Patent Number: 042082498
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to nuclear reactors and, more particularly, to hydraulic apparatus for absorbing shocks that are applied to fuel assemblies, and the like. 2. Description of the Prior Art To produce useful power from a nuclear reactor, it is necessary to assemble fissionable uranium in a sufficient concentration and in a physical configuration that will sustain a continuous sequence of neutron-induced fission reactions within the uranium nuclei. The heat generated through these reactions in this assembly, or reactor core, usually is absorbed in a stream of pressurized water. This heated pressurized water then is pumped to one or more heat exchangers in which the absorbed heat is transferred to secondary coolant water. It is, of course, this secondary coolant water that rises into the steam which drives the turbines, or other electrical power generating machinery. To provide a proper concentration of uranium for the reactor core, it has often been the practice to prepare pellets of uranium dioxide. These pellets are loaded into long, slender, hollow tubes which, when the tube ends are sealed off, are referred to as fuel rods. In order to enhance the structural integrity of the reactor core, these fuel rods are arranged into subgroups, each of about two hundred fuel rods, that are called fuel assemblies. The assemblies, in turn, are mounted in a generally right circular cylindrical array to form the reactor core. Naturally, the reactor core is environmentally hostile to the structural integrity of its component parts. The temperature, water flow velocity, pressure, radiation and the like within the reactor core all combine to place great stresses on the core materials. In addition to these environmental extremes, adequate provision also must be made to enable structural components of the reactor core to cope with other forces of a more unusual and, perhaps, of a more violent nature than those which are imposed through ordinary operating conditions. Seismic or earthquake shocks and the thermal shocks to physical structure that might attend an accident in which a significant portion of the pressurized water evaporates from or drains out of the reactor core are typical of the situations in which forces far in excess of those generated in the course of routine operation could be encountered. The customary response to this problem is the addition, in one way or another, of more materials and more metal to the reactor core. This direct approach although probably providing the needed structural protection, has a number of undesirable features. Additional materials in the reactor core, for example, exhibit a "parasitic" effect that absorbs a portion of the neutron population within the core. Neutrons, absorbed in this manner do not contribute to the energy production and hence, are used wastefully and inefficiently. Accordingly, there is a need for improvements to reactor core structures that will enable the core to safely attenuate or absorb shocks and other forces of unusual and major character without adding materials to the core structure that will not increase parasitical neutron losses. SUMMARY OF THE INVENTION These and other problems that have characterized the prior art are overcome to a large extent through the practice of the invention. Illustratively, the internal pads that brace the fuel assemblies to restrict longitudinal movement, bear against movable spring pads that are mounted on the upper end fitting of the respective fuel assemblies. At least some of these spring pads have plungers that are pinned to the respective pads. These plungers are slidably received within hollow tubular guide posts. Longitudinal slots formed in the guide posts accommodate the pins in order to permit the plungers to move in a longitudinal direction relative to the posts. The guide posts, in turn, are secured to the upper grids of the individual fuel assemblies. Within tubular guide posts, moreover, and partially blocking the open end of each of the posts are individual disk-shaped plates each with an orifice or hole. Thus, as an earth tremor, or the like, compels the plunger and spring pad assemblies to move in a longitudinal direction relative to the respective guide posts, pressurized water within the guide posts arrests this motion. This water, squirting through the holes in the plates, permits the guide posts and spring pads to move relative to each other in a longitudinal direction at a controlled rate in which the applied forces are absorbed in a safe manner. In accordance with a feature of the invention, however, the pin and plunger combinations, moving in a longitudinal direction through the respective hollow guide posts produce a progressive braking effect. Thus, as the pins which connect the plungers to their respective spring pads move longitudinally through the guide post slots, the pins and their associated plungers gradually block these slots, thereby continuously decreasing the discharge area through which the water within the guide post can flow. This technique provides a continuously increasing resistance to the further longitudinal relative movement of the plungers and orificed plates. The resistance increases, moreover, in a progressive manner that protects reactor core structure from sustaining severe damage that otherwise might occur through a more abrupt attenuation of imposed shocks, and the like. Thus, there is provided in accordance with the terms of the invention an improved technique for coping with the application of major forces that might be applied to the core of a nuclear reactor while eliminating substantial quantities of parasitical neutron absorbing material from the core. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operation and specific objects attained by its use, reference should be had to the accompanying drawing and descriptive matter in which there is illustrated and described a preferred embodiment of the invention.