Patent Number: 039754715
Section: description

The invention is further explained in the following examples. EXAMPLE 1 As fuel particles there were employed spherical kernels of UO.sub.2 having a grain diameter of 220 .mu.m. These particles were provided with a three layer coating of a pyrolytically deposited coating, the total thickness of the coating being 180 .mu.m. An intermediate layer of SiC having a thickness of 23 .mu.m was present between the two outermost carbon layers. (The thickness of the SiC layer is included in the total thickness of the carbon layers). The particles having a diameter of 580 .mu.m and a density of 2.3 g/cm.sup.3 contained 21.85 weight % uranium. The fertile particles of ThO.sub.2 having a grain diameter of 617 .mu.m were provided with a double layer coating of carbon (also pyrolytically deposited) having a total thickness of 144 .mu.m. The particles having a diameter of 905 .mu.m and a density of 3.99 g/cm.sup.3 contained 63.23 weight % thorium. As the molding powder there was employed a mixture consisting of 64 weight % natural graphite, 16 % graphitized petroleum coke and 20 % of a thermoplastic phenol-formaldehyde novolak resin as a binder. The fuel and fertile particles were overcoated in separate processes with the molding powder with addition of methanol in a rotating drum. The amounts supplied were so selected that in the finished compacts there was present a graphite matrix portion of about 55 volume %. There was sprayed on the finished overcoated particles stearic acid as a 5 % solution in trichloroethylene as the lubricant together with 1 % hexamethylenetetramine as the hardener. Based on the binder portion in the matrix powder the hardener addition amounted to 3 weight % and the lubricant addition to 15 weight %. The particles were dried at room temperature. 5.2 grams of overcoated fuel particles and 49 grams of overcoated fertile particles after admixing were filled into a cylindrical compression mold having a diameter of 16 mm and a length of 180 mm and held constant at 150.degree.C. and compressed from both sides. The amounts given correspond at a matrix density of 1.70 g/cm.sup.3 to a cylinder length of 100 mm. This length was attained at a compression of less than 20 kp/cm.sup.2. After expulsion, the compacts underwent a closed heat treatment at 1800.degree.C. Subsequent electrolytic decomposition of over 100 of the compacts in dilute nitric acid and fluorometric uranium analysis gave values of 7 to 15 micrograms of free uranium and the chemical thorium analysis of 10 to 25 micrograms per compact which is less than half the heavy metal content of one single particle. Thus, it is proven that no particles are destroyed by this process of the invention. The found values only depend on a surface contamination with uranium or thorium. EXAMPLE 2 Using the process of published German application DAS 2 215 577 as described on column 4, lines 1-34 FIG. 1 and Example 1, there was employed a molding powder of the same composition as for the overcoating and the composition was compressed in cavities of an elastic rubber disc to form three-dimensional isotropic granulates having a 1.5 mm particle size. The entire disclosure of the German application DAS 2 215 577 is hereby incorporated by reference. A mixture of 10.4 grams of overcoated fuel particles and 24.5 grams of overcoated fertile particles according to Example 1 and 15 grams of the granulates produced in the elastic rubber disc were coated wih hardener and lubricant and compressed in a manner analogous to Example 1. The length referred to (100 mm) at a matrix density of 1.7 g/cm.sup.3 was already obtained at about 20 kp/cm.sup.2. The uranium analysis carried out analogous to Example 1 showed 15 micrograms of free uranium. Free thorium was ascertained as 20 micrograms, which likewise proves there was a compression free of particle damage. The composition applied to the overcoat outer surface can consist of or consist essentially of the hardener and lubricant.