Patent Number: 042004926
Section: description

DESCRIPTION OF THE INVENTION Referring now more particularly to FIG. 1, there is shown a partially cutaway sectional view of a nuclear fuel assembly 10. This fuel assembly consists of a tubular flow channel 11 of generally square cross section provided at its upper end with lifting bale 12 and at its lower end with a nose piece (not shown due to the lower portion of assembly 10 being omitted). The upper end of channel 11 is open at 13 and the lower end of the nose piece is provided with coolant flow openings. An array of fuel elements or rods 14 is enclosed in channel 11 and supported therein by means of upper end plate 15 and a lower end plate (not shown due to the lower portion being omitted). The liquid coolant ordinarily enters through the openings in the lower end of the nose piece, passes upwardly around fuel elements 14, and discharges at upper outlet 13 in a partially vaporized condition for boiling reactors or in an unvaporized condition for pressurized reactors at an elevated temperature. The nuclear fuel elements or rods 14 are sealed at their ends by means of end plugs 18 welded to the cladding 17, which may include studs 19 to facilitate the mounting of the fuel rod in the assembly. A void space or plenum 20 is provided at one end of the element to permit longitudinal expansion of the fuel material and accumulation of gases released from the fuel material. A nuclear fuel material retainer means 24 in the form of a helical member is positioned within space 20 to provide restraint against the axial movement of the pellet column, especially during handling and transportation of the fuel element. The fuel element is designed to provide an excellent thermal contact between the cladding and the fuel material, a minimum of parasitic neutron absorption, and resistance to bowing and vibration which is occasionally caused by flow of the coolant at high velocity. A nuclear fuel element or rod 14 is shown in a partial section in FIG. 1 constructed according to the teachings of this invention. The fuel element includes a core or central cylindrical portion of nuclear fuel material 16, here shown as a plurality of fuel pellets of fissionable and/or fertile material positioned within a structural cladding or container 17. In some cases, the fuel pellets may be of various shapes, such as cylindrical pellets or spheres, and in other cases, different fuel forms such as particulate fuel may be used. The physical form of the fuel is immaterial to this invention. Various nuclear fuel materials may be used, including uranium compounds, plutonium compounds, thorium compounds and mixtures thereof. A preferred fuel is uranium dioxide or a mixture comprising uranium dioxide and plutonium dioxide. Referring now to FIG. 2, the nuclear fuel material 16 forming the central core of the fuel element 14 is surrounded by a cladding 17 hereinafter in this description also referred to as a composite cladding. The composite cladding container encloses the core so as to leave a gap 23 between the core and the cladding container during use in a nuclear reactor. The composite cladding is comprised of a zirconium alloy tube 21 which in a preferred embodiment of this invention is made of Zircaloy-2. The alloy tube has bonded on the inside surface thereof a metal barrier 22 so that the metal barrier forms a shield between the alloy tube 21 and the nuclear fuel material held in the cladding. The metal barrier forms about 1 to about 30 percent of the thickness of the cladding and is comprised of a low neutron absorption material, namely, moderate purity zirconium (such as sponge zirconium). The metal barrier 22 protects the alloy tube portion of the cladding from contact and reaction with gases and fission products and prevents the occurrence of localized stress and strain. The content of the metal barrier of moderate purity zirconium is important and serves to impart special properties to the metal barrier. Generally, there is at least about 1000 parts per million (ppm) by weight and less than about 5000 ppm impurities in the material of the metal barrier and preferably less than about 4200 ppm. Of these oxygen is kept within the range of about 200 to about 1200 ppm. All other impurities are within the normal range for commercial, reactor grade sponge zirconium and are listed as follows: aluminum--75 ppm or less; boron--0.4 ppm or less; cadmium--0.4 ppm or less; carbon--270 ppm or less; chromium--200 ppm or less; cobalt--20 ppm or less; copper--50 ppm or less; hafnium--100 ppm or less; hydrogen--25 ppm or less; iron--1500 ppm or less; magnesium--20 ppm or less; manganese--50 ppm or less; molybdenum--50 ppm or less; nickel--70 ppm or less; niobium--100 ppm or less; nitrogen--80 ppm or less; silicon--120 ppm or less; tin--50 ppm or less; tungsten--100 ppm or less; titanium--50 ppm or less; and uranium--3.5 ppm or less. The composite cladding of the nuclear fuel element of this invention has the metal barrier bonded to the substrate in a strong bond. Metallographic examination shows that there is sufficient cross diffusion between the materials of the substrate and the metal barrier to form a bond, but no cross diffusion to any extent away from the area of the bond. It has been discovered that sponge zirconium metal forming the metal barrier in the composite cladding is highly resistant to radiation hardening, and this enables the metal barrier after prolonged irradiation to maintain desirable structural properties such as yield strength and hardness at levels considerably lower than those of conventional zirconium alloys. In effect, the metal barrier does not harden as much as conventional zirconium alloys when subjected to irradiation, and this together with its initially low yield strength enables the metal barrier to deform plastically and relieve pellet-induced stresses in the fuel element during power transients. Pellet induced stresses in the fuel element can be brought about, for example, by swelling of the pellets of nuclear fuel at reactor operating temperatures (300.degree. to 350.degree. C.) so that the pellet comes into contact with the cladding. It has further been discovered that a metal barrier of sponge zirconium of the order preferably about 5 to 15 percent of the thickness of the cladding and a particularly preferred thickness of 10 percent of the cladding bonded to the alloy tube of a zirconium alloy provides stress reduction and a barrier effect sufficient to prevent failures in the composite cladding. Among the zirconium alloys serving as suitable alloy tubes are Zircaloy-2 and Zircaloy-4. Zircaloy-2 has on a weight basis about 1.5 percent tin; 0.12 percent iron; 0.09 percent chromium and 0.005 percent nickel and is extensively employed in water-cooled reactors. Zircaloy-4 has less nickel than Zircaloy-2 but contains slightly more iron than Zircaloy-2. The composite cladding used in the nuclear fuel elements of this invention can be fabricated by any of the following methods. In one method, a hollow collar of the sponge zirconium selected to be the metal barrier is inserted into a hollow billet of the zirconium alloy selected to be the alloy tube and then the assembly is subjected to explosive bonding of the collar to the billet. The composite is extruded at an elevated temperature of about 1000.degree. to about 1400.degree. F. (about 538.degree. to about 750.degree. C.) using conventional tube shell extrusion techniques. The extruded composite is then subjected to a process involving conventional tube reduction until the desired size of cladding is achieved. In another method, a hollow collar of the sponge zirconium selected to be the metal barrier is inserted into a hollow billet of the zirconium alloy selected to be the alloy tube and then the assembly is subjected to a heating step [such as 1400.degree. F. (750.degree. C.) for about 8 hours] to give diffusion bonding between the collar and the billet. The composite is then extruded using conventional tube shell extrusion techniques and the extruded composite is subjected to a process involving conventional tube reduction until the desired size of cladding is achieved. In still another method, a hollow collar of the sponge zirconium selected to be the metal barrier is inserted into a hollow billet of the zirconium alloy selected to be the alloy tube and the assembly is extruded using conventional tube shell extrusion techniques. Then the extruded composite is subjected to a process involving conventional tube reduction until the desired size of cladding is achieved. The foregoing processes of fabricating the composite cladding of this invention give economies over other processes used in fabricating cladding such as electroplating or vapor deposition. The invention includes a method of producing a nuclear fuel element comprising making a composite cladding container comprised of a metal barrier of sponge zirconium bonded to the inside surface of a zirconium alloy tube, which container is open at one end, filling the composite cladding container with a core of nuclear fuel material leaving a gap between the core and the container and leaving a cavity at the open end, inserting a nuclear fuel material retaining means into the cavity, applying an enclosure to the open end of the container leaving the cavity in communication with the nuclear fuel, and then bonding the end of the clad container to said enclosure to form a tight seal therebetween. The present invention offers several advantages promoting a long operating life for a nuclear fuel element, including the reduction of chemical interaction of the cladding, the minimization of localized stress on the zirconium alloy tube portion of the cladding, the minimization of stress corrosion and strain corrosion on the zirconium alloy tube portion of the cladding, and the reduction of the probability of a splitting failure occurring in the zirconium alloy tube. The invention further prevents expansion (or swelling) of the nuclear fuel into direct contact with the zirconium alloy tube, and this prevents the occurrence of localized stress on the zirconium alloy tube, initiation or acceleration of stress corrosion of the alloy tube and bonding of the nuclear fuel to the alloy tube. An important property of the composite cladding of this invention is that the foregoing improvements are achieved with no substantial additional neutron penalty. Such a cladding is readily accepted in nuclear reactors since the cladding would have no eutectic formation during a loss-of-coolant accident or an accident involving the dropping of a nuclear control rod. Further, the composite cladding has a very small heat transfer penalty in that there is no thermal barrier to transfer of heat such as results in the situation where a separate foil or liner is inserted in a fuel element. Also, the composite cladding of this invention is inspectable by conventional nondestructive testing methods during various stages of fabrication and operation. As will be apparent to those skilled in the art, various modifications and changes may be made in the invention described herein. It is accordingly the intention that the invention be construed in the broadest manner within the spirit and scope as set forth in the accompanying claims.