Patent Number: 052805042
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention relates to the fabrication of an improved fuel rod having a burnable-poison coating that is resistant to spalling and maintains a high coating integrity. Burnable poison materials of current interest include boron, gadolinium and erbium. While the preferred embodiments of the present invention use boron as a burnable poison, it should be understood that other burnable poisons may be appropriate, depending on the specific application. Naturally occurring boron includes roughly 20% boron-10 (B.sup.10) and 80% boron-11. (Bn). Boron-10, however, has a thermal absorption cross-section that is orders of magnitude greater then boron-11. Thus, the use of isotopically purified boron-10 will minimize the thickness of the burnable poison coating. 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.11 F.sub.3 reassociates more readily so that B.sup.11 concentrates in the vapor phase and B.sup.11 concentrates in the liquid phase. Varying degrees of enrichment of B.sup.10 can thus be produced by the Eagle-Picher process. Another method for enriching the boron of the boron-containing compound is by atomic vapor laser isotope separation (AVLIS). AVLIS was developed for large scale uranium enrichment applications at the Lawrence Livermore National Laboratory. AVLIS works by first heating and vaporizing a sample of interest followed by laser irradiation at a wavelength specifically selected to ionize only the selected isotope. Once ionized, the isotope is isolated using electric fields. Other separation methods include gas diffusion, centrifugal separation and liquid chromatography. According to an embodiment of the present invention, the burnable-poison coating is a ceramic or metal composition which is selected to bond securely to a zirconium alloy surface. The burnable-poison coating may have a thermal expansion coefficient similar to that of the zirconium alloy to further enhance the adhesion to the zirconium alloy surface. One material having this characteristic is zirconium diboride. As previously discussed, the required thickness of a boron-based burnable-poison coating will depend on the concentration of the boron-10 in the coating. For example, a 3/8-inch fuel rod will require a cladding tube having a naturally occuring zirconium diboride surface thickness between about 0.00004 and about 0.001 inches, and preferably about 0.0002 inches. If isotopically enriched zirconium diboride (having higher concentrations of boron-10) is used, the required coating thickness will be reduced. For example, since naturally occurring boron contains about 20% boron-10 and 80% boron-11, and since boron-10 has a thermal absorption cross-section several orders of magnitude greater than boron-11, isotopically purified boron-10 will provide approximately five times the performance of an equivalent amount of naturally occurring boron. Thus, a 3/8-inch fuel rod having a zirconium diboride surface which contains isotopically purified boron-10 will typically require a thickness between about 0.000008 and about 0.0002 inches, preferably about 0.00004 inches. FIG. 1 illustrates a cross-sectional view of a fuel rod 10 having a burnable-poison coating which is made in accordance with an embodiment of the present invention. The fuel rod 10 includes tubular member 11. A burnable-poison coating 12, such as zirconium dioboride, is provided in a thin layer on the inside surface of the tubular member 11. For purposes of this disclosure, the combination of the tubular member 11 and poison coating 12 will be referred to as the cladding tube. A fuel pellet 13, such as uranium dioxide, is enclosed within the cladding tube. The plasma-arc spray-head device for depositing the burnable-poison coating makes use of a weld head similar to that developed to weld sleeves to the inside of steel generator tubes. According to an embodiment of the present invention, the weld head is modified to perform the plasma-arc spraying of a fine ceramic or metal powder. FIG. 2 illustrates an embodiment of the plasma-arc spray-head device 20 of the present invention. The plasma-arc spray-head device 20 includes an electrode 21 which is preferably tungsten. Electrical power is brought to the electrode 21 by means of a conductor 22 which extends, for example, through the center of the plasma-arc spray-head device 20. The conductor 22 may comprise copper or other conductive material and is preferably tubular. A set screw 23 may be used, for example, to clamp the electrode 21 in place against the conductor 22. A portion of the conductor 22 is contained within a ceramic housing 24 which acts as an effective thermal and electrical insulator. An aperture 26 is formed in the ceramic housing 24 in the vicinity of the electrode 21. The electrode 21 is disposed within the aperture 26 so that it extends outwardly to a first maximum radius. This first maximum radius is less than a second maximum radius which corresponds to the outermost radius of the plasma-arc spray-head device 20. This prevents the electrode 21 from directly contacting the inside surface of the tubular member 11 and helps maintain a constant spacing between the electrode 21 and tubular member 11. A spindle 25, which acts as the supporting portion of the plasma-arc spray-head device 20, may be rotated at its base by means of a motor and gearing (not shown). Alternatively, the tubular member may be rotated. During operation, the plasma-arc spray-head device 20 is inserted into an appropriate tubular member 11. The tubular member 11 is preferably a zirconium alloy, with Zircaloy-2 and Zircaloy-4 being typical examples. An electrical arc is then established between the electrode 21 and the tubular member 11. At or near the same time, a burnable-poison-containing powder is introduced at the location of the electrical arc resulting in the deposition of a burnable-poison coating 12. The selected powder is provided by entraining the powder in an inert gas, preferably argon gas, and spraying the entrained powder into the cladding tube Il in the vicinity of the arc between the electrode 21 and the tubular member 11. The powder is selected to provide a boron-containing ceramic or metal burnable-poison coating 12 on the tubular member 11. The powder may be, for example, a metal boride such as a Group 11a boride, a transition-element boride, or aluminum boride, with zirconium diboride being preferred, or a ceramic boron-containing compound, with elemental boron, boron carbide, boron nitride and borosilicate glass being preferred. As previously stated, the boron compound is selected to provide a burnable poison layer which strongly adheres to the zirconium alloy cladding tube and preferably has a thermal expansion coefficient that closely matches that of the zirconium alloy. Other pertinent characteristics of the burnable poison layer include adequate corrosion resistance and melting point and, of course, thermal absorption cross-section. During the deposition process, one end of the tubular member 11 is sealed to help provide an inert atmosphere for the plasma-arc spray-deposition process. Moreover, the plasma-arc spray-head device 20 is designed such that it is inserted into the opposite end of the tubular member 11 while providing a fairly tight tolerance between the plasma-arc head device 20 and the tubular member 11. By providing a continuous flow of argon gas in the vicinity of the electrode 21, the region of the electrode 21 can be effectively purged of undesirable atmospheric gases such as oxygen and nitrogen, and an inert blanket can be maintained in that region. If necessary seals, one-way valves, bubblers and so forth can be provided to ensure the integrity of the inert blanket. The method for directing the entrained powder into the vicinity of the arc is selected to provide a high quality, uniform burnable-poison coating 12. For example, the entrained powder may be passed through the hollow conductor 22 and directed into the arc by means of grooves or small holes in the conductor 22 in the region of the electrode 21. Alternatively, the entrained powder may be introduced through the sealed end opposite the plasma-arc spray-head device 20, resulting in a continuous flow of the entrained powder down the length of the tubular member 11. Moreover, the entrained powder may be introduced using passageways (not shown) formed in the top of the plasma-arc spray-head device 20 above the electrode 21. A pure argon gas atmosphere may be necessary to establish the plasma arc. In that event, pure argon gas can be initially introduced into the system. Once the arc is established, the entrained powder can be introduced into the system through the same passageway as that used to provide the pure argon gas. Alternatively, it may be desirable to bring the pure argon gas and the entrained powder from separate passageways. To establish a uniform burnable-poison coating 12, the plasma-arc spray-head device 20 should be rotated with respect to the tubular member 11 as the powder is sprayed. Concurrently, the tubular member 11 should be gradually moved in an axial direction with respect to the plasma-arc spray-head device 20, further promoting the formation of a uniform burnable-poison coating 12 on the inside of the tubular member 11. The length of the plasma-arc spray-head device 20 which enters the tubular member 11 may be up to 12 feet in length, but this length can be reduced if the plasma-arc spraying is performed from both ends of the tubular member 11. It may be desirable to optimize several variables in the plasma-arc spraying process. In addition to the relative axial and rotational velocities between the plasma-arc spray-head device 20 and the tubular member 11, the mass flow rate of the entrained powder and the arc power between the electrode 21 and the tubular member 11 must be optimized. If the tubular member 11 is a zirconium alloy compound, the temperature of the tubular member 11 must be kept below about 600.degree. F., while at the same time maintaining proper arc conditions to bond the burnable-poison coating 12 to the inside surface of the tubular member 11. Depending on the application, it may be necessary to cool the tubular member Il to keep the temperature within the desired range. Thus, it will be seen that the invention provides a fuel rod cladding tube having a plasma-arc-sprayed burnable-poison coating on the inside surface. This coating is provided by means of a plasma-arc spray-head device which makes use of an electrical arc to provide a plasma for depositing a burnable-absorber-containing powder.