Patent Number: 046876278
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to nuclear reactors and, more particularly, is concerned with an improved water displacer rod containing hollow sealed pellets which prevent the rod from filling with reactor coolant in the event of rod failure. 2. Description of the Prior Art In the conventional designs of pressurized water reactors, an excessive amount of reactivity is designed into the reactor core at start-up so that as the reactivity is depleted over the life of the core there will still be sufficient reactivity to sustain core operation over a long period of time. However, since an excessive amount of reactivity is designed into the reactor core at the beginning of the core life, steps must be taken at that time to properly control it. One technique to control reactivity is to produce an initial spectral shift which has the effect of increasing the epithermal (low reactivity) part of the neutron spectrum at the expense of the thermal (high reactivity) part. This results in production of fewer thermal neutrons and decreased fission. Then, as fission decreases during extended reactor operation, a reverse shift back to the thermal part of the neutron spectrum at the expense of the epithermal part is undertaken. Such control technique is primarily accomplished through the use of displacer rods. As the name implies, these rods are placed in the core to initially displace some of the moderator water therein and decrease the reactivity. Then, at some point during the core cycle as reactivity is consumed, the displacement associated with these rods is removed from the core so that the amount of moderation and therewith level of reactivity in the core are increased. One approach for removing moderator water displacement is described in a U.S. patent application entitled "An Improved Water Displacer Rod Spider Assembly for a Nuclear Reactor Fuel Assembly" by Trevor A. Francis, filed Mar. 30, 1984 and assigned U.S. Ser. No. 595,154. Several constructions of a water displacer rod are illustrated and described in U.S. Pat. No. 4,432,934 to Gjertsen et al. Referring to FIGS. 12 and 13 of this patent, the displacer rod includes an upper first tube composed, for instance, of stainless steel and a lower second tube composed, for example, of Zircaloy-4, with the tubes being interconnected by a mid extension in the for of a rod of solid Zircaloy-4. The upper first tube is sealed at its upper end by a top end plug capable of being threadably attached to a spider, while the lower second tube is sealed at its lower end by a bottom end plug which can be bullet shaped to aid in insertion of the rod into a guide thimble within one of the fuel assemblies of the reactor. In one construction of the displacer rod shown in FIG. 12, the second tube is thin walled and contains a stack of annular pellets extending from the bottom end plug to the mid rod extension which provides structural support for the tube to prevent its collapse under reactor operating pressure. In the alternative, the pellets can be of a solid instead of a hollow construction. As another alternative construction, the second tube can be thick walled and pressurized with an inert gas to prevent collapse of the rod under reactor pressure. In still another construction, the rod contains a spring instead of the stack of pellets. In any event, the construction of the displacer rod is such that it provides a low neutron absorbing rod that is capable of displacing reactor coolant-moderator when inserted into a fuel assembly. While all of the above-cited prior constructions of the water displacer rod perform reasonably well and generally achieve their objectives under the range of operating conditions for which they were designed, a need exists for improvement in the design of the displacer rod so as to increase its general utility in fuel assemblies in a variety of slightly deformed structural states and to facilitate even more efficient use of nuclear fuel in the reactor core through employment of the spectral shift principle. SUMMARY OF THE INVENTION The present invention provides an improved water displacer rod designed to satisfy the aforementioned needs. Design activity leading up to the improved rod of the present invention has brought to light certain important considerations which affect the design of water displacer rods and are met by the rod construction of the present invention. First, the rod must be flexible to permit axial movement of the rod in a fuel assembly whose structure has become somewhat deformed axially, such as warped or bowed. Second, the amount of coolant flow needed to remove heat from the rod must be kept to a minimum. Third, to more precisely calculate nuclear peaking factors in the core, it must be assumed that there is no coolant in any of the rods. Each of the three above-mentioned considerations underlie, and are satisfactorily addressed by, the improved water displaced rod construction of the present invention. As will be explained hereafter, the problems of providing adequate rod flexibility and collapse resistance, of minimizing the need for increased coolant flow to avoid rod overheating, and of minimizing nuclear peaking in adjacent fuel rods are all treated in a satisfactory manner. First, flexibility of the rod can be increased through reduction in the moment of inertia of the rod. The most practical way to minimize the rod's inertia is to use a thin-walled cladding or tube in the rod. The use of a thin-walled tube, in turn, requires the provision of some means within it to prevent collapse of the tube due to external pressure. A stack of pellets therein bolsters the strength of the tube without adversely affecting its flexibility. Compared thereto, the use of another inner thin tube would augment resistance of the outer thin tube against collapse but would decrease its flexibility. Second, to minimize the amount of coolant flow required to remove heat from the rod and to prevent surface boiling due to gamma heating, the mass of the pellets selected to satisfy the first consideration--flexibility and collapse resistance--must be minimized. The use of hollow or annular pellets is a practical way of minimizing the mass of the pellets without deleteriously affecting the flexibility and collapse resistance of the tube. Third, without some additional steps being taken, the use of a thin walled tube increases the probability of clad failure and entry of coolant into the displacer rod and, to further compound the problem, the use of hollow annular pellets ensures that the entering coolant will flow throughout the empty spaces in the rod. Thus, in absence of some additional steps being taken, peaking factors will substantially increase. However, in accordance with the present invention, by individually sealing the center void of each annular pellet, substantially all of the empty space within the tube will be made impenetrable to coolant even if some of the thin walled tubes of the displacer rods should fail. Furthermore, the solid sealed ends of the individual pellets provide radial support at axially spaced locations along the rod which enhances its collapse resistance without adversely affecting its flexibility nor significantly increasing its overall mass. Accordingly, the present invention sets forth in a fuel assembly for a nuclear reactor including an organized array of nuclear fuel rods and moderator-coolant liquid flowing along the fuel rods, at least one improved water displacer rod disposed among the fuel rods of the assembly. The improved displacer rod includes: (a) an elongated hollow hermetically-sealed thin-walled tubular member; and (b) a plurality of pellets disposed in a stacked relationship within the tubular member. Each of at least a substantial number of the pellets have (i) a body with opposite ends and a hollow annular cross-sectional shape defining a central void through the pellet, and (ii) a web extending across at least one of the opposite ends of the body so as to close the void at the one body end such that the respective voids of the bodies of the pellets disposed in the stacked relationship are enclosed individually one from the next. More particularly, each of the pellets has a pair of webs extending across the respective ends of the pellet body so as to close the opposite body ends such that the respective voids of the bodies are sealed individually one from the next. In one form of the pellet, one of the webs is integrally connected with the pellet body at one of its opposite ends and the other of the webs is a cap attached with the pellet body at its other end. In an alternative form, each of the webs is integrally connected with the pellet body at a respective one of its opposite ends. In another alternative form, each of the webs is a cap attached to a respective one of the opposite ends of the pellet body. These and other advantages and attainments of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.