Patent Number: 052689468
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to the drawings wherein like numerals represent like parts throughout the figures, a fuel rod having an enhanced wear resistant coating in accordance with the present invention is generally designated by the numeral 10. Fuel rod 10 is employed in a nuclear reactor and includes a zirconium-alloy cladding tube 20 which contain pellets 30 of fissionable material such as for example, UO.sub.2. The tube is commonly made of zirconium-tin alloy such as Zircolay-2 or Zircolay-4. The fuel rods 10 are conventionally mounted in parallel fashion to a support structure including a support grid 12 of a fuel assembly 14 as schematically illustrated in FIG. 1. The lower portions of the fuel rods are in effect cantilevered into the flow of water which commonly contains debris comprising metallic particulate matter such as stainless steel or Inconel alloy steel (not illustrated). The debris particles are often hardened by irradiation and are known to rapidly fret the tubes 20 as the water flows in the direction indicated by the arrow. In accordance with the invention, a coating 50 is applied to the exterior surface of the cladding tube 20. The cladding tube coating 50 is comprised of a matrix of a ceramic material 52 and a glass binder 54, both of which are schematically illustrated. The relative dimensions of coating 50 is exaggerated in FIG. 2 for description purposes. The ceramic material preferably has a high degree of hardness, a high degree of thermal conductivity and a thermal expansion which is approximately equal to that of the zirconium-alloy substrate of the cladding tube 20. A preferred ceramic material is zircon which in a powder form has a particulate diameter on the order of approximately 10-60 microns. The ceramic material 52 is mixed with a glass 54 which also has a thermal expansion compatible with the zirconium-alloy cladding tube. A number of glass compositions are suitable. The selected glass should have a long term resistance to very hot water which for the reactor environment typically is on the order 400.degree. C. Calcium zinc borate, calcium magnesium aluminosilicate, and sodium borosilicate are all suitable glasses. Thermal coefficients of expansion for the various materials are set forth in the following table: ______________________________________ Material CTE .times. 10.sup.-7 .degree.C. ______________________________________ Zircaloy-4 48.9 Zircon 53 Calcium zinc borate 45-65 Calcium magnesium aluminosilicate 40-70 Sodium borosilicate 30-100 ______________________________________ The ceramic material 52 and the glass 54 are premixed in a ratio so that the there will be sufficient glass to bind the ceramic material and bond the ceramic material to the cladding tube substrate. The particles of the ceramic and glass material have diameters typically on the order of 10-60 microns. The glass particles are preferably significantly smaller than the ceramic particles so that the glass particles will heat rapidly and a large quantity of particles will be available for bonding to each ceramic particle. The zirconium-alloy cladding tube is heated to a temperature which is approximately in the range 300.degree. C. to 350.degree. C. It is naturally desirable to keep the processing temperature of the cladding tube below 400.degree. C. The mixture of the ceramic and glass particles is then flame sprayed onto the cladding tube. The flame spray characteristics are selected so that the glass particles will assume a semi-molten state while the ceramic particles are maintained in a non-molten state. The coating which is formed on the cladding tube substrate is primarily a ceramic composition with just enough glass to provide sufficient bonding of the ceramic material to the cladding tube. It is not required that the glass have a high degree of hardness, although glasses typically will be harder than metals. The ceramic particles are bonded to the tube by the glass. The glass which bonds the ceramic particles is primarily disposed on the back side of the ceramic particles where the glass is not subject to abrasion. Even if glass on the outer surfaces is abraided by metallic particles, it will eventually wear away to expose the ceramic substrate and will function as a wear resistant barrier to prevent further wear of the coating. The initial coating has some glass at the outer layer. The outer glass layer 56 may then be etched to remove the outer glass layer and expose the ceramic particles so that the outer surface 58 is substantially entirely composed of ceramic particles. While etching of the outer glass is not required, there may be some benefits to removing the glass since metallic particles of the debris could potentially strike the outer glass layer and initiate cracks and faults which would propagate throughout the glass matrix. The coating 50 can be applied to the cladding tube in a flame spray process which is efficient and cost effective. The thickness of the coating is preferably a few mils. For example, preferably the thickness of the coating is 5 mils or less. A relatively thick coating would potentially impede the coolant water flow around the cladding tubes. In addition, thicker coatings might undesirably function as a thermal barrier. EXAMPLE In one form of the coating 50, 6 kilograms of zircon ceramic material 52 having a nominal particle size diameter of 30 microns was premixed with 4 kilograms of calcium zirc borate glass 54 having a nominal particle diameter of &lt;10 microns. A zirconium-alloy tube having a nominal outside diameter of 0.4 inch was heated to a temperature of approximately 200.degree. C. The coating mixture was flame sprayed onto the outside surface of the cladding tube at a rate of 5 seconds per linear inch of tube so as to form an outer coating of approximately 5 mils in thickness. After the initial coating was applied, an etching solution of dilute HF acid was applied to remove the outer glass material. It should be appreciated that the coating 50--even though relatively thin in cross-section--provides an effective abrasion resistant coating for the cladding tube 20. The coating 50 is substantially harder than the metal particles present in the surrounding reactor debris. The coating 50 is resistant to the chemical environment in the reactor and does not impede the flow of coolant water around the tubes. The coating also does not constitute a significant thermal barrier. Moreover, the coating can be applied by a flame spray in a relatively inexpensive and cost effective manner which is suitable for mass production of the cladding tubes. While preferred embodiments of the foregoing invention have been set forth for purposes of illustration, the foregoing description should not be deemed a limitation of the invention herein. Accordingly, various modifications, adaptations and alternatives may occur to one skilled in the art without departing from the spirit and the scope of the present invention.