STEAM GENERATOR WRAPPER ACCOMMODATING TUBE SUPPORT MEMBERS OF HIGH THERMAL EXPANSION COEFFICIENT MATERIAL

Abstract: Slots (58) are provided in the wrapper (44) of a nuclear steam generator to increase the local flexibility of the wrapper (44) to allow for the difference in thermal expan­ sion between the carbon steel wrapper (44) and the high corrosion resistant alloy tube support plates (42). The slots preferably cut by a high speed industrial laser, are very thin and do not permit substantial commingling of cooler down­ comer water and the boiling mixture in the tube bundle re­ gion. The slots preferably have end portions which are curv­ ed away from one another to decrease stress concentra­ tions and increase fatigue life.

Description: STEAM GENERATOR WRAPPER ACCOMMODATING TUBE SUPPORT MEMBERS OF HIGH THERMAL EXPANSION COEFFICIENT MATERIAL This invention relates to heat exchangers and, more particularly, to a design which reduces thermally induced loads in the tube support plates and the tube bundle wrapper of a nuclear steam generator. A typical steam generator for a pressurized water nuclear reactor comprises a vertically oriented heavy wall rolled cylindrical shell, a thin wall round wrapper portion disposed within the shell portion and spaced therefrom, a plurality of U-shaped tubes disposed within the wrapper portion so as to form a tube bundle, a tube sheet for supporting the tubes at the ends opposite the bent sections thereof, a plurality of generally parallel tube support members longitudinally spaced within the wrapper portion and having a plurality of holes through which the tubes pass, and a plurality of individual radial support means angularly spaced about the wrapper portion and associated with the shell and wrapper for radially constraining the tube support members. Each radial support means includes a jacking block, which is welded into the wrapper, and jacking screws which are threaded through holes in the jacking block to align and support the wrapper within the shell, creating an annular space between the shell and wrapper. The jacking screws are then welded to the jacking block. The tube support members or plates are then positioned within brackets which provide longitudinal support. Wedges are driven between the jacking blocks and the side edges of the support plates while in the unheated shop condition and then welded, creating a structural load path between the support plates and the shell. The load path transmits dynamic loads, which may occur during steam generator handling and shipping or from seismic events, from the tube bundle to the shell, thereby inhibiting yielding of any of the tube bundle or tube support members. Nuclear steam generators are subject to a variety of corrosion mechanisms. At the intersections of the tube support members with the tubes, the corrodents present in the secondary side water tend to concentrate to the detriment of both the tubes and the tube support members. Use of a support member with improved corrosion resistance will also reduce tube corrosion. However, materials with higher corrosion resistance will generally contain higher amounts of nickel and chromium than carbon steel, resulting in higher coefficients of thermal expansion. As the steam generator is heated to operating temperature, the greater expansion of the corrosion resistant tube support members versus that of the carbon steel wrapper results in undesirable and usually unacceptable stress levels in the tube support members and in the wrapper. Such stresses could lead to local yielding. It is therefore the principal object of the present invention to provide a steam generator which can accommodate the difference in thermal expansion between the tube support members and the wrapper without inducing unacceptably high stresses in the steam generator components while maintaining a rigid radial structural load path between the tube support members and the shell. With this object in view, the present invention resides in a steam generator for a pressurized water nuclear reactor, comprising a cylindrical shell, a tubesheet extending across said shell and supporting a plurality of U-shaped tubes so as to form a tube bundle extending within said shell, a wrapper disposed within said shell around said tube bundle, a plurality of generally parallel tube support plates longitudinally spaced within said wrapper and having a plurality of apertures through which said tubes extend, and a plurality of individual radial supports associated with said shell and said wrapper for radially constraining said tube support plates, said wrapper consisting of steel having a predetermined coefficient of thermal expansion, and said tube support plates consisting of highly corrosion resistant alloy having a predetermined coefficient of thermal expansion which is greater than the coefficient of said thin wall wrapper means, characterized in that elongated expansion slots are provided in said wrapper proximate at least the majority of said radial supports to provide flexing capabilities for said wrapper during steam generator operation to compensate for the differences in thermal coefficients between said wrapper and said alloy tube support plates, said thin expansion slots being spaced from said radial supports to provide uninterrupted wrapper material immediately proximate said radial supports for structural protection during seismic occurrences. The invention will become more readily apparent from the description of a preferred embodiment shown by way of example only, in the accompanying drawings, in which: Figure 1 is a fragmentary elevational view, with parts broken away, of a steam generator for a PWR power plant; Figure 2 is a fragmentary elevational view with parts broken away taken along the line II-II in Figure 3; Figure 3 is a fragmentary elevational view of a preferred embodiment of the present invention; Figure 4 is a fragmentary cross-sectional view through a tube support means, wrapper, radial support means, and shell illustrating the relative positions of said components when the steam generator is in the cold, depressurized condition; ; Figure 5 is a fragmentary cross-sectional view corresponding to Fig. 4 but with the steam generator at operating temperature and pressure depicting the increased flexibility in the wrapper resulting from incorporation of the present improved design; Figure 6 is a fragmentary elevational view of another embodiment of the present invention; Figure 7 is a fragmentary elevational view of a further embodiment of the present invention; Figure 8 is a fragmentary elevational view of still another embodiment of the present invention; and Figure 9 is a fragmentary elevational view of yet another embodiment of the present invention. Referring to Figure 1, the steam generator 20 for a PWR has a generally cylindrical shell 22 which terminates below a horizontal tubesheet 24 in a hemispherical housing 26 having an inlet 28 and an outlet 30 for reactor coolant. Reactor coolant enters inlet 28, circulates through hundreds of U-shaped tubes 32 (only three shown for clarity), and is discharged through the outlet 30 in a manner wellknown in the art. Feedwater entering through feedwater inlet 34 distributes through feedwater ring 36, discharges through nozzles 38, and down through annulus 40 to just above the tubesheet 24. The water turns radially inward at the tubesheet, rises and circulates in the shell around the U-shaped tubes 32 which carry the hot reactor coolant and is converted to steam which rises into demisters (not shown) above the U-shaped tubes before being discharged for use in driving a turbine-generator combination. Within the shell 22 are tube support means 42 which preferably are formed as support plates. Surrounding the U-shaped tubes 32 and tube support plates is a cylindrical wrapper means 44 which ends just above the tube sheet 24 such that the mixture of feedwater and recirculating water is introduced within the wrapper into contact with the tubes. Collectively, these components are referred to as the internals. A direct load path between the wrapper 44 and the shell 22 for seismic forces is provided by radial support means 46, which are longitudinally positioned at the tube support means and angularly distributed around the gap or annulus 40 between the wrapper 44 and the shell 22. As depicted in Figure 2, the radial support means may be comprised of jacking blocks 48 welded into apertures 50 in the wrapper 44 and jacking screws 52 which are threaded into engagement with the shell 22 and welded in place. Wedges 54 may be driven between the support means 42 and wrapper 44 at the locations of the radial support means 46 to complete the structural load path between the tube support means and the shell 22. The wrapper 44 is preferably comprised of carbon steel having a typical mean expansion coefficient of 13 x 10 /OC in the temperature range of interest. Tube support means 42 is preferably comprised of material of higher corrosion resistance and will generally have a higher coefficient of thermal expansion than carbon steel, and example being Type 347 stainless steel which typically has mean expansion coefficient of 17.7 x 10 6 /OC in the temperature range of interest. Since the wedges 54 are driven between the tube support means 42 and the wrapper 44 while all parts are in the unheated condition, thermal interaction at operating temperatures between the higher coefficient of expansion tube support plates and the carbon steel wrapper results in undesirable and usually unacceptable stress levels in the support plates and wrapper. As depicted in Figure 3, very thin expansion slot means 58 are cut in the wrapper 44 according to this invention for increasing the local flexibility of the wrapper, thereby reducing the thermally induced stresses in the tube support means and wrapper while maintaining a structural load path between the tube support means and the shell 22, thereby preventing localized yielding of the wrapper 44. In the preferred form of the invention, expansion slots 58 approximately 0.9 mm wide in the shape depicted in Figure 3 are cut into the 0.95 cm thick wrapper 44 with a high-power industrial laser. As an example, the expansion slots 58 are vertically oriented and are cut into the wrapper 44 on either side of the jacking blocks 48 and within 2.54 cm therefrom and extend about 25.4 cm above and below the tube support means 42. The expansion slots may end in small curved portions 60 for improvement of fatigue life by reduction of stress concentrations at the ends of the slots. The expansion slots need not be made around each radial supporting means 46 but should be provided proximate at least two-thirds of the locations. As the steam generator is started up and the components become heated, the tube support means 42 expand more radially than can be accommodated by the lesser expanding circumference of the wrapper 44. As depicted in Figures 4 and 5, the expansion slots 58 permit flexing of the portions of the wrapper 44 surrounding the jacking block 48 between the slots with respect to the remainder of the wrapper while retaining sufficient strength in the wrapper for support of the tube support means 42. Pressure and thermal expansion cause the shell 22 to expand enough that little or no stress is imparted on the shell by the jacking screws 52 and the expanding tube support means 42. Due to the very small width of the slot achievable with a laser beam cutter, commingling of cold and heated feedwater by flow of water from the gap or annulus 40 through the slots in the wrapper is minimal. The expansion slots preferably extend through the full width of the wrapper. The slots can be cut at various angles through the wrapper with the preferred embodiment specifying slots cut parallel to the radius between the wrapper center to the center of the radial support device, such angle being observable in Figures 4 and 5. The expansion slots may be made narrower by machining the wrapper at the proposed slot sites before cutting. In another embodiment of the invention, which is shown in Figure 6, the expansion slots 62 are curved to more closely follow the contour of the jacking blocks 48, increasing local flexibility. Flexibility can be increased while maintaining support strength by using multiple slots as illustrated in Figure 7. Such an embodiment incorporates elongated curved slot portions 64 which are generally vertically disposed and short curved slot members 66 above and below the jacking blocks 48. Curved ends (not shown) can be employed to reduce stress concentrations, increasing fatigue life. Another embodiment, depicted in Figure 8, employs circumferential slots 68 rather than longitudinal slots. The slots may end in small curved portions 60 for fatigue purposes. Figure 9 represents still another embodiment with one continuous slot 70 around two or three sides of the radial support means 46. Small end radii (not shown) may be employed as previously described. While a high-power laser is preferred for machining the elongated and narrow slot members as described hereinbefore, it should be understood that other machining techniques could be substituted therefor.

Claims: CLAIMS: 1. A steam generator for a pressurized water nuclear reactor, comprising a cylindrical shell (22), a tubesheet extending across said shell (22) and supporting a plurality of U-shaped tubes (32) so as to form a tube bundle extending within said shell (22), a wrapper (44) disposed within said shell (22) around said tube bundle (32), a plurality of generally parallel tube support plates (42) longitudinally spaced within said wrapper (44) and having a plurality of apertures through which said tubes extend, and a plurality of individual radial supports (48) associated with said shell (22) and said wrapper (44) for radially constraining said tube support plates (42), said wrapper (44) consisting of steel having a predetermined coefficient of thermal expansion, and said tube support plates consisting of highly corrosion resistant alloy having a predetermined coefficient of thermal expansion which is greater than the coefficient of said thin wall wrapper means, characterized in that elongated expansion slots (58) are provided in said wrapper (44) proximate at least the majority of said radial supports (48) to provide flexing capabilities for said wrapper (44) during steam generator operation to compensate for the differences in thermal expansion between said wrapper (44) and said alloy tube support plates (42), said thin expansion slots (58) being spaced from said radial supports (48) to provide uninterrupted wrapper material immediately proximate said radial supports (48) for structural protection during seismic occurrences. 2. A steam generator according to claim 1, characterized in that said expansion slots (58) are machined in the wrapper by means of a high powered laser cutting device. 3. A steam generator according to claim 1 or 2, characterized in that said expansion slots (58) are provided at opposite sides of said radial supports (48). 4. A steam generator according to claim 1, 2 or 3, characterized in that said expansion slots (58) in said wrapper (44) are such that they extend substantially perpendicularly to said tube support plates (42) and above and below the wrapper intersection of said tube support plates (42) and said radial supports (48). 5. A steam generator according to any of claims 1 to 4, characterized in that the expansion slots (58) are provided with end portions curved away from one another to decrease stress concentrations, thereby increasing fatigue life.

Inventors: WEPPER, ROBERT MACKNESS
Applicants: WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1986
Main classification: F22B37
IPC codes: F28D7, F22B37, F22B1, F28F9