Patent Number: 062333003
Section: description

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 is a sectional view with parts cut away of a nuclear reactor pressure vessel (RPV) 10. RPV 10 includes a generally cylindrical side wall 12. A core shroud 14 of generally cylindrical shape is located within RPV 10 and surrounds the reactor core (not shown). Shroud 14 is supported by a shroud support structure 16. A core plate 18 is spaced below a top guide 20 within RPV 10. Core plate 18 and top guide 20 are coupled to shroud 14. Shroud 14 includes a shroud head 22 coupled to top guide 20. Particularly, top guide 20 includes a flange 24 and shroud head 22 includes a flange 26 configured to engage top guide flange 24. More particularly, top guide flange 24 engages shroud head flange 26 to form a shroud head to top guide interface 28. RPV 10 is shown in FIG. 1 as being shut down with many components removed. For example, and in operation, many fuel bundles and control rods (not shown) are located in the area between top guide 20 and core plate 18. In addition, and in operation, steam dryers and many other components (not shown) are located in the area above top guide 20. Also, steam separators 30 are permanently coupled to shroud head 22. Top guide 20 is a latticed structure including a plurality of top guide beams 32 defining top guide openings 34. Core plate 18 includes a plurality of openings 36 which are substantially aligned with top guide openings 34 to facilitate positioning the fuel bundles between top guide 20 and core plate 18. Fuel bundles are inserted into the area between top guide 20 and core plate 18 by utilizing top guide openings 34 and core plate openings 36. Particularly, four fuel bundles are inserted through a top guide opening 34, and are supported horizontally by an orificed fuel support (not shown) inserted in core plate opening 36, core plate 18, and top guide beams 32. Shroud 14, core plate 18, and top guide 20 limit lateral movement of the core fuel bundles. FIG. 2 is an enlarged exploded view of section A of shroud head to top guide interface 28 shown in FIG. 1. Interface 28 is formed by a top surface 38 of top guide flange 24 and a bottom surface 40 of shroud head flange 26. Top guide flange 24 includes a plurality of frusto-conical shaped guide pins 42 extending from top surface 38. Guide pins 42 are located around the circumference of top guide flange 24. Shroud head flange 26 includes a plurality of corresponding guide pin openings 44 extending from shroud head flange bottom surface 40 through shroud head flange 26. Each guide pin opening 44 is configured to receive a frusto-conical guide pin 42. FIGS. 3, 4, and 5 are enlarged exploded sectional views of section B of the shroud head to top guide interface shown in FIG. 2 and illustrate various positions of shroud head flange 26 in relation to top guide flange 24 during assembly. As described above, shroud head flange 26 includes a plurality of guide pin openings 44 configured to align with guide pins 42 located on top guide flange 24. Each guide pin opening 44 includes a frusto-conical portion 46, defined by an inside surface 48 of shroud head flange 26, that extends through shroud head flange 26 from bottom surface 40 and has a slope equal to the slope of frusto-conical guide pins 42. Each guide pin opening 44 also includes a cylindrical portion 50 that extends from the small base 52 of frusto-conical portion 46 of guide pin opening 44 to a top surface 54 of shroud head flange 26. The diameter of frusto-conical guide pin opening 44 at bottom surface 40 of shroud head flange 26 is configured to be larger than the diameter of frusto-conical guide pin 42 immediately adjacent top surface 38 of top guide flange 24. Because of the frusto-conical shape, guide pins 42 include a first base 56 and a second base 58, with first base 56 having a larger diameter than second base 58. Second base 58 is located immediately adjacent top surface 38 of top flange 24. During assembly, shroud head 22 is suspended from an overhead crane and lowered into engagement with top guide flange 24. Particularly, shroud head 22 is lowered so that each frusto-conical guide pin 42 extending from top guide flange 24 aligns with a corresponding guide pin opening 44 in shroud head flange 26. Shroud head 22 is lowered until bottom surface 40 of shroud head flange 26 is in surface to surface contact with top surface 38 of top guide flange 24. The conical shape of guide pins 42 and the conical shape of guide pin openings 44 provide greater clearance between guide pin 42 and guide pin opening 44 as shroud head flange 26 and top guide flange 24 approach engagement than the clearance when the guide pins are cylindrically shaped. In the assembled condition, a distance C between guide pin 42 and guide pin opening 44 is less than about 1.0 millimeters (see FIG. 5). At a position where the distance between the two flanges 24 and 26 is equal to the height of guide pins 44, distance C between each guide pin 42 and each guide pin opening 44 is about 4.0 millimeters when a cone angle D of guide pin 42 is 60 degrees (see FIG. 3). Cone angle D is measured in reference to first base 56 of guide pin 42. Of course, distance C is dependent on the value of cone angle D and the relative position of shroud head flange 26 and top guide flange 24. For example, at an intermediate position shown in FIG. 4, distance C is about 2.3 millimeters for a cone angle D of 60 degrees. Cone angle D may vary over a wide range, for example from about 20 to about 80 degrees. Preferably, cone angle D is about 40 to about 75 degrees, more preferably about 55 to about 65 degrees. If cone angle D is too high the conical shape of guide pin 42 approaches that of a cylinder and may not over come the inherent alignment problems of a cylindrical guide pin. If cone angle D is too low, guide pin 42 may not provide sufficient restriction of horizontal movement during a seismic event. The above described top guide to shroud head interface 28 includes frusto-conical guide pins 42 and guide pin openings 44 that provide for suitable clearances between guide pins 42 and guide pin openings 44 to accommodate flexing of shroud head flange 26 during installation. Additionally, frusto-conical guide pins 42 and guide pin openings 44 provide less than 1.0 millimeter of clearance in the installed position to minimize the impact loading on guide pins 42 and openings 44 caused by horizontal seismic accelerations during a seismic event. In an alternate embodiment, guide pin openings 44 do not extend through shroud head flange 26. In this embodiment, each guide pin opening 44 includes frusto-conical portion 46 configured to receive a guide pin 42, but does not include cylindrical portion 50. In another embodiment, guide pins 42 extend from shroud head flange 26 instead of top guide flange 24. In this embodiment, corresponding guide pin openings 44 are located in top guide flange 34. In still another embodiment, some guide pins 42 extend from top guide flange 24 and some guide pins 42 extend from shroud head flange 26. Additionally, each guide pin 42 has a corresponding guide pin opening 44 located in the opposing flange. Particularly, each guide pin 42 extending from top guide flange 26 has a corresponding guide pin opening located in shroud head flange 26, and each guide pin 42 extending from shroud head flange 26 has a corresponding guide pin opening 44 located in top guide 24. From the preceding description of various embodiments of the present invention, it is evident that the objects of the invention are attained. Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is intended by way of illustration and example only and is not to be taken by way of limitation. Accordingly, the spirit and scope of the invention are to be limited only by the terms of the appended claims.