Patent Number: 053902190
Section: summary

FIELD OF THE INVENTION The invention relates to steam generators used in nuclear installations for electric power generation, particularly those using one or more pressurized water reactors. The invention more particularly relates to a device placed in the secondary circuit of the steam generator. Prior Art In a nuclear power station, in which the power supplying means is constituted by a pressurized water nuclear reactor, a primary circuit is used for removing the energy produced by the reactor. Pressurized water is enclosed within the the primary circuit. Use is made of a heat exchanger, such as a steam generator, for utilizing the thermal energy extracted by the primary circuit from the nuclear reactor. Such a steam generator is shown in FIG. 1. The primary circuit has an inlet 1 and an outlet 2 located at the lower end of the steam generator. The pressurized water flowing in the primary circuit traverses, within the steam generator, a tube bundle 3, in the lower part of the steam generator. The inlet for the water for the secondary circuit 4 is lateral and is positioned in the upper part. The outlet 5 of the secondary circuit is located in the upper part of the steam generator. After entering through the inlet of the secondary circuit 4, the water from the latter is distributed by a supply ring 6 and is piped between the pressure envelope 7 of the steam generator and the envelope 8 of the tube bundle 3. It then rises along the latter and is vaporized by contact with it. On reaching the upper end 9 thereof, it is collected by the upper part 10 of the envelope 7 and by discharge pipes 12 equipped with centrifugal separators for separating the water from the steam. The steam is then discharged through the outlet 5 after passing through one or more dryers 13, which complete the separation of the water from the steam. The water in saturated form separated in this way from the steam is known as recirculated water and is piped via the pipe or pipes 15 before being mixed with the water from the supply ring 6. Bearing in mind the structure of that part of the secondary circuit traversing the steam generator, it is readily apparent that relatively small objects, enclosed within the secondary circuit may follow the path of the latter within the steam generator and may thus be jammed or wedged in the narrow passages. constituted by the spaces separating the tubes of the tube bundle 3. Bodies foreign to the steam generator and having different shapes, sizes and forms, referred to as migrating bodies, can be accidentally introduced into the steam generator via the secondary water supply system, or during maintenance operations. During the operation of the steam generator, these bodies may be carried along by the secondary fluid and then become fixed at different points of the steam generator, such as the tube plate 31, or enter the tubes of the bundle 3. These objects can damage the walls of the tubes by impact or friction. SUMMARY OF THE INVENTION It is an object of the invention is to limit the size and number of migrating bodies coming from the secondary water supply system and the upper part of the steam generator and which are liable to reach the tubular plate and damage the bundle of tubes. The subject of the invention is a device for trapping migrating bodies within the secondary circuit of a steam generator of a nuclear power generation installation, constituted by a plurality of grids or equivalent mechanical filtering systems which, for optimum efficiency, occupy the entire annular section of the secondary circuit, downstream of the water supply tubes for the secondary circuit and upstream of the introduction of the water into the bundle of tubes of the primary circuit. When the tubes of the bundle of tubes are spaced by a given distance, the efficiency of the device according to the invention increases if the grids have meshes whose maximum width is smaller than such distance, so as to not allow the passage of objects or bodies liable to jam between the tubes. In the main construction of the device according to the invention, the grids or the like are constituted by circular sectors fitted on two radial brackets common to two adjacent sectors and welded to the envelope of the tube bundle. In this case, the grids are preferably screwed to the brackets so as to be dismantlable. In the preferred embodiment of the device according to the invention, the width of the grids is slightly less than the width of the annular section of the secondary circuit at the point where the grids are installed so as to leave a clearance of a few millimeters, thus permitting expansions between the pressure envelope and the tube bundle envelope of the steam generator. In this case, first outer strips can be placed on the inner wall of the steam generator pressure envelope in the annular section in order to fill the external circular clearance between the grids and the pressure envelope. Second circular strips fixed on the outer periphery of the grids make it possible to avoid the reintroduction into the space between the pressure envelope and the tube bundle envelope of objects trapped on the grids. In the same way, inner strips can be placed between the grids and the outer wall of the tube bundle envelope. In order to be able to withstand the possible impacts of migrating bodies, the various hydraulic stresses and the weight of operators, the grids have a thickness of a few centimeters, so as to have an adequate mechanical strength.