Patent Number: 040452883
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 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 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 which in this invention is also referred to as a composite cladding. The composite cladding has a substrate 21 selected from conventional cladding materials such as a stainless steel and zirconium alloys and in a preferred embodiment of this invention the substrate is a zirconium alloy such as Zircaloy-2. The substrate 21 has metallurgically bonded on the inside diameter thereof a metal barrier 22 so that the metal barrier forms a shield of the substrate from the nuclear fuel material inside the composite cladding. The metal barrier preferably forms about 1 to about 4 percent of the thickness of the cladding and is comprised of a metal selected from the group consisting of niobium, aluminum, copper, nickel, stainless steel and iron. The metal barrier 22 has metallurgically bonded on the inside diameter thereof an inner layer 23 so that the inner layer is the portion of the composite cladding closest to the nuclear fuel material 16. The inner layer preferably forms about 5 to about 15 percent of the thickness of the cladding and is comprised of conventional cladding materials such as stainless steel and zirconium alloys and in a preferred embodiment of this invention the substrate is a zirconium alloy such as Zircaloy-2. The metal barrier serves as a preferential reaction site for gaseous impurities and fission products which have either diffused through or corroded through the inner layer 23 and protects the cladding from contact and reaction with such impurities and fission products. In another preferred embodiment of this invention, the substrate and the inner layer are comprised of the same material and a preferred material is a zirconium alloy such as Zircaloy-2. The composite cladding of the nuclear fuel element of this invention has a metal barrier metallurgically bonded to the substrate and an inner layer metallurgically bonded to the metal barrier. Metallographic examination shows that there is sufficient cross diffusion between the substrate and the metal barrier and between the metal barrier and the inner layer to form metallurgical bonds, but insufficient cross diffusion to alloy with the metal barrier itself. Also from FIG. 2 it is apparent that the metal barrier could be termed a "buried" metal barrier. It has been discovered that a metal barrier of the order preferably of at least about 1 to 4 percent of the wall thickness of the cladding metallurgically bonded to the substrate and the inner layer provides chemical resistance sufficient to prevent propagation of failures from the inner layer to the substrate of the cladding. The metal 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 substrate of the composite cladding protected by the metal barrier. For a typical fuel element the substrate of the composite cladding ranges in thickness from 24 to 30 mils, the metal barrier ranges in thickness from 0.5 to 1 mils and the inner layer is approximately 3 mils. 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 tube of the metal selected to be the metal barrier is inserted into a hollow billet of the material selected to be the substrate, a tube of the material selected to be the inner layer is inserted into the metal barrier tube, and then the assembly is subjected to explosive bonding of the tubes to the billet. The composite is extruded using conventional tube shell extrusion at elevated temperatures of about 1000.degree. to 1400.degree. F. (about 538.degree. to 760.degree. C.). Then the extruded composite is subjected to a process involving conventional tube reduction until the desired size of cladding is achieved. In another method, a tube of the metal selected to be the metal barrier is inserted into a hollow billet of the material selected to be the substrate, a tube of the material selected to be the inner layer is inserted into the tube of the metal barrier and then the assembly is subjected to a heating step (such as at 750.degree. C. for 8 hours) to give diffusion bonding between the tubes and the billet. The composite is extruded using conventional tube shell extrusion such as described above in the immediately preceding paragraph. Then 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 tube of the metal selected to be the metal barrier is inserted into a hollow billet of the alloy selected to be the substrate, a tube of the material selected to be the inner layer is inserted into the metal barrier tube and the assembly is extruded using conventional tube shell extrusion as described above. 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 gives 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 which is open at one end, the cladding container having a substrate, a metal barrier metallurgically bonded to the inside surface of the substrate and an inner layer metallurgically bonded to the inside surface of the metal substrate, filling the composite cladding container with nuclear fuel material having 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 the nuclear fuel element including the reduction of hydriding of the cladding substrate, the minimization of localized stress on the cladding substrate, the minimization of stress and strain corrosion on the cladding substrate, the reduction of the probability of a splitting failure in the cladding substrate and the prevention of the propagation of stress corrosion cracks through the composite cladding. The invention further prevents expansion (or swelling) of the nuclear fuel into direct contact with the cladding substrate, and this prevents localized stress on the cladding substrate, prevents initiation or acceleration of stress corrosion of the cladding substrate and prevents 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 to moderate neutron penalty (depending on choice of barrier material). Such a cladding is readily accepted in nuclear reactors since the cladding would have minimal eutectic formation (depending on choice of barrier material) in the substrate portion of the cladding during a loss of cooling 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 non-destructive testing methods during various stages of fabrication. In addition to the foregoing, when the zirconium alloy is selected as the substrate and the inner layer, the inside and outside surfaces of the composite cladding are compatible with manufacturing processes for light water nuclear reactor cladding and this enables the use of current manufacturing procedures, lubricants, etchants, etc. Those skilled in the art will gain a further understanding of this invention from the following illustrative, but not limiting, examples of this invention. EXAMPLES 1-4 Billets and inserts were machined, cleaned and assembled by standard procedures for example, and all dimensions were chosen so that the composite billets could be extruded into hot extrusion press. The billets were normal Zircaloy-2 conforming to ASTM B353, Grade RA-1, and the inserts were made of high purity niobium and 304L Stainless Steel (ASTM-A 312). All billet bores and inserts had an 8 mil per in. taper and were pressed together to ensure a good contact between the mating surfaces. The dimensions of the machined parts were as follows: __________________________________________________________________________ Inner Buried Diameter Billet Barrier Barrier Outer Inner Outer Inner Outer Inner Length X Dia. X Dia. Dia. Dia. Dia. Dia. __________________________________________________________________________ 1. Buried Nb Metal Barrier 9.5 .times. 5.74 .times. 2.59 2.59 - 2.44 2.44 - 1.66 2. Buried Nb Metal Barrier 9.5 .times. 5.74 .times. 2.59 2.59 - 2.44 2.44 - 1.66 3. Buried SS Metal Barrier 9.5 .times. 5.74 .times. 2.64 2.64 - 2.44 2.44 - 1.66 4. Buried SS Metal Barrier 9.5 .times. 5.74 .times. 2.56 2.56 - 2.44 2.44 - 1.66 __________________________________________________________________________ Prior to assembling the billets and inserts the mating surfaces were given a light etch to remove traces of impurities. The etchant used for the Zircaloy-2 was a solution of 70 ml H.sub.2 O, 30 ml HNO.sub.3, and PA1 5 ml HF; PA1 7.5 ml H.sub.2 SO.sub.4, PA1 4 ml HNO.sub.3, PA1 31 ml H.sub.2 O, and PA1 2 ml HF. PA1 Extrusion rate -- 6 in/min, PA1 Reduction ratio -- 6:1, PA1 Temperature -- 1,100.degree. F. and PA1 Extrusion force -- 3500 tons. PA1 Outer Diameter -- 2.500 inches, PA1 Inner Diameter -- 1.640 inches, and PA1 Length -- 5 Feet. and for the niobium a solution of 7.5 ml HCL, The stainless steel was polished with fine emery paper and cleaned with acetone and de-ionized water. To improve the chances for a satisfactory bond between the inserts and the billets during extrusion, it was decided to prebond the assemblies. This was accomplished by pressing the tapered inserts into the tapered bore in the billets in vacuum .ltoreq.20 .mu.m while maintaining the billet temperature at 1,400.degree. F. for 8 hours. Forces applied to the inserts during initial pressing ranging from 30-45,000 lbs. To reduce end-losses during the extrusion a 2 inch piece of Zircaloy-2 billet was welded on each end of the composite billets and machined flush. The extrusion of the billets into the tube shells was done using the following parameters: All billet surfaces except the bore and also the floating mandrel were lubricated with a water soluble lubricant which was baked on at 1,300.degree. F. for 1 hour. Both ends of the tube shells were cut clean and the inner diameter was honed to remove possible surface flaws and to improve the finish. Final dimensions for the tube shells were: The final reduction of the tube shells to fuel tubing followed the standard procedure which includes four reductions with cleaning and annealing between each step. The parameters for this process are listed in Table 1. TABLE 1 __________________________________________________________________________ CO-EXTRUDED TUBE REDUCTION PARAMETERS Inner Diameter Outer Thickness of Metal Barrier % Step Diameter Composite Insert Tube Reduction Qe* __________________________________________________________________________ Start with Tube Shell 2.500 .430 1.650 -- -- Clean for anneal (degrease - soap base caustic) Anneal - 1250.degree. F - 1 Hour First Pass 1.687 .270 1.147 57 1.2 Clean for anneal Anneal 1150.degree. F - 1 Hour Second Pass 1.125 .160 .805 60 1.4 Clean for anneal Anneal 1150.degree. F - 1 Hour Third Pass .750 .085 .580 64 1.7 Clean for anneal Anneal 1150.degree. F - 1 Hour Fourth Pass .495 .028 .439 70 2.3 Clean for anneal Anneal 1070.degree. F - 21/2 to 4 Hours Etch to .494 .028 .438 __________________________________________________________________________ *Qe is defined as the ratio of percentage of change in wall thickness to percentage of change in mean diameter. Dimensions of the final products are listed in Table 2. TABLE 2 ______________________________________ Dimensions in Mils of Inner Outer Metal Inner Diameter Diameter Barrier Layer ______________________________________ Example 1 0.438 0.494 1.0 .+-. .2 3.1 .+-. 0.6 Example 2 0.438 0.494 1.0 .+-. .2 3.2 .+-. 0.5 Example 3 0.438 0.494 1.4 .+-. .2 3.6 .+-. 0.2 Example 4 0.438 0.494 1.0 .+-. .1 3.0 .+-. 0.2 ______________________________________ 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.