Patent Number: 040295451
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 10 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 the 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 14 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 a 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 and as a composite cladding. The composite cladding 17 has an outer layer 21 selected from conventional cladding materials such as stainless steel and zirconium alloys and in a preferred embodiment of this invention the outer layer is a zirconium alloy such as Zircaloy-2. The outer layer has attached on the inside surface thereof a diffusion barrier 22 so that the diffusion barrier 22 forms a shield preventing any diffusion of other species through the diffusion barrier 22 to the outer layer. The diffusion barrier 22 is preferably about 0.00005 to about 0.001 inch in thickness and is comprised of a low neutron penalty material selected from the group consisting of chromium and chromium alloys. The diffusion barrier serves as a secondary reaction site for gaseous impurities and fission products and protects the outer layer from contact and reaction with such gaseous impurities and fission products. The diffusion barrier 22 has attached thereon a metal layer 23 so that the metal layer 23 covers the diffusion barrier 22 and also forms a shield for the outer layer against fission products and gaseous impurities emanating from the nuclear fuel material held in the container. The metal layer is about 0.0001 to about 0.002 inch in thickness and is comprised of a low neutron penalty metal selected from the group consisting of copper, nickel, iron and alloy thereof. The metal layer serves as a primary or preferential reaction site for gaseous impurities and fission products and also protects the outer layer from contact and reaction with such gaseous impurities and fission products. The purity of the metal layer and the diffusion barrier is important from a neutron penalty aspect. The total impurities in the two layers are limited to a boron equivalent of 40 parts per million or less. In addition impurities should be kept at a level of less than one weight percent and perferably below 1000 parts per million to maintain resistance to irradiation hardening. The composite cladding of the nuclear fuel element of this invention has the diffusion barrier bonded to the outer layer in a strong bond and the metal layer bonded to the diffusion barrier in a strong bond. When the composite cladding is heated in a diffusion step or cold worked the bond is metallurgical. Metallographical examination shows that there is significant physical bonding for the composite when the metal layer is electroplated. Tests to show the bond strength between the diffusion barrier and the outer layer show that the diffusion barrier remains firmly affixed when bent in the elastic region or when permanently strained to about 5%. Also, tests employing a metal stylus show that the underlying chromium layer cannot be scratched off, even in the as-plated condition. It is discovered that the metal layer (iron, nickel or copper or alloys thereof) has been more resistant to the deleterious effects of radiation hardening and damage than zirconium alloys under the conditions found in commerical nuclear fission reactors, e.g. at temperatures of 500.degree. F. to 750.degree. F. Thus, these materials have more ability to withstand plastic deformation without mechanical failure than zirconium and zirconium alloys under operating nuclear reactor conditions. Thus, these metals can deform plastically from pellet-induced stresses during power transients, relieving pellet-induced stresses. In addition, these metals will not rupture mechanically and thus will also shield the zirconium alloy outer layer from the deleterious action of fission products. It has further been discovered that a metal layer of the order of about 0.0001 inch to about 0.002 inch bonded physically or metallurgically to the diffusion barrier which in turn is bonded to the outer layer of zirconium or a zirconium alloy provides stress reduction and chemical resistance sufficient to prevent nucleation failures in the outer layer of the cladding. The metal layer, as well as the diffusion barrier, provides significant chemical resistant to fission products and gases that may be present in the nuclear fuel element and prevents these fission products and gases from contacting the outer layer of the composite cladding protected by the metal barrier. It has further been discovered that the composite clad will help to reduce the localization of stresses and strains in the cladding. Without a composite cladding, the zirconium alloy reacts with the oxide nuclear fuel to form ZrO.sub.2 on the internal cladding surfaces. At internal cladding temperatures and in the presence of radiation, this oxide can sinter to the oxide fuel thus bonding the fuel to the clad. During fuel rod power changes this bonding can localize stresses and strains in the clad to high levels at crack locations in the UO.sub.2. When the composite cladding is made with copper or nickel as the metal layer, the oxygen potential inside the fuel rod is such that the copper or nickel can not be oxidized, and thus bonding does not occur between the oxide nuclear fuel and the cladding. With iron as the metal layer, the oxides of iron are just marginally stable for the oxygen potentials inside the fuel rod and a strong bond does not form. All of the metals and alloys used in the metal layers herein will reduce the localization of stresses and strains in the cladding by having non-existent or weak bonding with the nuclear fuel. It has further been discovered that because the metal layer does not oxidize to any appreciable extent, the stoichiometry of the UO.sub.2 fuel can be stabilized. Without the metal layer, the zirconium or zirconium alloy will react with the oxide nuclear fuel forming ZrO.sub.2, thus changing the stoichiometry of the oxide nuclear fuel. The chemical state of various fission products is a very strong function of the oxide nuclear fuel stoichiometry. For example, at higher oxygen to uranium ratios, cesium forms a compound with the UO.sub.2 fuel. At lower ratios, this compound is not stable and cesium can migrate to the lower temperature regions of the fuel rod (e.g., inner surface of the cladding). Cesium, either alone or in combination with other fission products, may then promote stress corrosion of the cladding. In a fuel rod with an uncoated cladding, even if the oxide nuclear fuel has a high initial oxygen to uranium ratio, the oxygen consumed by the oxidation of the zirconium alloy will lower this ratio, and cesium can be released to migrate to the cladding surface. With the present invention using a diffusion barrier and a metal layer, the ratio will remain nearly constant or change at a reduced rate. Thus, an oxide nuclear fuel with any desired stoichiometry can be used in the composite cladding with the expectation that this stoichiometry will remain constant or change with time at a much slower rate. The composite cladding used in the nuclear fuel elements of this invention can be fabricated by any of the following methods. In one method, the chromium is electroplated on the zirconium or zirconium alloy outer layer so that the metal layer is uniform on the outer layer. A layer of copper, nickel or iron is then electroplated on the chromium layer by the following process. The outer layer zirconium alloy is first activated by exposure to an agitated solution of the following composition: NH.sub.4 FHF--10 to 20 gms/liter, H.sub.2 SO.sub.4 --0.75 to 2 gms/liter with the balance water to make one liter. The aformentioned bath should first be aged by exposure to a pickled zirconium or zirconium alloy for about ten minutes. The chromium can then be electroplated by employing conventional acid plating bath techniques. One method that has worked well uses a bath of 283 gms/liter of Cr.sub.2 O.sub.3, and 2.83 gms/liter of H.sub.2 SO.sub.4 with the balance H.sub.2 O and applied at a temperature of 66.degree. C. at a current density of 50 amps/sq. ft. After the chromium layer has been applied, the composite may be out-gassed in vacuum at 300.degree.-400.degree. F. for about 3 to 4 hours to remove hydrogen contamination from the electroplated metal. One or a combination of the metals of copper, nickel or iron is then electroplated on the chromium layer. One method that works well at this stage is to apply a nickel strike from an aqueous solution composed of 40 gms/liter of NiCl.sub.4.6H.sub. 2 O and 36 gms/liter of HCl. The strike is applied from a room temperature solution at 50 amps/sq. ft. A nickel, iron or copper layer may then be applied to this strike using standard acid bath plating procedures. A copper strike may also be substituted for the nickel strike. The entire composite cladding may then be vacuum outgassed at 300.degree.-400.degree. F. for 3-4 hours to remove hydrogen contamination. In another method, the above electroplating method is followed by a diffusion bonding step to metallurgically bond the diffusion barrier to the outer layer and the metal layer to the diffusion barrier. An example of this treatment would be a 2-5 hour treatment in vacuum at about 1350.degree. to 1400.degree. F. The foregoing processes of fabricating the composite cladding of this invention gives economies over other processes used in fabricating cladding such as vapor deposition. The invention includes a method of producing a nuclear fuel element comprising making a composite cladding container comprising an outer layer having two coatings on the inside surface with the first coating on the outer layer being a diffusion barrier and the second coating on the first coating being a metal layer, which container is open at one end, filling the composite cladding container with nuclear fuel material leaving a cavity at the open end, inserting a nuclear fuel material retaining means into the cavity, applying an end closure to the open end of the container leaving the cavity in communication with the nuclear fuel, and then bonding the enclosure to the container to form a tight seal therebetween. The present invention offers several advantages promoting a long operating life for the nuclear fuel element including the reduction of hydriding of the cladding substrate, the minimization of localized stress on the outer layer of the cladding, the minimization of stress and strain corrosion on the outer layer of the cladding, and the reduction of the probability of a splitting failure in the outer layer of the cladding. The invention further prevents expansion (or swelling) of the nuclear fuel into direct contact with the outer layer of the cladding, and this prevents or reduces localized stresses and strains on the outer layer of the, initiation or acceleration of stress corrosion of the outer layer of the cladding and bonding of the nuclear fuel to the cladding substrate. An important property of the composite cladding of this invention is that the foregoing improvements are achieved with a negligible additional neutron penalty. Such a cladding is readily accepted in nuclear reactors since the cladding would have essentially no eutectic formation during a loss of cooling accident or an accident involving the dropping of a nuclear control rod. No liquid eutectic forms in the claddings of this invention because chromium does not form a eutectic phase with either zirconium, zirconium alloys, copper, nickel or iron at temperatures experienced in postulated loss of coolant accident conditions in water-cooled and moderated nuclear reactors. 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 between the nuclear fuel and the cladding. Also the composite cladding of this invention is inspectable by conventional non-destructive testing methods during various stages of fabrication. The metal layer of the composite cladding reacts rapidly with fission product iodine to form iodides and thus chemically remove a known stress corrosion agent for zirconium and zirconium alloys from contacting the outer layer of the composite cladding. The metal layer of the composite cladding will resist radiation damage sufficiently at temperatures encountered by the cladding to have superior ductility and toughness properties to that of the zirconium or zirconium alloy. The metal layer prevents the bonding between the nuclear fuel and the outer layer of the cladding and reduces localized stress and strains. The metal layer also maintains the initial stoichemistry of the nuclear fuel. The diffusion barrier between the outer layer cladding and the metal layer serves to prevent the metal of the metal layer from reacting with the zirconium to form a liquid phase during a loss of coolant accident when the temperature of the cladding is elevated. 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.