Patent Number: 040175677
Section: description

Referring more specifically to the drawings the block fuel element 1 consists of a hexagonal prism made from graphite which contains the 18 fuel zones 2 made of a mixture of graphite and coated particles. Furthermore, there are provided 54 cooling channels 3 and a central loading channel 4. The cooling channels can be molded simultaneously with the molding of the block fuel element in order to avoid an inadmissible pressure build up of the cracking products inside the block in the carbonization. For this purpose corresponding metal rods are pressed in and are then removed after the molding. To carbonize the binder the fuel element is calcined in a nitrogen stream up to 800.degree. C. To balance the dimensional changes of the fuel containing zones with the fuel free zones during the carbonization there the degree of condensation of the phenol formaldehyde resin used for encasing is increased. This is obtained by the addition of small amounts of hexamethylene tetramine to the binder resin. To further explain the invention there are set forth several examples. Unless otherwise indicated, all parts and percentages are by weight. EXAMPLE 1 The Production Of A Cylinder Of Isotropic Granulate Without Fuel A molding powder was produced from a mixture of 60 weight percent natural graphite powder "FP", 20 weight percent graphitized petroleum coke powder and 20 weight percent of "1949 resin binder" dissolved as a 40 percent solution in methanol by kneading, drying and grinding. The FP powder is a nuclear pure natural graphite having an ash content of 200 ppm., an average particle diameter of 20 microns and a high crystallinity (crystal size Lc = 1000A). The graphitized petroleum coke is a needle coke graphitized at 3000.degree. C. having an extremely low ash content (ash less than 10 ppm), an average particle diameter of 20 microns and a crystal size Lc of 500 A. The 1949 binder is a phenol formaldehyde resin having a high degree of condensation (softening point of 100.degree. C., molecular weight 700), which remains stable without change in properties while molding at 150.degree. C. Spheres having a diameter of 62 mm. and a density of 1.9 g/cm.sup.3 were prepared by molding the molding powder in rubber molds at room temperature and a pressure of 3 t/cm.sup.2 (t being metric tons). In spite of the platelet shaped particles of the natural graphite powder, the molding in the rubber molds permitted the formation of an isotropic consolidation. The anisotropic factor of the thermal expansion measured on the spheres was only .alpha..perp..alpha..parallel.= 1.1. The isotropic granules used having a particle diameter of 3.15 &gt;d &gt;0.315 mm were obtained by comminuting the graphite spheres and subsequently sieving. Cylinders were molded from the isotropic granules at 150.degree. C. in steel dies. At a green density of 1.8 g/cm.sup.2 the required molding pressure was only 60 kg/cm.sup.2. The pellets were carbonized in an inert gas stream and finally calcined in a vacuum at 1800.degree. C. The following table sets forth the properties of the matrices produced from isotropic granules and from molding powder Table ______________________________________ From From Graphite Isotropic Molding Matrix Granules Powder ______________________________________ Bulk Density (g/cm.sup.3) 1.76 1.65 Specific electric resistance 1.39 1.18 (Ohm .times. cm) .times. 10.sup.-.sup.3 1.97 3.40 Ultimate Flexural strength 160 300 (kg/cm.sup.2) 120 150 Thermal conductivity 0.16 0.18 at 20.degree. C. (cal/cm sec. .degree. C) 0.13 0.07 Linear thermal 2.15 1.6 expansion (10.sup.-.sup.6 /.degree. C.) 3.21 5.6 Anisotropic factor of the thermal expansion 1.49 3.5 ______________________________________ = parallel to the grain orientation = right angle to the grain orientation The table clearly shows that according to the invention all properties of the graphite matrix in regard to isotropy are improved considerably. This follows most clearly from the anisotropic factor which is reduced from 3.5 to 1.49. Therewith the invention even permits the molding of quasi isotropic and homogeneous cylinders from a natural graphite powder having the advantage of high crystallinity with extremely unfavorable platelet shaped grains. EXAMPLE 2 The Production of Block Elements The isotropic granules were produced as in example 1. In the comminution of the spheres and the subsequent sieving about 30 weight percent were below the desired particle size (d &lt; 0.31 mm), which were returned to the molding powder and the mixture again molded to spheres. A cylinder having a diameter of 240 mm and a height of 450 mm was preliminary molded from the isotropic granulate at 70.degree. C. and a pressure of about 30 kg/cm.sup.2. There were pressed into the inside of the cylinder parallel to the longitudinal axis in a hexagonal arrangement 19 metal tubes having diameters of26 mm. After the removal of 18 tubes there were obtained channels for the admission of fuel. The fuel particles used were uranium-thorium oxide nuclei having a diameter of 500 microns which were coated with pyrolytic carbon andan intermediate coating of silicon carbide. The coated particles were encased with the molding powder in a rotating drum according to a kind of dragee process that their weight rose by a factor of 1.8. From the encased fuel particles there were preliminarily moded at about 70.degree. C. and 30 kg/cm.sup.2 cylinders having a diameter of 25.5 mm. After filling of the 18 channels with fuel cylinders, the entire block was heated to about 150.degree. C. and finally molded at a pressure of about 60 kg/cm.sup.2. After the molding the matrix density was 1.8 g/m.sup.3 at a 35 percent volume fraction of the coated particles in the fuel zone. Subsequently for the purpose of simplicity, there were bored 54 cooling channels in a hexagonal arrangement around the fuel columns. EXAMPLE 3 Production of Block Elements With Special Binder Pretreatment The fabrication of the isotropic graphite granules and the preliminary pressing of the block were carried out in the same manner as described in examples 1 and 2 respectively. The graphite matrix powder prepared to overcoated fuel particles was a mixture of 60 weight percent natural graphite powder. Grade FP, 20 weight percent of phenol formaldehyde resin, so for the same mixture as used for fabricating the granulate. However the resin used for overcoating contained 2 weight percent of hexamethylene tetramine as curing agent which in the heating step following after pressing at first leads to a large condensation of this resin and later on to a balanced carbonization shrinkage of the fuel body compared with the surrounding block matrix. Parallel thereto, under the same production condition, molded cylinders with a particle load in the fuel zone of 35 volume percent were, after final heat treatment, tested for damaged fuel particles. The graphite matrix of the samples was decomposed electrolytically (anode oxidation) and the electrolyte (dilute nitric acid) tested for free uranium. The total amount of uranium found amounted to 13 micrograms. This corresponds to only a third of the amount of uranium of a fuel particle. The result clearly shows that in production according to the invention, the coated particles remain completely uninjured.