Uranium Production

Uranium processing is closely related to uranium fuel cycle and consists of four steps which may be distinguished by processed isotopes. The main products are uranium compounds with natural isotopes ratio (the goal is concentration and refinement, preparation for isotopes separation and Pu manufacturing); compounds enriched by Uranium-235 (for nuclear power setups fuel elements in the form of uranium dioxide or alloys, as well as nuclear weapon production); compounds with depleted uranium for safe storage and alternative use; compounds obtained from irradiated nuclear fuel for radiochemical manufacturing: Plutonium and Neptunium separation, refining from fission products, preparation for isotopes separation and fuel elements recycling). Also the uranium-thorium nuclear fuel cycle technologies (high-temperature nuclear reactors which utilize the mixture of thorium-232 and Uranium-233 oxides or carbides) as well as the uranium-plutonium cycle are founded (fast neutron reactor reactors with Plutonium-239 and Uranium-238 dioxides mixture as fuel).

The isolation of the metal in a state of purity is accomplished only with difficulty. Three methods of procedure have been suggested, namely, (1) by reduction of urano-uranic oxide, (2) by reduction of uranous chloride, and (3) by electrolysis. The best results have been obtained by the first method, which also possesses an advantage in using as the starting material an oxide which is a commercial product.

Reduction of Urano-uranic Oxide, U₃O₈

Type of furnace used by Rideal for the preparation of uranium. AA, Magnesia tube. BB, Carbon resistance. CC, DD, Electrodes.

This was first accomplished by Moissan, who heated 500 parts of the oxide with 40 parts of sugar carbon in an arc furnace. Complete reduction only takes place above 1500° C. On a commercial scale, uranium of 96 to 98 per cent, purity can be obtained. The product always contains carbon. The oxide may also be reduced by heating with magnesium or aluminium powder. Rideal obtained uranium of 98 to 99 per cent, purity by heating, in a tube of pure magnesia, a mixture containing 80 per cent, of the oxide and 20 per cent, of magnesium powder which had been pressed into rods by means of an oil press. The tube was heated in an apparatus (fig.) which was a combination of resistance and arc furnace. The apparatus was first filled with hydrogen, and when the temperature had been raised sufficiently by resistance heating, the rods became conducting, and an arc was passed through the tube from a second pair of electrodes connected with 110-volt mains, whereupon the magnesium vapour reduced the uranium oxide to the metal. The finely divided "uranium black" produced was in a pyrophoric condition, so that it could not be immediately exposed to the air; the hydrogen was therefore first displaced by carbon dioxide. Excess of magnesium was removed by washing with dilute acetic acid. By heating urano-uranic oxide with sodium and magnesium, in presence of calcium chloride, in a steel cylinder, to bright red heat, Rideal obtained a greyish-black powder containing 99.4 to 99.6 per cent, uranium with traces of iron.

Reduction of Uranous Chloride

The metal was first produced by Peligot by the action of potassium on uranous chloride, and Zimmermann obtained the metal as a black powder by reducing with sodium in presence of sodium chloride. Rideal, repeating Zimmermann's experiments, and heating at 800° C. in a resistance furnace for forty-five minutes, obtained, after washing with alcohol and water, a brown powder containing 99.3 per cent, uranium. According to Moissan the product is liable to be impure to a varying degree with iron (up to 2 per cent.) and sodium chloride; he therefore used the double chloride, UCl₄.2NaCl, which is less hygroscopic than uranous chloride, and obtained a product which he considered was 99.4 per cent, uranium. Moore also obtained an exceedingly pure product by reducing carefully purified uranous chloride with pure sodium by heating in vacuo in a steel bomb, and extracting the metal in an atmosphere of dry nitrogen. Substitution of magnesium for sodium, and of uranous fluoride for uranous chloride, have been suggested, but the results are not satisfactory.

Electrolytic Methods

Moissan obtained the metal by electrolysis of the double sodium uranium chloride in an atmosphere of hydrogen, using carbon electrodes. Feree, by electrolysis of an aqueous solution of uranous chloride, using a mercury cathode, obtained a uranium amalgam, from which he isolated the uranium by distilling off the mercury in vacuo. However, uranium cannot be obtained in any quantity by this means.