The steam generators of pressurized water nuclear reactors comprise a bundle of parallel tubes whose cross-section is substantially circular. These tubes are arranged in a regular pattern in the cross-sections of the bundle and are fastened at their ends in a thick tube plate, circular in shape.
The tubes of the bundle in which pressurized water which constitutes the reactor coolant circulates are bent into a U in their upper part, so as to form two parallel arms whose ends are crimped by roller-expanding into the tube plate. The tubes pass through the entire thickness of the tube plate, being flush with one of the faces of this plate which forms the entry face, and projecting out in relation to the other face, or exit face, to enter the region of the steam generator which receives the feed water. The part of the steam generator which is situated below the entry face receiving the pressurized water which is the reactor primary fluid, forms a water box of hemispherical shape, connected to the tube plate and separated by a transverse partition bounding a delivery compartment for the pressurized water arriving from the reactor vessel and a recovery compartment for the pressurized water which has been circulated in the tubes.
In operation, the tubes of the bundle of the steam generator are subjected to high mechanical and thermal stresses and to the corrosive action of the primary fluid on their inner surface and of the feed water on their outer surface. Corrosion is particularly severe in the region of the tubes which is situated slightly above the exit face of the tube plate, which corresponds to the transition zone between the part which is distorted during the roller-expanding and the undistorted part of the tubes. Thus, cracks appear fairly frequently in this transition zone. The appearance of such cracks causes the entry of primary fluid, carrying radioactive products, into the feed water of the steam generator. The steam produced from this feed water is conveyed into the turbine of the nuclear reactor and circulates in many components situated outside the safety enclosure of the reactor. The steam produced may also be dumped into the atmosphere during certain stages of operation of the reactor.
It is important, for all these reasons, that the feed water should not contain any radioactive product. Any leakage through the wall of a steam generator tube must therefore be detected and remedied very promptly.
During the periods of normal stoppage for reactor maintenance, or during incidental stoppages, it is known to arrange for the defective tubes of the steam generators of the nuclear reactor to be put out of service. It is known, in particular, to insert a plug into at least one end of the defective tube and to perform the expansion of this plug to prohibit any circulation of pressurized water in the tube. The chief disadvantage of this kind of process of plugging the tubes of a steam generator is that it reduces the heat exchange surface of the steam generator in proportion to the number of tubes put out of service. In the case of a steam generator comprising a large number of defective tubes, the remaining exchange surface may become inadequate.
To extend the lifetime of steam generators, it has been proposed, therefore, to perform a lining of the part of the tubes in which cracks are present, by inserting, through the end of the tube which is flush with the tube plate, a liner which is then expanded inside the tube, near its ends. To ensure a leakproof connection between the liner and the tube to be repaired, the diametral expansion of this liner is followed by an operation of crimping, by roller-expanding the liner in its expanded regions. This operation of roller-expanding may be supplemented or replaced by a welding or brazing operation.
The lining of a steam generator tube is performed from the water box, which is accessible via a manhole, either manually or by using a complex, automatically controlled appliance.
In all cases, tube repairs which rely on lining are restricted to the middle region of the tube plate by the fact that it is necessary to use very long liners which cannot be inserted into the peripheral tubes because of the hemispherical shape of the water box which allows only a clearance which is inadequate for tubes to be inserted in the vicinity of the edges of the tube plates. Liners whose length is appreciably greater than the thickness of the tube plate must, in fact, be used, since the lower end of the liner must be placed in the tube, in its region which is flush with the entry face, and since its upper end must be situated above the transition zone in which the cracks generally appear.
In the case of steam generators of the nuclear reactors which are at present in service, the tube plate has a thickness of more than 0.50 meter and the lining liners employed have a length in the region of 0.70 m. This considerable length of the liner prevents use of the lining method in the case of the tubes situated in the peripheral rows, which represent approximately 25% of the bundle of tubes. Consequently, tube plugging remains the only method which can be used in the case of the peripheral tubes, whereas it would be desirable to have the possibility of repairing these tubes by lining, to avoid reducing the exchange surface of the steam generator, after a long period of use during which many peripheral tubes have become defective.