Patent Number: 046438688
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, a portion of the reactor core is shown which includes a pressure vessel head 10, a control drive mechanism (CDM) 12, a shroud 14, and a fuel assembly 16. A grid plate is indicated in dashed lines at 18. These components are, of course, conventional and the purpose of FIG. 1 is to illustrate the location of the module support arrangement of the invention, which is generally denoted 20, in relation to these components. As illustrated, the support arrangement 20 penetrates vessel head 10 and extends between CDM 12 and shroud 14. Referring to FIG. 2, which is a detail of the support arrangement 20 with the installation tool assembly in place, the support arrangement 20 basically comprises a cell support nut 22, which is generally cylindrical in shape and only one half of which is shown in FIG. 2, including an upper flange portion 24 having a downwardly facing bearing surface 24a that engages a corresponding upwardly facing bearing surface 10a of pressure vessel head 10 and a screw threaded portion 26 which engages a corresponding threaded portion 14a of shroud housing 14. A spring loaded spline lock, indicated at 28, prevents nut 22 from loosening during service. Spline lock 28 includes a spring member 28a having a spline 28b located at the free end thereof as illustrated. The head 27a of a bolt 27 located in an aperture 29 in shroud housing 12 is, as explained below, adapted to engage spring member 28a at a point intermediate the ends thereof during installation. The bore of nut 22 is splined at 30 to engage an outwardly projecting portion 32 of a torquing tool 34 described below. The inner surface of shroud housing 14 is shaped to engage a cell lifting and preloading tool 36 which is also described below and to this end, this surface includes an inwardly projecting gripping flange 38 which defines the inner bore of shroud housing 12. In a specific example, this bore is two inches in diameter. The module lifting and preloading tool 36 includes an expanding collet assembly including an elongate, cylindrical lifting member 40 which extends parallel to the longitudinal axis of the shroud housing 14. Lifting member 40 includes an intermediate enlarged portion 42 and an enlarged gripping portion 44 at the shroud (lower) end thereof. The lifting member 40 is bifurcated or otherwise split at the shroud end so as to form the expandable gripping arms of the collet assembly. Gripping portion 44 includes a bearing surface 44a which, when the gripping arms of the lifting member 40 are expanded and member 44 is suitably positioned, engages a corresponding bearing surface 38a of flange 38 of shroud housing 12. A central rod 46 is located in a longitudinal bore or opening formed centrally of lifting member 40 and extends along the length of member 40 as shown in FIG. 2 and FIG. 3. A tapered head 48 at the end of rod 46 cooperates with the expandable gripping arms of member 40 to actuate the expanding collet assembly and in particular, to control expansion and contraction of the gripping arms of lifting member 40 depending on the longitudinal position of head 48. The expanded position is shown in FIG. 2 wherein the gripping portions 44 of member 40 are forced outwardly by rod head 48 and engage flange 38 of shroud housing 12. The normal position is shown in FIG. 3. As illustrated in FIG. 3, the upper end of rod 46, i.e., the end opposite head 48, is threaded as indicated at 50 and a nut 52 controls the longitudinal movement of rod 46. As is also shown in FIG. 3, lifting member 40 further includes, near the upper end thereof, a piston 54 which cooperates with a hydraulic cylinder 56 to provide preloading. Cylinder 56 is suitably supported by struts indicated at 58 and includes a downwardly depending skirt portion 60 which is threaded at 60a and engages threading 62a of a spacer nut 62. Spacer nut 62 controls movement of torquing tool 34 as is described in more detail below. Turning now to a consideration of torquing tool 34, this tool comprises an elongate expandable sleeve member 64 which is somewhat similar in construction to lifting member 40 and which carries splines 32 referred to above at the lower, expandable end thereof. A handle 66 for rotating sleeve 64 is located at the upper end thereof. Spacer nut 62, which was referred to above, includes a lower, enlarged nut portion 67 and an upper tubular sleeve portion 68 which, as mentioned above, includes threading 60a. The lower end of spacer nut 60 abuts the upper end of sleeve member 64 and when nut 60a is screwed downwardly, sleeve member 64 of torquing tool 34 is forced downwardly so that the lower end thereof engages the intermediate enlarged portion 42 of lifting member 40 and is expanded thereby as illustrated in FIG. 2, whereby spline portions 32 are brought into engagement with cell support nut 22. Briefly considering the operation of the module support apparatus described above, assembly of the module support begins with insertion of the installation tool assembly, comprising torquing tool 34 and lifting and preloading tool 36, through the CDM 12. The CDM 12 has a restricted bore (assumed to be 2 inches in diameter in the specific example under consideration) and the installation tool assembly fits through this bore. Next, the cell lifting and preloading tool 36 is expanded into engagement with shroud housing 14, gripping portions 44 of lifting member 40 being expanded into engagement with flange 38 by action of head 48 in response to the upward longitudinal movement of rod 46 as described above. It should be noted that with gripping portions 44 so expanded, bolt 27 will be displaced thereby radially and the head 27a of bolt 27 will contact spring member 28a of spring lock 28 and force spring member 28a out of engagement with cell support nut 22 as shown in FIG. 2. Thus spring lock 28 is disengaged at this point in the operation and support nut 22 can be rotated relative to shroud housing 14. At this time, the cell is lifted into position and preloaded by actuation of the hydraulic piston 54 whose movement controls the longitudinal position of lifting member 40. At this point the lower end 32 of torquing tool 34 is expanded into engagement with the cell support nut 22, this expansion taking place by virtue of the downward movement of sleeve member 64 and the coaction therewith with the enlarged portion 42 of lifting member 40. The engagement of cell support nut 22 with shroud housing 14, and head 10, is tightened through the use of handle 66. This completes the cell installation. After the cell installation has been completed the installation tool assembly is removed by retracting the expanded portions and withdrawing the assembly through the bore in CDM 12. With the removal of the installation tool assembly, shroud housing 14 will be supported by cell support nut 22 and the spline lock 28 will be engaged so as to prevent loosening of the threaded connection between nut 22 and housing 14. The module support nut 22 is constructed so as to be capable of being disengaged through the use of a spanner wrench in the event that jamming occurs and, because of this jamming, the nut 22 cannot be disengaged by normal methods. This use of a spanner wrench requires removal of CDM 12. It will be appreciated that module support arrangement of the invention possesses a number of advantages compared with a conventional PWR module mounting arrangement. In particular, less structural metal is required in the fuel assembly, thereby resulting in improved neutron economy and higher core power and/or endurance. Further, there is less fuel assembly distortion, which results in freer control rod motion and simpler, more reliable refueling. Further, the module support arrangement of the invention also possesses important advantages over other head-supported core arrangements including the features that the CDMs do not have to be removed to gain access to the module suspension hardware and the head thickness can be greatly reduced because only small head penetrations are required. Although the invention has been described relative to an exemplary embodiment thereof, it will be understood by those skilled in the art that variations and modifications can be effected in this exemplary embodiment without departing from the scope and spirit of the invention.