Patent Number: 046438688
Section: summary

TECHNICAL FIELD This invention relates to pressurized water reactors and in particular, to improved module support apparatus for the core modules of such reactors. BACKGROUND ART Most commercial pressurized water reactors (PWRs) have core modules which are axially supported at the bottom by a core support plate and are held down at the top by springs. Thus, the modules are subjected to column-type compression loading. Since it is desired that the fuel assemblies contain a minimum of structural material in order to minimize parasitic neutron absorption and maximize fuel content, such modules are relatively weak and tend to buckle under this column loading. The resulting distortion has been the cause of difficulties during refueling wherein a distorted module has interfered with removal of an adjacent module. Other potential difficulties include lack of straightness of control rod guide tubes and the resultant interference with control rod travel. In designing a light water breeder reactor based upon PWR technology, the problem of module column loading has been exacerbated by special requirements of the breeder core design. The amount of structural material in the fuel assembly has had to be further reduced in order to enhance the neutron economy required to achieve breeding. The low water fraction necessitated by breeding considerations causes a high core flow resistance, thereby requiring higher compression hold-down spring forces to resist the resulting high upward pressure forces. Thus, with previous constructions, column loading increased while the structure designed to carry this loading became weaker. In some nuclear reactors the core is supported by the pressure vessel head. Examples include the original core and present breeder core installed in the Shippingport reactor. These reactors have required removal of their control drive mechanisms (CDMs) for access to the modle support hardware and the disconnecting of all module supports prior to removal of the reactor vessel head. Relatively large pressure vessel head penetrations have been required to accommodate this support hardware. In typical commercial PWRs the fuel modules are arranged in a parallel array, with relatively small pressure vessel head penetrations through which the CDM leadscrews pass. There are on the order of one hundred CDMs in such installations. The removal of so many CDMs for access to core suspension hardware is excessively time consuming, and the small leadscrew penetrations in the head could not accommodate this hardware. In this regard, it is noted that in a typical commercial PWR the CDMs remain installed on the pressure vessel head when the head is removed for refueling and the tops of the CDM pressure housings are removed for the access required to disconnect and remove the leadscrews prior to head removal. As mentioned above and discussed in detail below, the present invention relates to a module support arrangement, and patents of possible interest in this regard include U.S. Pat. Nos. 3,163,585 (Metcalf et al); 3,604,746 (Notari); 3,857,599 (Jones et al); 3,905,634 (Johnson et al); 4,030,973 (Hoffmeister et al); 4,038,133 (Bitterman et al); and 4,279,699 (Kuhn). These patents relate to various forms of gripper, latching and support devices and arrangements for nuclear reactor components. SUMMARY OF THE INVENTION In accordance with the invention, a fuel cell module support arrangement is provided which possesses a number of significant advantages over prior art support arrangements and, in particular, over conventional PWR module mounting arrangements. As will become evident from the discussions below, these advantages include a reduction in the amount of metal required in the fuel assembly, and the attendant improvement in neutron economy and higher core power and/or lifetime resulting therefrom. Additionally, there is less fuel assembly distortion, resulting in freer movement of the control rod and more reliable refueling. Further, 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 of the small head penetrations necessary. In this latter regard, the support arrangement of the invention provides for axial support of the individual core modules from the pressure vessel head and permits attachment and detachment of the core modules from the head through the control drive mechanisms after the leadscrews are removed. According to a preferred embodiment thereof, the arrangement comprises a module support nut, engaged with the pressure vessel head and supported therefrom, including a downwardly depending screw threaded portion, and a shroud housing for the fuel cell module including a screw threaded portion engaged with the screw threaded portion of the support nut such that the shroud housing is suspended from the support nut and thus from the pressure vessel head, the module support nut and the shroud housing including locking means for, when engaged, locking the nut and housing against relative rotation so as to prevent loosening of the nut during operation. The locking means preferably comprises a spring mounted lock member which is associated with the shroud housing and which is normally biased into engagement with a spline lock member of the support nut to provide locking of the support nut. In accordance with a further important aspect of the invention, an installation tool assembly is provided which is adapted to be inserted through the bore in the control drive mechanism for the fuel cell module and into the bore of the shroud housing. This installation tool assembly includes a lifting and preloading tool for, when actuated, engaging and lifting the shroud housing and a concentrically mounted torquing tool for, when actuated, engaging the cell support nut and applying torque thereto. Preferably, the lifting and preloading tool includes an expandable gripping portion for, when expanded, gripping the shroud housing, and a central rod member for controlling expansion of the expandable gripping portion. Advantageously, the shroud housing includes an inwardly extending flange and the gripping portion includes an upwardly facing support surface which, in the use of said lifting and preloading tool, engages a downwardly facing surface of the flange. The lifting and preloading tool includes an elongate lifting member which includes the gripping portion referred to above and a hydraulic means is provided for controlling longitudinal movement of the lifting member. Preferably, this hydraulic means includes a hydraulic piston formed integrally with the lifting member. The torquing tool preferably includes expandable gripping means for, when expanded, gripping the module support nut and means for applying a torque to the gripping means. Advantageously, the elongate lifting member of the lifting and preloading tool is positioned concentrically within the expandable gripping means of the torquing tool and includes means for expanding this expandable gripping means. The latter preferably comprises an expandable sleeve, and the torque applying means comprises a handle connected to the upper end of the sleeve. A spacer nut is preferably provided which is concentric with the sleeve and in abutment with the upper end thereof, and controls longitudinal movement of the expandable sleeve of the torquing tool. The means for expanding the expandable gripping means comprises an enlarged portion of the elongate lifting member located intermediate the ends thereof and the module support nut includes an inwardly extending spline portion for engagement by this expandable gripping means. As noted above, the module support nut and the shroud housing preferably include spline locking means for, when engaged, locking the support nut and housing against relative rotation. In a preferred embodiment, the gripping portion of the lifting and preloading tool, when engaged with the shroud housing, provides disengagement of this spline locking means. Advantageously, the spline locking means includes, as noted above, a spring loaded spline locking member, and in this preferred embodiment, the shroud housing includes a bolt member which is positioned to be engaged by the gripping portion of the lifting and preloading tool and which, when engaged by the gripping portion, contacts the spring loaded spline locking member and causes disengagement thereof from the module support nut. Other features and advantages of the invention will be set forth in, or apparent from, the detailed description of the preferred embodiments which follows.