Patent Number: 048805971
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 a boron-containing alloy on the inside of a cladding tube. The burnable poison is deposited as an alloy using an electroless nickel-thallium-boron plating process that utilizes boron-containing reducing agents 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, typically, 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. U.S. Pat. No. 3,625,821 discloses an electroplated inside tube coating of a matrix meta and boron compound of, for example, nickel, iron manganese or chrome. Boron nitride (BN), titanium boride (TiB.sub.2) and zirconium boride (ZiB.sub.2) are specifically named. Electroplating boron compounds onto the Zircaloy substrate, as described in U.S. Pat. No. 3,625,821, has been shown to cause the substrate to hydride. This pickup of hydrogen causes the material to embrittle effecting its physical properties. U.S. Pat. No. 4,695,476 shows vapor deposition of volatilized boron compounds on the inside of fuel rod cladding. 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 alloy on the inside surface of the zirconium alloy cladding tube of the fuel rod. The preferred boron-containing alloy is an electroless nickel-thallium-boron plating or coating known in the art as SAE AMS 2433. It offers a unique combination of hardness, ductility and low coefficient of friction while not having any significant effect on tensile properties of the zirconium alloy cladding tube. The adhesion of the nickel-thallium-boron coating to the zirconium alloy cladding tube is dependent on tube preparation, i.e. minimizing surface contamination. The substrate material is compatible with the coating and adhesion is excellent. Therefore, the coating is less likely to deteriorate under irradiation than would similar coatings on the UO.sub.2 pellets. A suitable thin layer or coating of homogeneous amorphous nickel-thallium-boron on the inside surface of the cladding tube is applied by using a method of electroless plating, or chemical deposition using sodium borohydride reducing agents, on the inside of nuclear fuel rod cladding. The liquid process bath contains nickel, approximately 5 percent by weight boron, 2.5 to 6 percent by weight thallium, and the reducing agents. The boron percent by weight content and isotopic content, together, essentially determine the end product burnable poison characteristics. The boron is preferably initially enriched in the B.sup.10 isotope to a level in the range of 50 to 80 percent by weight, typically 50 percent. (However, both natural and enriched boron will work). Eagle-Picher Industries, Inc., Quapaw, Okla. 74363, enriches the boron by a process of fractional distillation. Boron trifluoride (BF.sub.3) dimethylether complex is dissociated in a fractional distillation column. B.sup.10 F.sub.3 -dimethylether reassociates more readily so that B.sup.11 concentrates in the vapor phase and B.sup.10 concentrates in the liquid phase. Any enrichment of B.sup.10 can be produced by the Eagle-Picher process.