Patent Number: 059329300
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is a process for fabricating weapons-grade plutonium into mixed-oxide fuel for use in a nuclear reactor. The preferred form of plutonium feed material for processing in the fuel fabrication plant is plutonia, PuO.sub.2. Therefore, the metallic weapons-grade plutonium must be converted into ceramic grade plutonia, which is delivered to the site of the fuel fabrication facility. Plutonia made from weapons-grade plutonium is of sufficiently high purity that no further refining to further remove impurities is necessary, with the possible exception of gallium. There are several alternative methods for converting the plutonium into plutonia: (1) controlled burning of the metal to form the oxide and followed by a mechanical milling step; (2) controlled burning of the metal followed by dissolution and precipitation process steps; and (3) controlled hydriding, dehydriding and oxidation process. The preferred method for converting plutonium into plutonia is controlled burning and subsequent milling. The potentially reduced activity of plutonia may tend to limit the mixed oxide composition to a maximum of about 10% plutonia and allows pellet densities in the low 90s range. This process provides the lowest cost approach and is expected to produce plutonia which meets the fuel performance requirements for plutonium disposition. If plutonia of higher activity or finer particle size is required, then the second and third alternative processes may be used. However, the hydride-dehydriding process coupled with a controlled oxidation operation would also provide plutonia with higher activity or finer particle size. The hydriding step allows better control of the final oxidation process. After the plutonium has been converted into plutonia powder, the plutonia powder is pressed into high-density pellets and fired in a sintering furnace for transport to the fuel fabrication facility. Conventional comminution techniques are used to reduce the plutonia pellets to a fine powder for blending with urania powder to produce mixed oxide feed material acceptable for reactor operation. More specifically, the high-density plutonia pellets were broken up and ground to produce a fine powder which does not tend to agglomerate. The plutonia was ground until it passed through a 325 mesh screen (&lt;44 microns). Then urania powder was blended with the ground and sized plutonia to produce a mixed oxide powder blend in accordance with conventional mechanical blending techniques. The process for fabricating fine plutonia feed powder into oxide fuel pellets has been utilized to fabricate both urania and mixed-oxide fuel for light water reactors, mixed oxide fuel for liquid metal reactors and uranium nitride fuel for space reactor applications. The basic mill-slug-granulate process has been well developed for over 20 years. In this process the feed powders are blended together in the desired concentration, milled to improve inhomogeneity and pre-pressed and granulated to form free-flowing powder for pellet pressing. The resulting pellets are sintered in a reducing atmosphere for 1 to 5 hours at 1600.degree. C. to 1750.degree. C., resulting in pellets which are acceptable from the plutonium homogeneity standpoint. The pellets are then ground to size, inspected and loaded into fuel rods. The fuel rods are sealed by welding, inspected and assembled into fuel bundles. The completed fuel bundles are stored at the fabrication facility until they are needed to provide initial or reload assemblies for a reactor. The relatively low radioactivity associated with weapons-grade plutonium will allow the fuel fabrication activities to be performed in a glove box arrangement. The requirement for minimized worker exposure is handled by mechanizing all unit processing operations, which will reduce the time the operator is in close proximity to the nuclear material. Also, localized shielding will be installed and lead glass and sheet added as needed to the exterior of the glove boxes. The feed materials for the fuel fabrication process which utilizes powder blending are the oxide forms of uranium, plutonium and gadolinium. The urania (either natural or depleted) will be supplied as an active powder capable of sintering to high density. The plutonia must meet the requirements of ASTM C757, Nuclear-Grade Plutonium Dioxide Powder, Sinterable. In the fuel pellet fabrication process the feed oxides are transferred from storage to geometrically safe holding bins. Weight feeders are used to load the appropriate amount of materials into a vibromill where the powders are blended and milled together. The resulting blend is split into as many as eight increments and the subsequent eight increments are cross blended to form a large uniform feed lot of powder. The blend splits are stored in a geometrically safe arrangement and removed for continued processing one at a time. During reblending or cross-blending, each blend split is vibromilled to smooth blend differences and to produce an agglomerated powder for feed to pellet pressing. Small hydraulic pellet presses developed for glove box use are utilized to press pellets. Each press can optionally be provided with multi-cavity dies, depending on the throughput requirements. The green pellets are loaded into molybdenum sintering boats. Specially designed sinter furnaces with removable refractor sections are utilized to sinter the mixed-oxide pellets. The oxygen atmosphere of the sintering furnace is controlled to obtain fuel pellets of the desired stoichiometry. All of the sintered pellets are passed through a centerless grinding station after inspections for sintering characteristics to control the pressing operations. The grinder has an automatic control feedback loop which will automatically adjust the grinding to maintain the pellet within limits. The column of pellets which exit the grinding operation pass directly to the automated pellet inspection station. At the station, the pellets are inspected and sorted for geometric properties (diameter, length, density, surface finish, cracks and pits). After sampling for chemical characteristics, the pellets are placed into storage to await certification approval for fuel loading. The certified fuel pellets are aligned into columns, where final column geometry is obtained prior to loading into fuel cladding (tubes with one end plug welded in place).