Uranium is present in the Earths crust at an average concentration of 2 parts per million. Acidic rocks with high silicate, such as granite, have higher than average concentrations of uranium, while sedimentary and basic rocks have lower than average concentrations. Uranite or pitchblende (U3O8), the most common uranium-containing ores, are mixtures of UO2 (basic) and UO3 (amphoteric) oxides. The richest ores are found in the western United States, Canada, Australia, South Africa, the former Soviet Union, and Zaire (the former Belgian Congo). The concentration of U3O8 in ores can vary from 0.5% in Australian ores to 20% in Canadian ores.
Extraction of uranium is often difficult and the metallurgical procedures vary with the geological environment of the ore. The ore is first crushed and ground to liberate mineral particles. The amphoteric oxide is then leached with sulfuric acid.
UO3(s) + 2H+(aq) --> UO22+(aq) + H2O
UO22+(aq) + 3SO42-(aq) --> UO2(SO4)34-(aq)
The basic oxide is converted by a similar process to that of a water soluble UO2(CO3)34-(aq) ion.
Two methods are used to concentrate and purify the uranium: ion exchange and solvent extraction. Solvent extraction, the more common method, uses tertiary amines in an organic kerosene solvent in a continuous process.
First the amines, R3N, react with sulfuric acid:
2 R3N(org) + H2SO4(aq) --> (R3NH)2SO4(org)
Then the amine sulfate extracts the uranyl ions into the organic phase while the impurities remain in the aqueous phase. In the case of the uranyl sulfate ion, the following reactions occur:
(R3NH)2SO4(org) + UO2(SO4)34-(aq) --> (R3NH)4UO2(SO4)3(org) + 2SO42-(aq)
The solvents are removed by evaporating in a vacuum and ammonium diuranate, (NH4)2U2O7, is precipitated by adding ammonia to neutralize the solution. The diuranate is then heated to yield a purified, solid U3O8, known as yellow cake.
Refining and converting U3O8 to UF6
At the refinery, the yellow cake is dissolved in nitric acid. The resulting solution of uranium nitrate, UO2(NO3)2· 6H2O, is fed into a continuous solvent extraction process. The uranium is extracted into an organic phase (kerosene) with tributyl phosphate, and the impurities remain again in the aqueous phase. After this purification, the uranium is washed out of the kerosene with dilute nitric acid and concentrated by evaporation to pure UO2(NO3)26H2O. Heating yields pure UO3. The initial separation and refining processes generate large volumes of acid and organic waste.
It is necessary to enrich the U-235 isotope concentration from its natural composition of 0.7% for use in either reactors or bombs. Reactor grade uranium contains from 3.5 to 4.0% U-235, while the Hiroshima uranium bomb contained more than 80% of the lighter U-235. The process used for enrichment involves gaseous diffusion and thus the uranium must be converted to a gaseous compound, uranium hexafluoride (UF6).
Conversion to the hexafluoride involves the following sequence of reactions.
The UO3 is reduced with hydrogen in a kiln:
UO3(s) + H2(g) --> UO2(s) + H2O(g)
The uranium dioxide is then reacted with hydrogen fluoride to form uranium tetrafluoride:
UO2(s) + 4HF(g) --> UF4(s) + 4H2O(g)
The tetrafluoride is then fed into a fluidized bed reactor and reacted with gaseous fluorine to obtain the hexafluoride:
UF4(s) + F2(g) --> UF6(g)
The hexafluoride is now suitable feedstock for the gaseous diffusion process.
Production of uranium metal
Uranium metal is produced by reducing the uranium tetrafluoride with either calcium or magnesium, both active group IIA metals that are excellent reducing agents.
UF4(s) + 2Ca(s) --> U(s) + 2CaF2(s)
This reduction may be done before or after the enrichment process, depending on the intended use of the uranium. Reactors use both enriched (3 to 5% U-235) uranium metal and uranium oxide as fuel while weapons use more highly enriched uranium (up to 90% U-235).
Reactor fuel assembly
Complete Bibliography on Uranium from the ALSOS Digital Library for Nuclear Issues