Patent Number: 046719270
Section: description

DETAILED DESCRIPTION The present invention provides a hybrid burnable absorber-containing nuclear fuel composition and fuel rod assemblies containing such a composition which combines the benefits of both gadolinium oxide and boron carbide as absorbers into a single fuel pellet. As illustrated in FIG. 1, a nuclear fuel rod 1, for use in a nuclear fuel rod assembly, comprises a metallic elongated tubular cladding 3, usually formed from zirconium alloys, having a top end plug 5 and a bottom end plug 7 which provides an enclosed chamber 9. In the enclosed chamber 9, a plurality of fissionable fuel pellets 11 are disposed in end-to-end abutment biased against the bottom end plug 7 by the action of a spring 13. It is the fissionable fuel pellets 11 that are formed containing the hybrid burnable absorber according to the present invention. The diameter of the pellets 11 is slightly smaller than the interior diameter of the tubular cladding 3 such that a clearance space 15 is provided therebetween. Both the spring 13 and the clearance space 15 accommodate thermal expansion of the pellets 11 during operation. The nuclear fuel is preferably uranium in the form of uranium dioxide enriched in the U-235 isotope. In place of the use of enriched uranium dioxide, a mixture of uraniumplutonium dioxide may be used. The nuclear fuel composition of the present invention includes the nuclear fuel, preferably uranium dioxide, in admixture with gadolinium oxide and coated boron carbide particles. These materials are mixed together, compacted to the desired size and shape and sintered to produce dense pellets for use in a nuclear fuel rod. The nuclear fuel pellets, typically having dished ends and chamfered edges, will normally by cylindrical in shape with a length of about 0.4-0.6 inch, and a length to diameter ratio of less than 1.7:1, and preferably of about 1.2:1. The nuclear fuel pellets, in addition to the uranium dioxide contain between 1 to 20 percent by weight gadolinium oxide, based on the weight of the pellet. As previously described, the incorporation of gadolinium oxide, as a burnable absorber, into a fissionable material for use as fuel pellets is known. Generally, the preferred amount of gadolinium oxide used in the pellets of the present invention will preferably be between about 2 to 10 percent. In addition to the gadolinium oxide, there is added to the fuel composition between about 0.02 to 1.0 percent by weight of coated boron carbide particles. The boron carbide particles are of a size in the range of betwen 20 to 100 microns in diameter and have a coating thereon of a thickness of between 0.5 to 10 microns. A coating thickness of about 5 microns on the boron carbide particles is preferred. The coating on the boron carbide particles is of an attrition resistant, helium impervious coating material. Such materials are known, for example, a coating of niobium, zirconium, nickel, graphite, alumina, and the like. Such coated boron carbide particles may be produced by various methods and are commercially available. It is believed that such coated particles have been previously prepared for use in separate burnable absorber rods of a nuclear reactor system. The coating prevents helium gas, formed during operation of a reactor, from escaping to the environment within the rod outside the coated particles. In formation of the pellets 11, conventional nuclear fuel pellet formation techniques are used. The uranium dioxide, gadolinium oxide, and coated boron carbide particles are intimately mixed, the mixture compacted to the desired shape and size, and the pellets sintered to produce nuclear fuel pellets containing a hybrid burnable absorber composition throughout the pellet. The benefits of use of the pellets of a nuclear fuel containing the hybrid burnable absorber composition is graphically illustrated in FIG. 2, which shows the results of neutronics spatial calculations using the PHOENIX transport-depletion code. This code is described in the literature and used in FIG. 2 relative to a pressurized water reactor containing about a 3 percent enrichment of uranium dioxide fuel. In FIG. 2, the effective reactivity constant (Keff) is plotted against the reactor lifetime as measured by megawatt days/metric tons of uranium (MWD/MTU). The full-line represents the rate of burn-out with use of uranium dioxide pellets containing 4 percent by weight (4.0 w/o) of gadolinium and no boron carbide (containing B.sup.10). The dash-line curve represents the rate of burn-out with use of uranium dioxide pellets containing a hybrid burnable absorber consisting of 4 percent by weight (4.0 w/o) of gadolinium oxide and 0.05 percent by weight (0.05 w/o) of the boron isotope, B.sup.10, corresponding to the presence of about 0.3 percent by weight of boron carbide. As illustrated, the addition of only 0.05 w/o of B.sup.10 (.about.0.3 w/o B.sub.4 C) will cause the burn-out rate to be delayed about 2000 MWD/MTU (megawatt days/metric tons of uranium). This is equivalent to increasing gadolinium enrichment 1.5 w/o. Higher amounts of boron carbide would result in proportionally greater burn-out delays. Also, it should be noted that the peak reactivity value is reduced about 4 percent .DELTA.K. This is equivalent to reducing assembly powers 10 to 16 percent. Gadolinium burnable absorber assemblies generally limit the core at their peak reactivity time in life. Thus, the present hybrid burnable absorber fuel pellets will substantially reduce the power peaking control problems associated with gadolinium. The final line on the graph, the dotted line, illustrates an intermediate fuel-hybrid absorber composition containing 2.5 percent by weight (2.5 w/o) of gadolinium oxide and 0.05 percent by weight (0.05 w/o) of B.sup.10. In addition to providing moderation of the burn-out rate of a gadolinium-containing fuel pellet, the residual penalty of the hybrid burnable absorber rod is less than a standard boron carbide rod and only slightly higher than a gadolinium rod with the same weight percent of absorber. The residual penalty may be described as the affect of minor amounts of undepleted absorber in the fuel on the lifetime of the reactor before refueling is required. When absorber is added to the fuel, some absorber will still remain active after the desired time has passed for its use, and will thus compete with the fuel in absorbing neutrons. With the present hybrid burnable absorber composition the reduction in the loading of gadolinium possible by trading off gadolinium and boron carbide loadings could result in a product with even less residual penalty than either a gadolinium-containing fuel rod or a boron carbide-containing fuel rod alone. Thus, the use of the gadolinium oxide - boron carbide hybrid absorber in uranium dioxide could provide a synergistic effect and provide a lower residual penalty than the use of comparable amounts of either gadolinium oxide or boron carbide by themselves.