation to the tubular adaptor and with a certain radial clearance, a thermal sleeve which includes a diametrically widened portion resting on a bearing surface located at the upper portion of the bore of the adaptor. In order to increase the operational reliability and safety of nuclear reactors and to extend the lifetime of these reactors, the operators are obliged to carry out an increasing number of inspections on the various elements constituting the nuclear reactor. For example, the state of the parts for penetrating the vessel head is inspected in order to be sure of the integrity of these parts, after a certain time of operating the reactor, in particular in the area where these tubular parts are welded to the vessel head. These inspections and repairs are carried out during a shutdown of the nuclear reactor, the vessel head being dismounted and placed on an inspection and repair stand. The head is placed on the inspection and repair stand in a position corresponding to its surface position, the convex external surface of the head facing upwards and the concave internal surface facing downwards. The presence of defects in the adaptors, which may, in some cases, be constituted by cracks, can also be detected during hot pressure testing of the nuclear-reactor vessel. In some exceptional cases, defects constituted by cracks penetrating the wall of the adaptor may be present. In the case where defects are detected, it is necessary to carry out a repair, on the inspection and repair stand, before replacing the head in the service position on the nuclear-reactor vessel. In some cases, in particular in the very rare case where a crack of an adaptor would be penetrating, it is necessary either to replace the vessel head completely or to replace the defective adaptor by a new adaptor. Obviously the complete replacement of the head is an extremely expensive operation, one which requires having a complete head in reserve on the nuclear-reactor site. The operation of replacing a defective adaptor by a new adaptor, which requires removal by machining of the lower portion of the adaptor to be replaced and of the weld for fixing the head, as well as welding of the replacement adaptor, is lengthy and tricky to perform, in particular on account of the fact that the internal surface of the head is highly radioactive. Such a repair, which requires the installation of a protective structure, can turn out to be lengthy and expensive. The object of the invention is therefore to provide a method of replacing an adaptor for penetrating the vessel head of a nuclear reactor, of rounded shape, disposed, after dismounting, so as to have a convex outer surface facing upwards and a concave inner surface facing downwards, each of the adaptors having a vertical disposition and being fixed in a penetration opening in the head by means of a weld in the vicinity of the concave inner surface and carrying an item of equipment resting on one end of the adaptor, this end projecting outside the head, the method consisting in machining a lower portion of the adaptor to be replaced, in the vicinity of the inner surface of the head, and at least one portion of the weld for fixing the adaptor, in extracting the adaptor to be replaced from the opening in the head, in inserting a replacement adaptor into the opening and fixing it therein, and in welding the replacement adaptor to the head, this method being able to be implemented in a simple manner and enabling the cost of the replacement operation to be reduced. With this object in mind, the replacement is performed from a repair area located above the convex face of the head and at least one of the operations of machining the adaptor to be replaced after dismounting the item of equipment resting on its upper end and/or of welding the replacement adaptor is carried out by inserting an appropriate tool via the upper end of the adaptor.