Patent Number: 054220475
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to carbonaceous fuel particles and method for making same, particularly for making such particles containing fertile or fissile materials. 2. The Prior Art The most promising or advantageous fuels for high efficiency nuclear power plants or for rocket propulsion have included non-graphite, spherical fuel particles made from sol-gel such as uranium carbide spheres (UC.sub.2). Fuel made of such spherical particles however has a molten temperature at about 2654.degree. C. and thus needs to operate below, eg. 2200.degree. C. to avoid significant deformation. Attempts have been made to find a nuclear fuel that has a significantly higher melting temperature, to reduce the threat of melting and to achieve greater engine efficiency, e.g. for a gas turbine or for improved rocket engine specific impulse. Accordingly there is a need and market for such improved higher temperature nuclear fuel that lessens or obviates the above prior art shortcomings. There has now been discovered a nuclear fuel that has a significantly higher melting temperature than seen in the prior art and that concurrently can provide for greater efficiency of nuclear engine operation, including that of gas turbines and rocket engines for space vehicles. In related prior art, U.S. Pat. No. 4,035,452 to Davis et al (1977) discloses impregnating fuel particles with polymer, adding nuclear fuel and then joining the particles into a cohesive mass with a carbonaceous binder. U.S. Pat. No. 4,963,758 to Noren (1990) and U.S. Pat. No. 5,037,606 to DeVelasco, et al (1991) disclose coatings for fuel spheres made by others. In further prior art, an Article entitled "Preparation of carbon microbeads containing fine platinum particles from aqua-mesophase" Oct. 8, 1991, "Letters to the Editor" in Carbon, vol. 30, 1992 at pp. 120 & 122 by K Esumi, et al., discloses adding platinum to spheres made from aqua-mesophase in silicone oil for use, e.g. as a catalyst. Such microbeads are then heated to 1000.degree. C. to form carbon microbeads. There is no suggestion of preparing a fuel from such process, nor are carbon microbeads converted to graphite microbeads and the size of the microbeads ranges from 10-20 microns. SUMMARY OF THE INVENTION Broadly the present invention provides a method for making fuel particles having metal carbides dispersed in spherical graphite skeletons comprising: a) adding pitch-derived aqua-mesophase to a desired amount of alkaline solution (pH=9-12), to form a first solution, PA1 b) adding a metal salt to the first solution to form a fuel solution, PA1 c) adding said fuel solution to an oil bath to form an emulsion of aqueous spheres in oil and heating and stirring same until spheres with metal salts dispersed therein, dry to form solid spheres in the oil bath, PA1 d) filtering the solid spheres out of the oil bath and rinsing and drying the spheres, PA1 e) heating the solid spheres to 700.degree. C. to 1100.degree. C. for 1-24 hours to carbonize such spheres from pitch to amorphous carbon and convert the metal salts to metal oxides, PA1 f) heating the solid spheres to 2000.degree. to 3000.degree. C. for 2-10 hours, to carburize the metal oxides to form fertile or fissile carbides and graphitize the amorphous carbon to form the fuel particles and PA1 g) depositing a protective carbon-based coating on the fuel particles. PA1 a) a spherical graphite skeleton, PA1 b) metal carbides dispersed in the skeleton and PA1 c) a deposit of carbon-based coating covering the skeleton and the metal carbides dispersed therein. Definitions: The invention also provides a fuel particle comprising: By "dispersed in the skeleton" as used herein, is meant, both in and on the skeleton. By "pitch" as used herein, is included natural pitch and synthetic pitch. Natural pitch includes pitch from coal tar and petroleum residues and is discussed at length in Chemistry and Physics of Carbon, vol. 15, 1978, pp. 229-286, by H. Marsh and P. L. Walker and references therein, which Article is incorporated herein by reference. For discussion of synthetic pitch see Carbon, vol. 30, 1992, pp. 55-61 by I. Mochida et al and references therein, which Article is incorporated herein by reference. Aqua-mesophase is a carbon-containing material derived from mesophase pitch, which can be of natural or synthetic origin. For a further discussion of aqua-mesophase material and a discussion of the preparation thereof, see the above cited Article by K. Esumi, which Article is incorporated herein by reference. By "fissile metal salt" as used herein is meant salts of uranium (U) and plutonium (Pu), including any cation and anion containing U or Pu, e.g. uranyl nitrate, UO.sub.2 (NO.sub.3).sub.2, uranyl acetate, UO.sub.2 (CH.sub.3 COO).sub.2 and uranyl acetylacetonate, UO.sub.2 (C.sub.5 H.sub.7 O.sub.2).sub.2. By fertile metal salt as used herein is meant salts including any cation and anion containing Th, e.g. Th(NO.sub.3).sub.4 and Thorium nitrate. The fuel solution is aqueous and when added to the above oil bath, forms an emulsion of liquid pitch-derived spheres in oil. The bath is stirred under heated to, e.g. 80.degree. C. for several hours to drive off most of the water or water vapor, from the pitch-derived spheres (eg to dry and solidify the spheres), many of which remain suspended in the oil bath. Thus in a so dried pitch-derived sphere, e.g. only 10.0 to 0.1 wt % water remains. By "carbon-based" coating, as used herein, is meant a carbon or carbide coating.