1. Field of the Invention
The present invention relates in general to primary divider plates for recirculating steam generators, and in particular to a method for replacement of a primary divider plate and a new floating one-piece curved replacement divider plate.
2. Description of the Related Art
In nuclear power stations, steam generators, such as recirculating steam generators and once-through steam generators, are used for heat exchange purposes in the generation of steam to drive the turbines. This application relates to recirculating steam generators. Primary fluid which is heated by the core of the nuclear reactor passes through the primary head (10) as shown in FIG. 1 through a bundle of U-shaped tubes (4) in a recirculating steam generator (2). In the construction of a steam generator, the primary head (10) is welded to the tubesheet (8) of a recirculating steam generator. In the United States, the term "primary head" is often referred to as "channel cover". Secondary fluid, normally water, which is fed into the space surrounding the tubes receives heat from the tubes and is converted into steam for driving the turbines. After cooling and condensation has occurred, the secondary fluid is directed back into the space around the tubes to provide a continuous steam generation cycle.
In the primary head (10) of the steam generator (2) there is normally one or two inlets (12) and one or two outlets (16) for the primary fluid with a divider plate (14) separating the inlet (12) from the outlet (16) to direct the flow of the primary fluid up through one side of the U-shaped tubes where the primary fluid rises in the tubes and turns in the U-shaped tubes and then exits from the outlet (16). The primary fluid is pumped through the system to return to the reactor where it is heated once again. The primary divider plate (14) provides a means for directing the flow of the primary fluid through the tubes (4) of the steam generator (2) as well as separates the heated inlet flow of the primary fluid in the primary head (10) from the "cooled" outlet flow.
In Canada, the steam generators referred to as CANDU steam generators have tighter space constraints than those employed in the United States. The CANDU steam generators usually employ heavy water (D.sub.2 O) as the primary fluid. In the United States, the primary fluid is water. The primary fluid causes erosion of the divider plate. Erosion damage to the primary divider plate has generated concerns over loose parts potential and thermal performance degradation of the steam generator.
FIG. 2 shows in a frontal sectional view a segmented bolted primary divider plate. The divider plate is positioned in approximately the middle of the primary head and acts as a full partition or a baffle. The individual segments or plates (18) that make up the primary divider plate are inserted through a manway, which is an opening to provide access to the primary head with the opening being approximately 14 inches.times.18 inches. The segments (18) are fastened to each other usually with fasteners (19), for example, by being bolted together. The segments (18) are positioned on tubesheet and primary head seat bars and clamped together with a clamping dog also referred to as a clamping block (20). Experience has shown that the primary fluid erodes the fasteners as well as the area around them and can cause loose pieces to fall off. This can cause damage to the primary heat transport pump and line and can present problems in a loss of cooling accident (LOCA) by shedding loose parts due to the shock wave which results during this type of accident. A rapid change in pressure of the fluid forms the shock wave which can cause the divider plate to break apart and cause damage in the primary head and system downstream.
Another primary divider plate design that has been used in Canada is the one-piece floating welded divider plate (22) shown in cross sectional view in FIG. 3. The one-piece floating welded divider plate (22) includes a main portion of the divider plate (24) which is positioned in the primary head (10) on a seat bar (26) during fabrication and prior to welding the primary head (10) to the tubesheet (8). The one-piece floating welded divider plate (22) has a flat construction and is supported at its edges by the primary head seat bar (26) which is a tongue-in-groove joint received in the groove (24a) at one end and by a channel (28) in the tubesheet (8), or alternatively, a tubesheet seat bar (27) at the other end. The tubesheet seat bar (27) is of a similar construction as the primary head seat bar (26). The one-piece floating welded divider plate is installed in the fabrication process of new steam generators by positioning the main portion (24) of the divider plate into the primary head (10) onto the seat bar (26) prior to welding the primary head to the tubesheet (8) . After the primary head (10) is welded to the tubesheet (8), the thinner upper portion (30) is placed in position by way of three pieces through a manway. There is a long center piece and two small corner pieces. These pieces are placed into position in channel (28) with the thinner upper portion (30) having still even a thinner portion (30a) to facilitate installation in channel (28). A shim (32) is employed in the channel (28) to firmly hold the upper portion (30) in place. The upper portion (30) is then strength welded to the lower portion (24) with a full penetration weld (34) for strength across the entire length. A space or gap (36) is left at each end of the divider plate (22) to provide a floating design which compensates for the thermal and pressure induced dilation of the primary head (10) and the tubesheet (8). The primary head (10) dilates due to thermal and pressure effects while the tubesheet (8) has a tendency to bulge in the middle due to pressure. The flat construction one-piece floating welded divider plate (22) is designed for the bending moment which has been found to be less away from mid-span. This explains why the lower portion (24) is approximately 50 millimeters (mm) thick while the upper portion (30) is approximately 32 mm thick with the materials being made from carbon steel. The problems of erosion still affect the one-piece floating welded divider plate. Also, the primary divider plate (22) shown in FIG. 3 is not suitable for retrofit or replacement when it becomes necessary to replace the primary divider plate due to erosion. Erosion is less of a concern with a single piece design because there is much less leak path length than for the segmented design. One of the advantages of a single piece design is that there are no loose parts which can cause damage.
There is still a need for a primary divider plate which is suitable for retrofit in existing steam generators. Preferably, this retrofitable primary divider plate would be a floating design and be constructed to reduce the amount of material and welding. Reducing field welding is important in order to minimize radiation exposure to personnel and to minimize distortion due to field welding. Also, the replacement primary divider plate would have an increase in the load carrying capacity with an improved rim to limit the amount that the boundary moves and have no loose parts.