Patent Number: 047175346
Section: description

DETAILED DESCRIPTION The present invention provides a nuclear fuel cladding and a fuel rod containing the same, where the cladding is a composite material having a boron-containing burnable absorber integrally incorporated therein. Referring now to FIG. 1, there is illustrated a transverse cross-section of the composite cladding 1. The composite cladding 1 is composed of an outer tubular layer 3 of zirconium alloy having a first thickness, and an intermediate layer 5 having a thickness less than the thickness of tubular layer 3, formed from a mixture of a boron-containing material, such as zirconium boride, and a zirconium alloy, bonded to the inner wall 7 of the outer tubular layer 3. An inner layer 9 of zirconium metal, which has a thickness less than the thickness of the intermediate layer 5 is bonded to the inner surface 11 of the intermediate layer 5. The outer tubular layer 3 is formed from a zirconium alloy which contains less than about 5 percent by weight of alloying elements, usable in nuclear reactors. Such zirconium alloys contain elements which increase the mechanical properties of zirconium metal and/or the corrosion resistance of zirconium metal. The elements that are used in the formation of such alloys include niobium, oxygen, tin, iron, chromium, nickel, molybdenum, copper, vanadium and the like. Especially useful alloys are a zirconium alloy containing about 2.5 percent niobium and the zirconium alloys known as Zircaloy-2 and Zircaloy-4. Zircaloy-2 contains, by weight, about 1.2-1.7 percent tin; 0.07-0.20 percent iron; 0.05-0.15 percent chromium; and about 0.03-0.08 percent nickel; the balance being zirconium. Zircaloy-4 contains, by weight, about 1.2-1.7 percent tin; 0.12-0.18 percent iron, and 0.05-0.15 percent chromium, the balance being zirconium. The intermediate layer 5 is also formed from a zirconium alloy, of the alloys defined relative to the outer tubular layer 3, and preferably of the same alloy as that of the outer tubular layer, and has admixed therewith a boron-containing burnable absorber. The boron-containing burnable absorber is selected from boron compounds such as natural boron, enriched boron (boron having a higher percentage by weight of the isotope B.sup.10 than natural boron) zirconium boride (ZrB.sub.2), boron carbide (B.sub.4 C), boron nitride (BN), and the like. The boron-containing burnable absorber is dispersed throughout the zirconium alloy, in an amount of less than 3 percent by weight of the alloy, and the mixture formed into an intermediate layer that is bonded to the inner surface of the outer zirconium alloy tubular layer. The inner layer 9 is a layer of zirconium metal and has a thickness less than the thickness of the intermediate layer 5, and is bonded to the inner surface of the intermediate layer such that a composite tubular cladding is produced that has a boron-containing burnable absorber integrally incorporated therein. The inner zirconium layer prevents a problem of stress corrosion and possible failure of the tubular cladding by "pellet-clad interaction". This term is used to describe the attack on the cladding by volatile fissile materials such as iodine, cadmium, or other volatile elements released by the fuel during operation of the reactor. Such attack, coupled with cladding operating stresses, can produce stress crack corrosion of the metallic cladding and eventual penetration of the wall of the tubular cladding. Also, during irradiation, the boron-containing burnable absorber results in helium gas being produced in the intermediate layer, and the inner layer of zirconium metal prevents the penetration of such helium gas into the interior space of the tubular cladding. Since the intermediate layer has a thicker zirconium alloy tubular layer 3 on its outer surface, and a thinner zirconium layer 9 on its inner surface, this intermediate layer 5 is subject to neither the coolant for the reactor which contacts the outer tubular layer 3, nor the fuel pellets and emission products thereof which contact the inner layer 9. The outer tubular layer 3 has the largest cross-sectional area and, as such, serves the normal function of a cladding, the mechanical integrity and strength to contain the fuel pellets and resist corrosion from the fuel, emission products, and the coolant in which the fuel elements are positioned. As illustrated in FIG. 3, the outer zirconium alloy tubular layer 3 has a thickness A of at least about 15 mils. The intermediate layer 5, the zirconium alloy containing the boron-containing burnable absorber has a thickness B, less than the thickness A, and is preferably a thickness of about 3-5 mils. The thickness will vary dependent upon the type of boron-containing burnable absorber used and the amount of burnable absorber desired. The inner zirconium metal layer 9 has a thickness C, less than the thickness B, and is preferably a thickness of about 1-2 mils. This layer is the thinnest layer, and is used to isolate the intermediate layer from the fuel pellets. The overall thickness of the component cladding, A+B+C, is preferably between 18-22 mils, with A&gt;B&gt;C. In most instances, due to the large thickness of the outer tubular layer A, the thickness of the outer tubular layer A will be at least twice the thickness of the sum of the thicknesses of the intermediate layer and the inner layer, i.e. A&gt;2(B+C). The intermediate layer, which contains the boron-containing burnable absorber can be fabricated to provide specific desired burnable absorber contents. The concentration of the burnable absorber in a specific application will depend on the requirements of the nuclear system, manufacturability, and the irradiation behavior during operation of the reactor in which the fuel element is used. For the same nuclear system requirements, the concentration of the burnable absorber can be adjusted by either altering the thickness of the intermediate layer or the B.sup.10 enrichment of the boron-containing burnable absorber. As an example of how the amount of boron-containing absorber can be determined, using a 5 mil thick intermediate layer of zirconium boride (ZrB) in a Zircaloy-4 alloy, the following describes the key parameters. The symbols and their typical values are as follows: ______________________________________ (Typical) Symbol Unit Description Value ______________________________________ D in Clad OD 0.374 .tau. mil Thickness of Zr--ZrB.sub.2 layer 5 B mg/cm B-10 concentration per 0.6 unit length E B-10 enrichment 0.19 (Nat) 1.0 (Enriched B) .rho.1 gr/cc ZrB.sub.2 density 6.09 .rho.2 gr/cc Zircaloy-4 density 6.55 F.sub.B Fraction of Boron in ZrB.sub.2 0.1917 C w/o w/o of ZrB.sub.2 in Zircaloy-4 &lt;3 V v/o v/o of ZrB.sub.2 in Zircaloy-4 &lt;3 ______________________________________ C and V are related as ##EQU1## The B-10 content per unit length is obtained by ##EQU2## For typical light water reactors, ZrB.sub.2 concentration in the mixture layer can be easily in manufacturability range by adjusting the layer thickness and boron-10 enrichment. The composite tubular cladding may be formed by various processes. For example, an intermediate layer can be formed by powder metallurgy techniques in a thicker construction than that desired in the final tube and inserted in an outer tubular member also of a thicker construction than that desired in the final product and the two structures subjected to cold working to reduce the same, such as pilgering, to give the desired diameter and thickness, and bonding, of these two layers and then the zirconium inner layer coated on the inner surface of the intermediate layer, and bonded thereto, to form the desired component tubular cladding. As illustrated in FIGS. 2 and 3, a nuclear fuel element 13 using the cladding 1 of the present invention is used to hermetically seal fuel pellets 15. The fuel pellets 15, as is conventional, are preferably sintered pellets of enriched uranium dioxide, or mixed uranium-plutonium dioxide. The pellets are retained within the cladding 1 by a bottom Zircaloy end plug 17 which has previously been welded to the composite tubular cladding, and a welded Zircaloy top end cap 19. A void space or plenum 21 is provided between the top pellet and the Zircaloy top end cap 19 and a biasing means, such as spring 23 restrains the pellets 15 within the cladding 1, with clearance spaces 25 (FIG. 3) left between the pellets and the inner layer 9 of the composite cladding 1. The clearance space and plenum are filled with a high purity, inert atmosphere having high thermal conductivity, such as high purity helium pressurized to about 2 to 5 atmosphere (STP). The present composite cladding provides the benefits associated with use of a boron-containing burnable absorber in a fuel rod while separating the burnable absorber handling from the fuel pellet manufacturing line. Also, helium gas resulting from irradiation of the burnable absorber is prevented from entering the interior space of the tube and improved properties are provided to prevent pellet clad interaction. In addition, the present invention provides more flexibility as the amount of burnable absorber usable and the pattern of such absorber in the fuel rod later in the manufacturing stage of the fuel rod.