Patent Number: 046363521
Section: description

DETAILED DESCRIPTION The present invention provides a nuclear fuel rod which incorporates a pellet-clad interaction fix with a burnable poison concept. As illustrated in FIG. 1, a nuclear fuel rod 1, comprises a metallic tubular cladding 3, which may be formed from known metal cladding materials such as zircaloy, the tube being closed at both ends, as is conventional, by closure means, not shown. Positioned within the metallic tubular cladding 3, are a plurality of nuclear fuel pellets 5. The nuclear fuel pellets generally comprise sintered pellets of uranium dioxide that is enriched in the U-235 isotope. In place of the use of enriched uranium dioxide, a mixture of uranium-plutonium dioxide may be used. These fuel pellets are generally formed by enriching the uranium dioxide and either alone or in a mixture with plutonium dioxide, compacting the material to a desired size and shape and sintering the same to produce dense pellets for use in the nuclear fuel rod. The nuclear fuel pellets will normally be of a length on the order of 0.4-0.6 inch, and have a length to diameter ratio of less than 1.7:1, and preferably of about 1.2:1. As formed, the nuclear fuel pellts have concavities 7, therein, to provide concave faces on the confronting faces of the pellets arranged in the tubular cladding in an axial relationship. During the operation of a reactor containing the nuclear fuel, volatile fission products are released. Because such a release is generally temperature dependent, it has been found that the greatest release of such volatile fission products occurs at the concave faces of the fuel pellets, as indicated by the arrows shown in the drawing. Normally only a small amount of such volatile fission products are released from the sides of the fuel pellets directly confronting the wall of the metallic cladding. Thus, the present invention substantially reduces or eliminates attack on the cladding which could cause a pellet-clad interaction failure by freezing out the fission products released from the concave faces of the fuel pellets. In order to freeze out the volatile fission products, and to also assist in controlling the power shaping of the reactor system, a plurality of ceramic wafers 9, are each axially disposed between a major portion of adjacent fuel pellets, the wafers being formed from gadolinium oxide and natural or depleted uranium dioxide. The uranium dioxide used in the formation of the ceramic wafers 9 have no more uranium-235 isotope than is present in natural uranium dioxide. Generally, natural uranium dioxide will contain on the order of 0.71 percent by weight of the uranium-235 isotope. In the production of enriched uranium dioxide for use as a nuclear fuel, the same is processed so as to increase the uranium-235 isotope content. The scaling or residue from such processing is depleted uranium which contains an amount of uranium-235 isotope lower than that of the natural uranium, and can be essentially devoid of that isotope. It is this depleted uranium dioxide, or natural uranium dioxide which is usable in the formation of the ceramic wafers of the present invention. The natural or depleted uranium dioxide has added thereto gadolinium oxide, and the mixture is sintered to form the ceramic wafers. The amount of gadolinium oxide added to the natural or depleted uranium dioxide is between about 1-8 percent by weight of the mixture. The ceramic wafers 9 so formed should have a diameter substantially the same as the diameter of the fuel pellets 5, as indicated in the drawings but will be of a much smaller length. The length of the wafers should be between about 10-100 mils. Wafers of less than 10 mils in thickness would be difficult to produce and could not retain an integral structure during handling and use, while wafers of a thickness of more than 100 mils would space the fuel pellets so far apart as to tend to cause problems relative to power peaking. It is not necessary that a ceramic wafer 9 be disposed between all of the adjacent fuel pellets 5, but a major portion of the adjacent fuel pellets should have a wafer disposed therebetween. By varying the number of ceramic wafers present in the fuel rods, flexibility in the power shaping of the system is achieved. For example, a ceramic wafer could be disposed between each of adjacent fuel pellets in the zone of high power generation, generally the middle region of the rod, while adjacent fuel pellets at the end regions of the rods, or other areas of low power generation, would not have a ceramic wafer therebetween. Also, as an aid to power shaping of the system, particular wafers throughout the rod could vary in the amount of gadolinium oxide present, in the range of 1-8 percent by weight. The use of the present fuel rods, in addition to reducing or eliminating pellet-clad interaction failures, can thus eliminate the need for a burnable poison in the fuel pellets and eliminate the need for use of separate burnable poison rods in the reactor system. In the present fuel rod, containing the ceramic wafers of natural or depleted uranium dioxide and gadolinium oxide, power generation from the ceramic wafers is very low and the wafers freeze out volatile fission products and prevent such fission products from reaching the cladding and causing pellet-clad interaction failures. After the gadolinium oxide has burned out, the natural or depleted uranium dioxide in the wafer is still low in power production, and will act to freeze out the harmful fission products.