Patent Number: 048246344
Section: summary

BACKGROUND OF THE INVENTION This invention relates to nuclear fuel elements and in particular, the provisions of fuel elements with a burnable poison coating in the form of a thin layer of boron-containing compound particles on the inside of a cladding tube. The burnable poison particles are deposited alone or with lubricity providing graphite particles from a liquid suspension on the inside of a zirconium-alloy cladding tube. A nuclear fuel element of the type involved in the invention is part of a fuel assembly. Heretofore, typical fuel assembly designs have employed fixed lattice burnable poison rods to control early-in-life reactivity and power peaking. These rods have become a necessary design feature for the fuel management of first cores of light water reactors as well as in schemes to achieve extended burnups and reduced radial neutron leakage. Such rods displace fuel rods within the assembly lattice which increases the core average linear heat generation rate and local peaking factors. Alternate approaches have been proposed that place burnable poison material inside the fuel rods so that much less fuel material is displaced, for example, as boride coatings on the UO.sub.2 pellets. Such coatings, however, while adhering when first applied, tend to spall off under the stresses of the irradiation environment in the nuclear reactor core, in part because of difficulty in matching the thermal expansion behavior of the coating to that of the fission material or UO.sub.2 pellet. Attempts to incorporate boron compounds as mixtures within the UO.sub.2 pellets have not been successful because of volatilization of boron species during high temperature fabrication processes and redistribution of the boron under irradiation. For further background, see U.S. Pat. Nos. 3,925,151; 4,372,817; 4,560,575; 4,566,989; 4,582,676; 4,587,087; 4,587,088; and 4,636,404. SUMMARY OF THE INVENTION The invention involves an improved fuel element with a burnable poison coating which substantially overcomes problems of spalling and coating integrity because of the closely matched thermal expansion coefficients of the substrate and coating material and the action of fission sintering to enhance adhesion of the coating to the substrate. The invention includes coating a thin layer of a boron-containing compound on the inside surface of the zirconium alloy cladding tube of the fuel rod. The preferred boron-containing compound is zirconium diboride (ZrB.sub.2) because its thermal expansion coefficient most nearly matches that of the zirconium-tin alloy cladding tube. The adhesion between the coating and cladding, therefore, is less likely to deteriorate under irradiation than would similar coatings on the UO.sub.2 pellets. Also, the fission sintering phenomenon that has been observed in irradiated compacts of boron-containing compound powders at cladding temperatures (approximately 400.degree. C.) is more likely to promote adhesion between the ZrB.sub.2 and the metallurgically-related zirconium-tin alloy cladding tube substrate than would be the case for a UO.sub.2 substrate. That is, fission sintering will not only join ZrB.sub.2 particles to each other, but is also likely to form a bond of the particles to the zirconium-tin alloy substrate under irradiation. A suitable thin layer or coating of ZrB.sub.2 particles on the inside surface of the cladding tube is applied by a method analagous to that used for graphite lubricant coatings developed by the laboratories of the assignee of the instant invention for nuclear fuel rod cladding. A liquid suspension which includes isopropanol, an acrylic polymer binder material and the boron-containing compound particles in a range of from 0.1 to 1.5 microns, with or without colloidal graphite particles, has its solids content adjusted to provide the desired viscosity for the coating process (approximately 16% by weight solids). Each fuel tube is then filled with the liquid suspension and drained at a controlled rate, leaving a thin film on the inside surface of the cladding tube. The film is dried at room temperature and cured in a vacuum at temperatures up to 427.degree. C. (800.degree. F.). The resulting thin layer containing ZrB.sub.2 (and perhaps graphite) at a density of approximately 50% of theoretical, along with a small residue from the decomposition of the binding material. The ZrB.sub.2 is preferably initially enriched in the B.sup.10 isotope to an 80% level.