Patent Number: 052805042
Section: summary

FIELD OF THE INVENTION This invention relates to fuel rods for use in a nuclear fuel assembly which have a burnable-poison coating disposed on the inside surface of the fuel rod cladding tube. More particularly, this invention relates to a fuel rod cladding tube having an internal burnable-poison coating which consists of zirconium diboride or other boroncontaining material comprising naturally occurring boron or isotopically enriched boron-10. This invention also relates to a method and apparatus for applying a ceramic or metal coating on the inside of a small-diameter tube. More particularly, this invention relates to a method and apparatus for plasma-arc spraying a ceramic or metal layer on the inside of the tube. BACKGROUND OF THE INVENTION General background materials on fuel rods, claddings and absorber materials are available. See, e.g., Frank J. Rahn et al., A Guide to Nuclear Power Technology, pp. 429-438 (1984). In light water reactor (LWR) designs, fuel is formed into oxide pellets, which consist of uranium oxide or mixed uranium/plutonium oxide. These pellets are then placed in long tubes called cladding tubes to form fuel rods. The cladding tube forms a barrier against radioactive fission products released in the fuel pellets during irradiation. Proper fuel design requires an economical fuel cycle, while providing the necessary fuel characteristics for safe plant operation. Thus structural materials must be selected that have low neutron cross-section and low cost, while providing adequate mechanical corrosion resistance characteristics. Fuel assembly design should accordingly allow for the operation of the reactor at the design power and for the highest possible burn-up without breaching the cladding and releasing radioactive products to the primary coolant. Zirconium alloys are used in fuel designs because they combine desirable nuclear, physical and mechanical properties. Because nuclear-grade zirconium is expensive, its alloys are used only in the active zone of the nuclear core where its neutron economy is most advantageous. Zircaloy-2 and Zircaloy-4 are two slightly different alloys which were developed for nuclear applications. Zircaloy-2 typically contains about 1.4 wt % tin, 0.15 wt % iron, 0.1 wt % chromium and 0.06 wt % nickel, 1,000 ppm oxygen and the balance zirconium. Zircaloy-4 typically contains about 1.4 wt % tin, 0.21 wt % iron, 0.11 wt % chromium, 30 ppm nickel, 1,200 ppm oxygen and the balance zirconium. Zircaloy-2 has a small content of nickel, while in Zircaloy-4 the nickel content is essentially replaced by iron. This small change in composition reduces the hydrogen absorption rate during service in high-temperature water. The physical and mechanical properties of the two alloys are nearly identical. Pressurized water reactor (PWR) fuel rods are typically made with Zircaloy-4 cladding, while boiling water reactor (BWR) fuel rods utilize Zircaloy-2. Continuous operation of a reactor requires that the core remain critical. However, to compensate for the gradual depletion of fissile material with time, as burn-up accumulates, and to compensate for other phenomena such as the buildup of fission products, excess reactivity must be built into the nuclear core. This excess reactivity, however, must be controlled at any given time to keep the reactor critical for steady-state operation. This task is accomplished by the use of materials that are strong neutron absorbers or "poisons." Control elements constructed from neutron absorbers regulate power generation according to demand, provide quick shutdown, account for short-term and long-term reactivity changes that result from temperature changes, and adjust for fission product accumulation and fissile material depletion. The foremost characteristic of a control material is its neutron absorption properties. These vary with the energy of the impinging neutrons but one can gather together the detailed absorption features into a "thermal absorption cross-section," which is of interest in LWR's. The dominant absorber used in control rods in LWR's is boron. In addition to the movable control rods used in all LWR's, present LWR designs utilize burnable poisons. These are solid neutron absorbers which are placed in the reactor. As it is subjected to neutron irradiation, the burnable absorber material is gradually depleted. Thus the depletion of the burnable poison corresponds, roughly, to the depletion of fissile material. Burnable-poisons are used to counterbalance excess reactivity at the beginning of the fuel cycle and to provide a means for power shaping and optimum core burn-up. Burnable poison compounds currently of interest include erbium, gadolinium and boron. LWR fuel designs employ burnable absorber rods to control power peaking or moderator temperature coefficient in a number of ways. In some designs, burnable absorber rods are placed in fuel assembly lattice locations, thereby displacing fuel rods. Other designs employ burnable absorber rod inserts and fuel assembly guide thimbles. Still other designs involve the formation of burnable-absorber coatings on the inside diameters of cladding tubes, on fuel pellet surfaces, or involve distribution of the burnable absorber within the fuel pellet. The use of a burnable-poison which is provided on the inside surface of the fuel cladding tube has several advantages. For example, such a configuration can be used with uranium dioxide fuel pellets provided inside the cladding so that the fuel rod produces as much (or almost as much) power as a regular fuel rod. Moreover, the burnable-poison can be applied to the cladding tube prior to the introduction of the uranium dioxide pellets into the tube, allowing the burnable-poison to be applied to the cladding in a cold (non-nuclear) area. This allows the burnable-poison to be applied by the tubing fabricator or by the fuel-rod fabricator and reduces the costs associated with the manufacture of the cladding tubes containing the burnable poison. Furthermore, when the burnable poison is applied to the inside of the fuel cladding tubes, it is relatively easy to adjust the axial gradient of the burnable poison. This provides an advantage over associated methods which involve putting burnable poison on the pellet and mixing pellet types. Finally, the use of cladding tubes having a burnable-poison layer provides for improved quality control. For example, the burnable-poison coating depth can be accurately determined by bombarding the tubing with neutrons and measuring the fraction of the neutrons which are not absorbed by the burnable poison. It is proposed that plasma-arc spraying be used to apply the burnable poison to the inside of the fuel cladding tube. The plasma-arc spray-coating process is used to provide a securely bonded layer which will resist abrasion or spalling. The plasma-arc spray-coating has not been used to deposit a burnable poison coating on the inside diameter of fuel rod cladding tubes. Moreover, it is anticipated that fuel cladding tubes of extremely small diameter (i.e., approximately 3/8 inch) will be used. The inside diameter of such tubes have apparently never been plasma-arc spraycoated prior to the present invention for either nuclear or non-nuclear use. SUMMARY OF THE INVENTION In view of the foregoing, it is an object of the present invention to provide a zirconium alloy fuel rod cladding tube having a burnable-poison which has been plasma-arc spray-coated on the inside surface. As a further object of the invention, the burnable poison is preferably naturally occurring boron or isotopically enriched boron-10. For a 3/8 inch rod, the boron coating may be provided in a thickness on the order of 0.0002 inches for naturally occurring boron and in a thickness on the order of 0.00004 inches for boron-10. It is another object of the present invention to provide a method for depositing a ceramic or metal coating on the inside surface of a small tube using a plasma arc spraying technique. Accordingly, an electrical potential is applied across an inert atmosphere between an electrode and a tubular member such that an electrical arc is established between the electrode and the inner surface of the tubular member. A burnable-poison-containing powder is then entrained in an inert atmosphere and provided in the vicinity of the arc. At the same time, the electrode is rotated and axially moved with respect to the tubular member while maintaining a predetermined spacing from the tubular member. It is yet a further object of the present invention to provide a plasma-arc spray-head device for depositing a ceramic or metal coating on the inside surface of a small tube. Thus, a device is provided comprising an annular ceramic housing having an aperture; an electrically conductive member disposed within the housing; an electrode in electrical contact with the electrically conductive member, the electrode being disposed such that it projects into the aperture in the ceramic housing and extends outwardly to a first maximum radius which is less than a second maximum radius corresponding to the outermost radial extent of the plasma-arc spray-head device; and means for supplying a gaseous composition to the vicinity of the electrode.