Patent Number: 046844970
Section: description

DETAILED DESCRIPTION Cadmium oxide has been found by the inventor to be similar in chemical behavior to the lead and zinc oxides, but generally forms higher melting compounds than lead oxide. For example, cadmium silicate melts at about 1240.degree. C., whereas lead silicate melts at about 750.degree. C. A cadmium oxide/boron oxide eutectic melts at about 900.degree. C., whereas a lead oxide/boron oxide eutectic melts at about 500.degree. C. In accordance with the present invention, glasses suitable for use as a glaze on nuclear fuel elements and containing a burnable absorber can be made using cadmium oxide instead of lead oxide, except that such glazes must be fired at higher temperatures than the lead-based glazes. In particular, it has been further found that there are a group of glasses known as borosilicate glasses which include boron trioxide as a suitable glass forming component. Since cadmium oxide, like lead oxide, forms a series of glasses with silicon dioxide, the present invention broadly contemplates the substitution of cadmium oxide, wholly or in part, for the boron trioxide in these borosilicate glasses. Referring to the sole FIGURE, there is disclosed a nuclear fuel pellet 100 of fissionable material, that is, a material fissionable by neutrons of thermal energy such as U-235, U-233 and Pu-239. Coating the exterior of the nuclear fuel pellet 100 is a glaze 102 which includes a burnable absorber, which in accordance with the present invention, comprises an oxide of cadmium-113 isotope which has a neutron capture cross section of about 20,000 barns per atom. The cadmium-113 isotope is about five times more effective as a burnable absorber than the boron-10 isotope which has a neutron capture cross section of only about 3850 barns per atom. The glaze 102 containing cadmium-113 isotope is useful as a burnable absorber in effectively controlling the reactivity of a reactor core and ultimately extending the operating life cycle of the nuclear reactor. The camdium-113 isotope, as a constituent of the glaze 102 coating the nuclear fuel pellet 100, functions as a burnable absorber which burns out at a rate which reduces the negative reactivity introduced into the reactor by the cadmium-113 isotope at a rate approximately equal to the decline in excess reactivity due to fissionable material depletion. The glaze 102, in addition to containing the oxide of cadmium-113 isotope as a burnable absorber, contains any of the common constitutes of glass such as silicon dioxide (SiO.sub.2), aluminum oxide (Al.sub.2 O.sub.3), boric oxide (B.sub.2 O.sub.3), sodium monoxide (Na.sub.2 O), potassium oxide (K.sub.2 O), lead monoxide (PbO), and mixtures thereof. However, in accordance with the present invention, it has been found that for the glaze 102 to be useful as a burnable absorber, the oxide of the cadmium-113 isotope should be present in greater than about 0.5 percent by weight. For example, the glaze 102 may contain cadmium oxide in the range of about 50 to 95 percent by weight; preferably in the range of about 70 to 95 percent by weight cadmium oxide; the preferred range being about 82 to 90 percent by weight cadmium oxide; and the balance being, for example, silicon dioxide. The use of boron-10 isotope as a burnable absorber in a coating on a nuclear fuel pellet contemplates a concentration of the order of 3.2 milligrams of natural boron per centimeter of pellet length. The corresponding quantity of cadmium-113 isotope is 11.0 milligrams of natural cadmium per centimeter of pellet length. When cadmium-113 isotope is substituted for the boron-10 isotope, such substitution would, for example, be approximately in the ratio of 11 parts by weight of cadmium to 3.2 parts by weight of boron. This substitution of the oxide of cadmium-113 isotope for the boron-10 isotope, increases the rate of burnout of the burnable absorber, reduces the amount of undesirable gases produced by the burnout of the boron, and produces a harder and more refractory glaze coating. In this regard, the cadmium-113 isotope produces no gaseous products as a result of neutron capture. Thus, the use of the oxide of cadmium-113 isotope as a burnable absorber burns out more rapidly than the boron-10 isotope and leaves less residual burnable absorber at any given time than that of the boron-10 isotope. Referring again to the sole FIGURE, a typical nuclear reactor pellet 100 of fissionable material, such as enriched uranium dioxide or mixed oxides, might be of the order of 0.5 inches (1.3 centimeters) in length. The nuclear fuel pellet 100 is expected to have a cadmium silicate glaze coating containing about 16 milligrams of the oxide of cadmium-113 isotope, i.e., about 87 percent cadmium by weight. However, greater or lesser amounts of cadmium oxide may be used in coating such nuclear fuel pellets as the present invention broadly relates to the use of cadmium oxide as a burnable absorber in a glaze for such nuclear fuel pellets, wherein cadmium-113 isotope is substituted, wholly or in part, for boron-10 isotope. Further, although there has thus far been described the use of the oxide of cadmium-113 isotope as a burnable absorber in a glaze for nuclear fuel pellets, it is also contemplated that a combination of two burnable absorbers, each having different neutron capture cross sections, may be incorporated into the glaze for controlling the reactivity of the reactor core and ultimately extending the operating life cycle of the nuclear reactor. The incorporation of more than one burnable absorber having different neutron capture cross sections, provides an extra degree of freedom for the nuclear engineer in the design of a reactor core. The two burnable absorbers burn out at different rates so that the reactivity of the reactor core can be controlled with more finesse. The use of such a sophisticated control can result in savings of fissionable material and produce more energy per unit of fissionable material loaded into a reactor core. In accordance with the present invention, a cadmium borosilicate glaze may contain from about 50 to 75 percent by weight cadmium oxide, two (2) to three (3) percent by weight boric oxide, three (3) to six (6) percent by weight potassium oxide and the balance silicon dioxide. The boron-10 isotope can also be present as sodium borate (Na.sub.2 B.sub.4 O.sub.7.10H.sub.2 O). The glaze 102 is applied to the nuclear fuel pellet 100 by a dip coating process. Generally, the constitutes of the glaze 102 are ground to a fine powder and made into a thin slurry with water. The pellets 100 to be glazed are dipped into the slurry which can be thickened or thinned to produce the ultimate coating of the proper thickness. The wet glaze containing cadmium oxide is dried to about 70.degree. to 90.degree. C. and subsequently fired to melt the glaze to hard refractory coating upon cooling. However, it should be noted that the nuclear fuel pellet 100 may be dipped into the slurry one or more times as required to produce the ultimate coating thickness, each dip being followed by a drying step. Thus, several dips can be applied to provide greater coating thicknesses as required. The following example is illustrative of the present invention in applying a glaze 102 containing the oxide of cadmium-113 isotope as a burnable absorber of predetermined thickness to a nuclear fuel pellet 100 containing fissionable material. EXAMPLE I A cadmium silicate glaze composition for glazing nuclear fuel pellets in accordance with the present invention was prepared by grinding cadmium oxide powder and pure quartz powder in a porcelain ball mill with porcelain balls for 48 hours. The resulting mixed powders containing 89 percent by weight cadmium oxide, the balance silicon dioxide, was made into a slurry using water. Cylinders of uranium dioxide were dipped into the slurry and dried at about 70.degree. to 90.degree. C. and subsequently weighed. The dipping process was repeated until the cylinders had picked up the desired weight of dry slurry, that is, about 18 mg per 1.3 centimeters of cylinder length. The coated cylinders were fired at 1350.degree. C. in an inert atmosphere furnace for three (3) hours to produce ceramic cylinders with a nearly uniform coating of cadmium silicate glaze of about five (5) microns thick. The glaze cylinders were heated and cooled in the furnace at a rate less than 15.degree. C. per minute to prevent thermal shock to the cylinders. The furnace cycle was about two (2) hours for heat up, three (3) hours at glazing temperature, and twelve (12) hours for cool down. Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made in the illustrative embodiments and that other arangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims. In particular, although an exemplary application of the present invention would glaze nuclear fuel pellets (each having a generally cylindrical configuration with an approximately one-third inch diameter and an approximately one-half inch length) for placement in fuel rods which make up fuel assemblies, the glazing of nuclear fuel plates, columns or other nuclear fuel shapes is considered to be equivalent to the glazing of nuclear fuel pellets, which has been hereinbefore described.