Patent Number: 043326404
Section: summary

The invention relates to refueling mechanisms for reactors, particularly to a fuel grapple for removing spent fuel assemblies from a reactor core, and more particularly to a vibrating fuel grapple for enabling additional withdrawal capability. Reactor refueling operations remove fuel assemblies at refueling by applying an axial force greater than the assembly weight. With significant friction forces due to the design, environment and material behavior, the axial force will have to be large enough to overcome these friction forces also. The refueling approach to these concerns has been to limit allowable withdrawal loads to an assumed safe level. In liquid metal fast breeder reactor (LMFBR) core restraint systems, the current design trend is for a restrained system which utilizes above-core and top load pads and formers along with a nozzle/receptacle restraint at the bottom of the core assemblies. The operating environment of an LMFBR exposes core structural materials to temperature and fast-flux irradiation gradients. Core structural materials exposed to fast-flux irradiation exhibit swelling and creep behavior as a function of irradiation and time. The combined effects of the restrained components, the environment and the material behavior, result in core assemblies with permanently bowed shapes which interact within the core restraint system. This condition results in load interactions, on the cores fuel assemblies, which, as a result of friction, produce retraining loads on the assemblies. This tends to prevent easy removal of assemblies for refueling at end of life (with the design trend to larger cores for commercial reactors, these loads could be significantly large). In addition, the existence of these friction loads can cause damage to remaining adjacent assemblies. Significant changes to remaining fuel assembly surfaces can cause undesirable operational stick-slip behavior and create difficulties for future refueling operations. It is thus seen that it is extremely difficult to predict accurately the load pad normal forces and ultimately the force required to withdraw or insert an LMFBR core fuel assembly at refueling, since the accuracy of such load predictions is subject to the uncertainties in the operational environment, creep and swelling correlations, and the uncertainties of pad-to-pad and nozzle-to-receptacle friction coefficients. Thus, it is possible that actual withdrawal loads will exceed core fuel assembly withdrawal force design limits. Various mechanisms exist in the prior art for removing assemblies, fuel and control, from the core of nuclear reactors. These prior efforts are exemplified by U.S. Pat. No. 3,151,033 issued Sept. 29, 1964; No. 3,175,854 issued Mar. 30, 1965; No. 3,801,148 issued Apr. 2, 1974; No. 3,856,621 issued Nov. 24, 1974; and No. 3,950,020 issued Apr. 13, 1976. Apparatus is known for dislodging stuck elements, such as well pipe, drilling bits, etc. where accoustic energy or vibration is utilized. These prior approaches for removing stuck items are exemplified by U.S. Pat. No. 3,132,707 issued May 13, 1964; No. 3,399,724 issued Sept. 3, 1968; and No. 4,058,163 issued Nov. 15, 1977. The latter patent, for example, uses an eccentric weight which is rotated by means of a pressurized fluid, wherein the rotation of the eccentric weight results in vibration of the apparatus and the well bore member. Thus, while the use of apparatus for dislodging stuck components by vibration, for example, is known in the field of well drilling, there is no known apparatus, as pointed out above, which utilizes a technique such as vibration, to loosen a stuck fuel assembly in the core of a reactor, such that the axial force applied to the fuel assembly does not increase beyond a safe limit. SUMMARY OF THE INVENTION The present invention provides an improved reactor refueling method utilizing a fuel grapple mechanism that incorporates therein a pneumatic vibrator which enables additional withdrawal capability without exceeding the allowable axial force limit. Thus, the present invention provides increased capability for removing spent fuel assemblies wherein the friction loads thereof have increased due to the environment and the material behavior resulting from the fuel assemblies being irradiated in the reactor core. Vibration has been shown to be an effective means to reduce the effective coefficient of friction. By incorporating a pneumatic vibrator into the fuel grapple head, the withdrawal capability is increased. The only moving part in the vibrator is a steel ball, pneumatically driven by a gas around a track. Centrifugal force created by the ball is transmitted through the grapple to the fuel assembly handling socket, causing vibration of the fuel assembly and a reduction in the friction load involved in removable of the fuel assembly. Therefore, it is an object of this invention to provide a reactor refueling method which utilizes vibration of the fuel assemblies being removed from a reactor core for reducing functional loads thereon. A further object of the invention is to provide an improved refueling fuel grapple mechanism. Another object of the invention is to provide a fuel grapple mechanism utilizing vibration techniques for reducing friction loading of the fuel assemblies during withdrawal from a reactor core. Another object of the invention is to provide a refueling grapple mechanism utilizing a pneumatic vibrator in the grapple head enabling additional withdrawal capability without exceeding the allowable axial force limit. Another object of the invention is to provide a vibrating fuel grapple for reducing the effective coefficient of friction resulting from the removal of fuel assemblies from a reactor core. Another object of the invention is to provide a vibrating fuel assembly grapple wherein the only moving part in the vibrator is a steel ball pneumatically driven by gas, such as argon, around a track, such that centrifugal force created by the ball is transmitted through the grapple to the fuel assembly handling socket. Other objects of the invention will become readily apparent to those skilled in the art from the following description and accompanying drawings.