Patent Number: 041994043
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows an enlarged view of a very small portion of a fuel pellet made according to the present invention. Area 10 is a relatively high-density (greater than about 95 percent of theoretical) uranium carbide particle. Fabrication and surface-area consideration dictate that the particle size, on the average, be greater than around 50 microns. This particulate region contains depleted or naturally occurring uranium. According to the invention, any fertile material, such as uranium-238 silicide, uranium-238 nitride, or uranium-238 oxide, can make up the particles. Since carbide fuels are noted both for high density and high swelling rate and are of interest in the American fast-breeder effort, it is thought that the beneficial characteristics of the invention will be exhibited most advantageously in a carbide fuel, so carbide is chosen for the preferred embodiment. In addition to the fertile component, fissionable uranium is also present in the particles because natural, and even depleted, uranium always has some U.sup.235 in it. For that matter, fissionable plutonium or other materials could be included in the particles; according to the invention, as much as 10 percent of the fissile content of the pellet may be contained in the particles. However, the advantages of the present invention are greatest when the fissile component of the particles is kept low, since it is desired that gaseous fission products not be produced in quantity in the high-density regions of the fuel. Uranium carbide in this particulate form makes up between 60 percent and 90 percent of the total volume of the pellet in the preferred embodiment. The remainder of the pellet, between 10 percent and 40 percent by volume, is made of uranium-238-plutonium carbide, a fissile-fertile blend, in the form of a low-density powder 12 having a stable microstructure. The powder is, by methods known to the art, to have a surface area fabricated greater than about 0.5 m.sup.2 /g. This insures an interconnected pore structure that will allow the fission gases to escape. The powder must be fine enough to fill in between the particles in a manner similar to that in which mortar fills in between bricks, because this will allow a relatively homogeneous particle-powder mixture. This is among the reasons why the particle size is specified as being greater than about 50 microns; a smaller particle size could result in too great a surface area to be covered by the powder. According to the invention, the density of the powder is less than 85 percent of theoretical density and contains 90 percent or more of the fissile material. In the preferred embodiment some fertile component is also included in the powder because the melting point of pure plutonium carbide is low and tends to decrease as the fuel is burned up. The addition of uranium both increases the melting point and decreases the effect of burn-up. In order to keep the sintering and temperature-dependent properties of the powder similar to those of a uranium ceramic powder, the fissile-component percentage of the powder should be less than about 40 percent, but to maintain a high enough total-pellet density, the percentage should be above 20 percent. The preferable percentage range is 25 percent to 35 percent. The addition of uranium also increases the amount of powder in the pellet, which is desirable because the powder has a greater tendency than the particles do to give under pressure, so the tendency of pellet chips to damage the clad is reduced. Another advantage of the fertile-fissile blend is that the uranium content, together with the stable microstructure, reduces the amount of diffusion of plutonium into the high-density uranium region and the diffusion of the uranium from the high-density region to the low-density region during use of the fuel. Thus, a fissile-fertile blend is preferred for the powder. It is to be noted that the term blend is used. This is because blend is thought to have no definite chemical meaning, and it can therefore be defined, for present purposes, to refer to a structure having a crystal structure like that of the fertile component (uranium-238 carbide) with some of the fertile atoms (U.sup.238) being replaced by fissile atoms (Pu). Such a structure has been described by several terms, such as alloy, solid solution, and even mixture, but it is thought that each of these terms may be chemically incorrect. The term blend is therefore used for want of a definitely correct term. Since it is necessary for the fission gases to escape, it is a requirement that the powder have a stable microstructure. This means that the pores must remain in the fuel during burnup. Experience in the art of fabricating fuel for conventional reactors has taught methods of producing fuels having stable microstructures, and successful practice of the present invention requires that now-conventional techniques for guaranteeing a stable microstructure be employed in the fabrication of the fuel pellets. The particles are generally made by agglomerating the fertile material, produced as a highly sinterable powder, into particles greater that about 50 microns in diameter. "Burnt" (not highly sinterable) powders of fertile-fissile material are mixed to a high degree of homogenity with the agglomerated fertile particles. This mixture is then pressed into pellets and sintered. The process is arranged to produce a pellet structure containing high-density particles of fertile material dispersed in a low-density powder of fertile-fissile material having a stable microstructure. It is possible for experienced practitioners of this art to arrange the process so that the particulate fertile-material regions have densities greater than about 95 percent of theoretical, while the powder regions have densities below about 85 percent. Through the use of the fuel pellet of the present invention, maximum effect can be given to the density of carbide fuels, and an associated decrease in cladding damage can be effected.