As illustrated in FR-A-2 333 200, a steam generator equipping a nuclear power station conventionally comprises a vertically axed, outer envelope, whereof the inner space is subdivided into two portions, by a horizontal plate known as a tube plate or sheet. The ends of the tubes of a bundle of inverted U-tubes are fixed to the tube plate and issue below the same respectively into an admission collector or manifold and into a discharge collector or manifold for the water flowing in the primary circuit of the reactor, known as primary water. The water flowing in the secondary circuit of the steam generator, known as secondary water and is a mixture of so-called feed water coming from outside the steam generator and so-called recirculated water from the upper part of the steam generator. The feed water is injected into that part of the steam generator located above the tube plate by a toroidal or semitoroidal supply distributor. The supply distributor is normally placed above an annular recirculation space formed between the outer envelope and an inner envelope covering the tube bundle and having a lower edge is spaced from the tube plate.
In such a steam generator, it is essential to prevent to the greatest possible extent the stagnation of secondary water in certain areas adjacent to the tube plate. Thus, the water may contain mud or slurry, whose deposition on the tube plate around the tubes of the bundle can lead to rapid corrosion of the tubes necessitating their sealing. When an excessive number of steam generator tubes becomes inoperative, it is necessary to replace the entire steam generator. This is obviously undesirable from the standpoint both of cost and the resulting nuclear power station shutdown time. In order to limit these secondary water stagnation areas in the vicinity of the tube plate, distribution plate having an opening in its central portion is positioned in the lower part of the inner envelope and surrounding the tube bundle.
Another problem which arises in the area of the steam generator located immediately above the tube plate relates to the homogenization or equalization of the temperature of the mixture between the feedwater introduced into the generator by its supply manifold and the recirculation water from the water/steam separators located in the upper part of the generator and which drops again through the annular recirculation space. It is also desirable to ensure a good regulation of the flow rates of the mixture before it rises in the inner envelope around the tubes of the bundle.
This problem is generally solved by placing on the tube plate in the space normally known as a waterway and which is formed between said plate, the flow distribution plate and the hot and cold branches of the tubes of the tube lane blocking device.
Moreover, it is also desirable to have a purge immediately above the tube plate, so as to allow removal or sampling of the secondary fluid in this area and to carry out purging mainly in the areas where the secondary fluid flow is at a lower speed, particularly in the center of the tube plate.
In order to fulfil this function, in the waterway there are generally two pipes having holes in the lower part and connected to a purging circuit positioned outside the steam generator. This purging circuit makes it possible to check the chemical concentrations of the secondary fluid, so as to maintain them at acceptable values. IL also makes it possible to sample suspended materials contained in the secondary fluid in the vicinity of the tube plate.
U.S. Pat. No. 4,664,178 describes a steam generator equipped with a purging pipe in the vicinity of the tube plate and having L-shaped deflecting means above the purging pipe, at the base of the inner envelope covering the tube bundle.
Although prior art deflection and purging devices operate in a satisfactory manner , they suffer from the disadvantage of requiring the addition within the steam generator of a relatively large number of separate parts, which complicates the assembly of the generator, increases its price and makes maintenance more difficult.