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Method of Fabricating a Nuclear Reactor Fuel Element

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Method of Fabricating a Nuclear Reactor Fuel Element 3,7,184 United States Patent Office Patented July 2, 1974 3 809 731 Other objects and advantages of my invention will be- METHOD OF FABRICATING A NUCLEAR come apparent from the following detailed description. REACTOR FUEL ELEMENT In the present invention I have provided a method of Charles C. Woolsey, Jr., Calabasas, Calif., assignor to the protecting a uranium-zirconium hydride fuel from inter- United States of America as represented by the United action with cladding material, which comprises prepar- States Atomic Energy Commission ing a zirconium-uranium alloy, enclosing said alloy in a No Drawing. Filed Mar. 31, 1961, Ser. No. 99,701 Int. CI. G26c 21/10 sleeve of zirconium metal and then hydriding the result- U.S. CI. 204—154.2 7 Claims ing composition to yield a uranium-zirconium alloy hy- dride fueled core clad with a protective zirconium hy- My invention relates to an improved method of fabri- 10 dride outer region. The resulting fuel rod may then be cating a uranium-zirconium hydride nuclear reactor fuel enclosed in a conventional protective tubing, such as of element, and more particularly to a method of protecting stainless steel. Thus, the fuel-moderator body is surround- such a fuel element from interaction with cladding metals ed by a region of unfueled zirconium hydride, providing at elevated temperatures. a barrier material between the fuel-moderator material Zirconium hydride is an excellent moderating material and the cladding, and preventing eutectic formation with 15 for a nuclear reactor core, particularly where cores of the uranium fuel. small diameter or high power density are required. Hydro- In this method of preventing interaction between ura- gen has the greatest neutron slowing down ability of any nium and cladding materials the moderating character- element, and combined with zirconium, a structural metal istics of the composition are retained, no problems due of relatively low thermal neutron absoprtion cross sec- 20 to differential thermal expansion of the fueled and un- tion, the hydrogen is in a relatively stable, high density fueled protective regions are experienced due to the com- form adapted for high temperature utilization. Zirconium mon use of zirconium hydride, possibilities of either fis- hydride may be used as a moderator material in both sion product release or coolant attack on uranium due heterogeneous and in homogeneous nuclear reactor cores. to outer tube rupture are significantly reduced, thereby In the heterogeneous core, the fuel and moderator are 25 providing an added safety factor, and any ratio of fueled separated, and in the homogeneous core, a uranium-zir- to unfueled region thickness can be made as required conium hydride alloy composition is typically employed. for a particular application. The homogeneous composition has many nuclear, fabri- The range of uranium concentration in uranium-zir- cation, and heat transfer advantages, and has been em- conium alloys which will yield readily workable alloys ployed to date in a number of reactor systems. For in- 30 is very wide, typically ranging up to 50 weight percent formation on zirconium hydride and its use in reactors, uranium. The uranium content of the alloy will vary, as reference is made to the following representative publica- a practical matter, with the specific reactor design, and tions: "Nucleonics," January 1960, page 104; U.S. Pat. the degree of enrichment of the uranium employed. In 2,929,707; and U.S. Pat. 2,940,916 and the abandoned order to make a fuel element with an unfueled zirconium application S.N. 664,706 cited therein. 35 hydride outer region directly exchangeable for an exist- Present methods for the fabrication of uranium-zir- ing uranium-zirconium hydride element of a given size conium hydride fuel elements generally involve the for- in a reactor without design modification, either the weight mation of a uranium-zirconium alloy, followed by the percent of the uranium in the alloy or the enrichment massive hydriding of the alloy at an elevated temperature of the uranium should be increased. For example, in a in a hydrogen atmosphere to yield the final hydride com- 40 1.25 inch uranium-zirconium hydride rod having an outer position. The resulting composition, when fashioned into unfueled zirconium hydride region of 0.15 inch, the core the desired fuel element configuration, is normally en- would contain 18.6 weight percent uranium instead of cased within a protective cladding, typically of stainless the 10 weight percent uranium of the same enrichment steel or other high temperature alloy, to prevent release used for a rod fueled throughout its section. The 18.6 of fission products and to protect the fuel from corrosive 45 weight percent alloy is readily fabricated in the same or erosive attack by reactor working fluids. manner as the 10 weight percent alloy. A particular difficulty has been encountered at elevated The fuel-moderator member with the unfueled outer temperatures with uranium-zirconium hydride fuel slugs region can be fabricated using a number of fabrication encased in ferrous and nickel alloys (e.g., stainless steel methods. Therefore, the following method should be re- and a Hastelloy). Uranium metal forms low melting 50 garded as illustrative rather then restrictive of my inven- eutectics with iron and with nickel (melting points just tion. The core alloy (zirconium-uranium) is prepared by above 1300° F.) and such melting can quickly rupture casting or arc melting. A sleeve of zirconium metal of thin-walled cladding tubes in reactors operating at tem- suitable wall thickness is then slipped around the cast peratures approaching such eutectic temperatures. alloy billet. The resulting assembly is next coextruded Accordingly, the principal object of my invention is to 55 to provide a zirconium-clad, zirconium-uranium alloy rod provide a method of protecting a uranium-zirconium hy- of required dimension. Bonding between the zirconium- dride fuel from interaction with its cladding. uranium core and the zirconium sleeve is accomplished Another object is to provide a method of preventing by coextruding at temperatures between about 800-1300° interaction between uranium-zirconium hydride alloy and F., with a temperature of about 800-1000° F. being very a ferrous or nickel cladding material. 60 satisfactory in obtaining bonding. Bonding of fuel plate Another object is to provide a protective, unfueled coating of common properties for a uranium-zirconium assemblies or other geometric configurations may be ac- alloy which can be coextruded to provide zirconium- complished by hot rolling or pressure bonding in the same uranium alloy rods of required dimension. temperature range as extrusion. The hydriding is then Still another object is to provide such a protected fuel accomplished by heating the resulting composite rod in hy- 65 member which can be hydrided massively, such that the drogen. For example, the composition may be heated at fueled core and the cladding together maintain the mod- temperatures of 1500-1750° F. in a hydrogen atmosphere erating properties of the composition. for periods necessary to give the required degree of hy- A further object is to provide such a fuel element drogenation, usually until hydogen equilibium is reached. which is substitutable, in terms of dimensions and nuclear The resulting hydride rod is then provided with protec- characteristics, for a conventional uranium-zirconium hy- 70 tive metal cladding by conventional means. For example, dride element. the hydride rod may be placed in a tight- fitting stainless >9,731 4 steel tube and the end closures made by welding. Thermal and then hydriding the resulting assembly to yield said contact between the rod and tube may be made in a num- fuel-moderator core. ber of different ways. For low power applications metal- 2. The method of claim 1 wherein said zirconium-ura- lurgical bonding is not necessary and satisfactory heat nium alloy member and said enclosing zirconium metal transfer occurs across an air annulus between the fuel and 5 member are bonded togther by coextrusion. cladding. Metallurgical bonding for higher powder applica- 3. The method of claim 2 wherein said coextrusion is tion may be accomplished by hydrostatic pressing. conducted at temperatures of 800-1300° F. The following example illustrates my invention in 4. A method of preparing a zirconium-uranium alloy greater detail. hydride fuel core enclosed in a protective enclosure of An extrusion billet of a zirconium-uranium alloy is 10 zirconium hydride, which comprises forming a zirconium- made by a double arc melting. Weighed amounts of zir- uranium alloy, enclosing said alloy in a zirconium metal conium sponge and uranium pellets to yield a zirconium member, coextruding the resulting assembly, and then hy- alloy containing 18.6 weight percent of uranium enriched driding the extruded assembly. to about 90 percent in U-235 are pressed into an elec- 5. The method of claim 4 wherein said coextrusion is trode. A W2" diameter rod is cast from the consumable 15 conducted at temperatures of 800-1000° F. electrode; this rod is in turn used as a consumable elec- 6. The method of claim 4 wherein said hydriding is trode to cast a 3" diameter extrusion ingot. The latter is conducted at temperatures of about 1500-1700° F. in a machined to 2.75" diameter and a zirconium tube, 2.75" hydrogen atmosphere. ID with a 0.50" wall, slipped around the cast alloy billet. 7. A method of preparing a zirconium-uranium hydride The resulting assembly is then coextruded at temperatures 20 fuel-moderator element, which comprises forming a mix- of 800-1000° F. to effect bonding between the fueled core ture of uranium and zirconium, forming an electrode of and the unfueled cladding. The extrusion reduces the rod the resulting mixture, consumably arc melting the elec- to 1.25" diameter, a reduction in area of about 9 to 1, trode into an extrusion billet, enclosing said billet in a with a 0.15" diameter unfueled outer zirconium section.
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