Patent Number: 043115608
Section: summary

BACKGROUND OF THE INVENTION This invention relates to nuclear reactors having control rods reciprocable from above into a reactor core, and in particular to control rods reciprocable in guide tubes having a liquid flowing upward within the tube. In a typical nuclear reactor the core is composed of a plurality of elongated fuel assemblies each containing a plurality of elongated fuel elements. A liquid coolant is pumped upward through the core in order to extract the generated heat for the production of useful work. The heat output of the core is usually regulated by the movement of control rods containing neutron absorbing material such as B.sub.4 C. In reactors of the pressurized-water type, each fuel assembly typically includes a plurality of cylindrical guide tubes through which cylindrical control rods are reciprocated. Some of the coolant flow is usually diverted into the lower end of the guide tube in order to cool the control rod, which generates heat in the nuclear transformation associated with its neutron absorbing function. During typical power operation, most of the regulating control rods are maintained in a unique withdrawn position in which the lower tip of the control rod is within the guide tube at the upper end of the assembly. For reasons that are not fully understood, significant wear has been found on the inner walls of the guide tubes at precisely the elevation corresponding to the tip of the control rod in the withdrawn position. Flow tests on a laboratory model of the guide tube and control rod indicate that flow-induced vibration of the rod results in an oscillatory contact of the rod tip against the guide tube wall. It is believed that the source of the driving force is located well above the control rod tip, in the vicinity of the guide tube exit. Continuous wear of the guide tube wall may lead to perforation of the tube and significant weakening of the fuel assembly frame. Thus, there has arisen the need to either eliminate the source of the flow induced driving force, or significantly mitigate its affect. Guide tube wear must be greatly reduced not only in nuclear reactors to be built in the future, but also in existing reactors. SUMMARY OF THE INVENTION The present invention provides a spring device on the lower end of the control rod which not only prevents the rod tip from impacting the guide tube wall, but also reduces the oscillatory behavior of the rod over its entire length. The base of the device is connected to or integral with the lower end seal of the control rod. A plurality of springs project from the base and are sized to provide a uniform, resilient interference fit against the guide tube wall. This maintains the rod tip substantially centered in the guide tube and also absorbs energy without impacting the guide tube walls. The portion of the springs that contact the tube walls have a large surface to distribute the biasing force over a large area on the guide tube, which helps prevent localized wear even with a substantial radial biasing force. The invention provides enough interference to avoid significant guide tube oscillation, without decreasing the scram time of the control rod. In the preferred embodiment, a plurality of laterally spaced, elongated extensions are cantilevered downward from the base of the device. Each extension has a shoulder for contacting the guide tube, and the plurality of shoulders as a group has a transverse dimension that is preset to be larger than the inner diameter of the guide tube such that an interference fit is obtained when the control rod is inserted in the tube. The elongated extensions form an open-ended, substantially hollow member through which most of the liquid coolant flows, and the spaces between adjacent extensions allow the flow to bypass the shoulders without experiencing a significant pressure drop. Furthermore, the natural frequency of such a device a higher than that of the forcing functions and the control rod itself, so that the spring device will act as a damper rather than a contributor to the control rod oscillations. The present invention has many desirable features in addition to its effectiveness in preventing guide tube wear. It is inexpensive to manufacture, compact, and easily installed on existing control rods. In the preferred embodiment, it can be built from a single piece of material, and has no moving parts. BRIEF DESCRIPTION OF THE DRAWING These and other objects and advantages of the invention will be evident from the following detailed description in which: FIG. 1 is partially-sectioned elevation view of the upper end of a nuclear fuel assembly having the inventive control rod therein; FIG. 2 is a partially-sectioned elevation view of the inventive control rod in interference fit with its guide tube; FIG. 3 is a cross section view along the line 3--3 of FIG. 2; FIGS. 4a and 4b are graphic displays of the relative acceleration amplitudes of a typical control rod with and without the invention shown in FIG. 2; FIG. 5 is an elevation view of an alternate embodiment of the invention, showing the spring and shoulders in their neutral positions; FIG. 6 is a cross section view along the line 6--6 of FIG. 5; and FIG. 7 is a partially-sectioned elevation view of the alternate embodiment of the invention in interference fit with the guide tube.