Patent Number: 051924927
Section: summary

FIELD OF THE INVENTION The invention relates to a method and apparatus for sealing communicating co-axial tubes containing a fluid so that the fluid level in one of the tubes can be higher than the fluid level in the other of the tubes and will permit operations intermediate the tubes without being encumbered by fluid intermediate the tubes. The invention relates particularly to a method and apparatus for permitting repair operations between the inner and outer shells of a steam generator which is supplied with a heated fluid from a nuclear reactor vessel. BACKGROUND OF THE INVENTION Known types of steam generators supplied with heated fluid from a nuclear reactor vessel have an outer shell or tube and an inner shell or tube with heat exchanger tubes within the inner shell for circulating the heated fluid within the steam generator. A liquid, such as water, is contained within the shells, which are interconnected for liquid flow, so that the upper level of the liquid is the same both in the inner shell and the outer shell and covers the heat exchanger tubes during operation of the steam generator. At times, it can be necessary to perform repair operations within the space intermediate the inner and outer shells, e.g. repair of the girth weld in the outer shell which is adjacent the upper level of at least one of said heat exchanger tubes. It would appear to be a simple matter merely to suspend operation of the steam generator and the reactor and lower the fluid water level in the generator to permit such repairs, but radiation is still omitted from inside the inner shell to which workmen in the space between the inner and outer shells are exposed which is undesirable. In the past, under such conditions, lead sheathing has been applied to the outer surface of the inner shell to reduce such exposure. However, the application of such sheathing is time consuming, resulting in high radiation exposure of the workmen, and is not as effective as keeping the tubes immersed in water within the inner shell. Since the spaces within the shells are interconnected for water flow, the water level between the shells ordinarily would be the same as the water level within the inner shell so that merely raising, or permitting, the water level within the inner shell to be high enough to provide the desired shielding would be unsatisfactory because the areas on which repairs are to be performed would be immersed in the water. The problem then arises as to how to maintain a water upper level between the shells which is low enough to permit such repairs while permitting a water level within the inner shell which is higher than the water level between the shells and which will cause said heat exchanger tubes to be immersed. With steam generators of the type which have been installed in pressurized water, reactor power plants, the steam generator cannot, as a practical matter, be opened up to provide easy access to the space between the shells where repairs are to be made. Access to such space is normally limited to relatively small access openings in the outer shell which are above such space. SUMMARY OF THE INVENTION In accordance with the preferred embodiment of the invention, the water level within the inner shell, and hence, in the space between the inner and outer shell, is adjusted so that the water covers said heat exchanger tubes within the inner shell, yet is at a small distance below the area where the repairs are to be made so that during the installation steps described hereinafter, which can be accomplished within a relatively short time, the workers are protected by the water within the inner shell as well as the water in the space between the shells. In this case, desirably, the workmen wear wet suits because the subsequent steps require that parts of the bodies of the workmen be immersed in the water between the inner and outer shells. However, if desired, with less safety, the water level in the inner shell may be temporarily reduced below the installation positions for the equipment hereinafter described, and after such equipment is installed, the water level within the inner shell can be raised to a level at which the heated fluid tubes within the inner shell are totally immersed in water. In the area of concern, the outer shell and the lower part of the inner shell usually have the shape of truncated cones with the lower ends having the smaller diameter. At its upper part, the inner shell can have the shape of a cylindrical annulus. Thus, the shells are nearer each other at the lower ends of the truncated cones. A resilient, flexible tube of a diameter which will fill the space between the inner and outer shells at the lower ends of the truncated cones is inserted between the inner and outer shells to provide a support for workmen and to restrain debris and dropped tools. Thereafter, a plurality of arcuate, metal, seal support sections of a size which can be passed through the access openings in the outer shell are inserted between the shells above the flexible tube and below the level where the workmen will stand while making repairs and are clamped against the surfaces of the inner and outer shells to provide a seal support deck encircling the inner shell. Then, two sections of uninflated, but inflatable tubing, the combined length of the sections being substantially equal to the peripheral size of the inner shell, are passed through an access opening and applied over the seal support sections with flexible sealing blocks between the adjacent ends thereof. After the inflatable sealing tubes are installed, a plurality of arcuate, metal, seal retention sections which can be passed through said access openings are installed over the sealing tubes, clamped together and, by means of jack screws engaging the outer surface of the inner shell, positioned within the space between the shells to provide a ring of sections encircling the inner shell. The inflatable tube sections are then inflated, e.g. by air under pressure, to provide a water-tight seal between the inner and outer shells. Preferably, after the tube sections are inflated, but optionally, before they are inflated, a plurality of arcuate metal sections which can pass through an access opening are applied on top of the seal retention sections to provide a work platform encircling the inner shell on which workmen can stand during the making of repairs. If water is between the shells and above the work platform after the steps described, preferably, such water is pumped out from between the shells so that the space between the shells and above the work platform is free of water. However, because of the seal provided by the sealing tube sections, the water level within the inner shell will not be lowered by the pumping operation.