Patent Number: 039880753
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 with a lifting bale 12 extending above channel 11 and a nose piece at the lower end of channel 11 (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 14 is enclosed in channel 11 with one fuel element 14 being shown in partial section, and the array is 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 elevated temperatures. Referring now to FIG. 2 in addition to FIG. 1, a nuclear fuel element or rod 14 is shown in partial sectional view constructed in accordance with the teachings of this invention. The fuel element includes 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; 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. The container is sealed at its ends by means of end plugs 18 which may include studs 19 to facilitate the mounting of the fuel rod in the assembly. A cavity or plenum 20 is provided at one end of the fuel element to permit longitudinal expansion of the fuel material and accumulation of gases released from the fuel material. A helical member 21 is positioned within space 20 and serves to maintain the position of the fuel during handling and transportation of the fuel elements. Cladding 17 is secured to end plugs 18 by means of circumferential welds 22. Common cladding materials are stainless steel alloys, aluminum and its alloys and zirconium and its alloys. The fuel element is designed to provide an excellent thermal contact between the fuel 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. Referring to FIGS. 2 and 3, there is positioned in the plenum 20 inside helical member 21 (preferably a stainless steel helical member 21), a hollow container 23, preferably a metallic container such as a stainless steel container, having a multiplicity of gas permeable openings in one portion of the container, preferably one end or cap 25 of the container, permitting gases and liquids entering the plenum 20 to enter the container 23. In container 23 is disposed an additive of a getter 24 comprised of barium or barium alloys containing one or more metal alloying components in addition to barium such as aluminum, zirconium, nickel, titanium and combinations of the foregoing. The getter is preferably in particulate form, to maximize the surface area per unit weight of the getter available to react with the gases and liquids entering container 23. Generally the alloying components will constitute up to 15 weight percent of the alloy with the balance being barium. However certain advantageous alloys above 15 weight percent are also contemplated in this invention. One such preferred alloy is about 50 weight percent aluminum with the balance being barium; another such alloy is about 10 weight percent nickel, about 40 weight percent aluminum with the balance being barium; and still another such alloy is about 15 to about 20 weight percent zirconium with the balance being barium. While FIGS. 2 and 3 present a preferred embodiment of the getter of this invention, additional physical forms of the getter can be utilized in plenum 20 including foil, sheet, films, wire, rod, bar and combinations of the foregoing. These other physical forms may be placed in the plenum, preferably inside the helical member 20 and preferably in a container such as stainless steel container 23. The container 23 in FIGS. 2 and 3 is preferably in the form of a right circular cylinder although any other configuration for the container is suitable. One end or cap 26 and the cylindrical wall portion 28 are solid metal, preferably a stainless steel, and the other end or cap 25 is preferably a screen material and preferably stainless steel screen of about 400 to about 32 mesh. The container is assembled by welding, brazing or otherwise sealing the solid end and the screen end into the hollow cylindrical wall portion 28. The ends or caps 25 and 26 are preferably concave or recessed into the cylinder as shown in FIG. 3 to facilitate welding. An effective amount of the getter is charged into the container with one end open, preferably the screen end open, and an end closure is then effected typically by spot welding. Preferably about 5 .+-. 1 grams of getter are used in a fuel rod containing about 5 kilograms of sintered nuclear fuel material (or generally about one gram of getter per kilogram of fuel material). Larger quantities of getter are used in powder fuel rods and in fuel rods suspected of containing large amounts of deleterious gases. The preferred use of the getter container 23 disclosed herein results in additional advantages. The container 23 insures retention of the particulate getter and any reaction products resulting from reaction of the getter with reactive gases in the fuel element. In this manner particulate material from the plenum will not be capable of entering the portion of the fuel element occupied by the nuclear fuel and the getter reaction products will be retained in the plenum, the lower temperature portion of the fuel element. This keeps the getter reaction products at lower temperatures and minimizes the chance of exposing the reaction products to higher temperatures tending to release the reactive gases combined to form the reaction product. The container 23 is easy to load, can be fabricated within very close dimensional tolerances, and has excellent dimensional stability due to the strength of the metal forming the cylindrical wall portion. Further the strength of the metal forming the cylindrical wall portion minimizes deformation of the container during handling and assembly of the fuel element. In another embodiment of the container, one or more openings can be made in one portion of the cylindrical wall portion 28 to give gases access to the getter. This embodiment may retain the screen end cap 25 or have a solid end cap replace the screen end cap 25. The getter used in the nuclear fuel element of this invention and its properties will now be described in detail. It has been discovered that a material suitable for controlling moisture and other reactive gases by chemically combining with such gaseous materials, namely a getter, should have a combination of properties. One desirable property is the minimization of any free hydrogen after the chemical reaction of the getter with water, as the minimization of free hydrogen prevents any possible hydride failures of cladding for nuclear fuel elements. Thus the getter should react approximately stoichiometrically with the water and water vapor (both herein called water) and in such a way that there is a negligible net source of hydrogen from the reaction. The getter should also rapidly react with the water at the temperature prevailing in the system in which the getter is utilized. The getter should generally have a low neutron cross section and be inexpensive to fabricate. The getter should also have the property of reacting with hydrogen, other reactive gases such as carbon monoxide, carbon dioxide, oxygen, nitrogen, and hydrogen-containing compounds such as hydrocarbons. Barium and the barium alloys disclosed herein have the foregoing properties and can be readily purchased or fabricated in a form of small particles having a Tyler Screen mesh size in the range of about No. 1 to No. 8, giving a high surface area for reaction with any reactive gases present in the fuel element. Barium alloys containing one or more metals in addition to barium such as aluminum, zirconium, nickel, titanium and combinations thereof can be readily obtained commercially and when available in the foregoing size range provide a high surface area reactive with any reactive gases present in the fuel element. The impurity content of the barium-containing materials is not critical to the development of the foregoing getter properties and substantial amounts of impurities can be included in the fabricated barium-containing materials as long as the surface of the barium-containing materials has barium effectively exposed for reaction. In practice it has been discovered that oxygen contents up to several thousand parts per million in the barium-containing materials are tolerable. Nitrogen contents up to about 750 parts per million are tolerable in utilization of the barium-containing materials. The other impurities found in the barium-containing materials used in this invention which do not hinder their use as getters in nuclear fuel rods include hydrogen and carbon. Metallic impurities found in the barium-containing materials which do not hinder use of the barium-containing materials as getters are hafnium in amounts up to about 1000 parts per million or more, iron in amounts up to about 1000 parts per million or more and chromium in amounts up to about 1000 parts per million or more. The fact that the impurity content of the barium-containing materials is not critical to their utilization as moisture getters enables fabrication of the barium-containing materials from corresponding low-grade metallic components. Since the barium-containing materials are utilized in the plenum of the fuel elements, small amounts of impurities of high neutron absorption cross section offer negligible interference. The barium-containing materials used in this invention have the property of reacting with water for long periods of time at a rapid rate of reaction over a temperature range of about room temperature (typically about 70.degree. F) up to fuel element plenum temperature (typically 650.degree. .+-. 100.degree. F) without becoming passive. During reaction with water, the barium-containing materials leave substantially no free hydrogen so cladding used in association with the getters of this invention would be exposed to substantially no hydrogen thereby eliminating formation of metallic hydrides which ultimately lead to weakening or failure of the cladding. This minimum release of hydrogen during the reaction of the barium-containing materials with water indicates a substantially stoichiometric reaction of the barium-containing materials with water. Studies indicate that the barium-containing materials used in this invention readily react with hydrogen over a temperature range of room temperature to reactor operating temperatures so that these materials are efficient hydrogen getters. The barium-containing materials also react with hydrogen-containing compounds such as hydrocarbons and with other gases such as nitrogen, carbon dioxide, carbon monoxide and oxygen. The barium-containing materials have a low neutron cross section required for use in nuclear applications when the impurities having high neutron cross section are minimized. 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.