The discovery of fission led to two potential routes to the production of fissile material for the first nuclear weapons by the United States in the 1940s. The first involved separating uranium-235 from uranium-238 isotopes in natural uranium by gaseous diffusion. The second path produced plutonium-239 by bombarding fertile uranium-238 in a nuclear reactor. But both approaches began with mining of uranium ore. Today, the production of fissile fuel for nuclear power reactors uses many methods originally developed for producing nuclear weapons. This unit addresses the metallurgy of uranium, its conversion into gaseous uranium hexafluoride required for enrichment processes, and the fabrication of fuel rods from the enriched uranium hexafluoride. The enrichment processes are covered in a separate unit.
Mining and Preparation of Yellowcake
Figure 1 – Drums of Yellowcake
Courtesy of the U.S. Department of Energy
The ore is first crushed and ground to liberate mineral particles (Figure 2). An amphoteric oxide is then leached with sulfuric acid.
A basic oxide is converted by a similar process to the water-soluble UO2(CO3)34-(aq) ion.
Figure 2 – Preparation of Yellowcake
Courtesy of the Uranium Information Center
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.
2 R3N(org) + H2SO4(aq) → (R3NH)2SO4(org)
At the refinery, the yellowcake 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.
Because the uranium isotopes have identical chemical properties, the processes employed for enrichment must use physical techniques which take advantage of the slight differences in their masses. The two enrichment methods used today, centrifugation and diffusion, require that the uranium be in a gaseous form, uranium hexafluoride, UF6(g). Although enrichment involves physical processes, chemistry plays an important role in synthesizing UF6 gas and returning the UF6 enriched in U-235 to a solid, UO2.
Uranium hexafluoride is now suitable feedstock for the gaseous diffusion or centrifugation enrichment processes.
Production of uranium metal
Production of solid fuel rods from uranium hexafluoride gas enriched in U-235 requires another series of chemical and metallurgical processes (Figure 3).
Figure 3- Production of Fuel Rods from UF6
Courtesy of the U.S. Nuclear Regulatory Commission
The uranium hexafluoride is first reduced to uranium tetrafluoride with hydrogen.
Reactor fuel consists of ceramic pellets formed from pressed uranium oxide, which is sintered (baked) at a high temperature (over 1400°C). The pellets are then placed in metal tubes made of a zirconium alloy or stainless steel and sealed in an atmosphere of helium to form fuel rods. The fuel rods are then grouped in clusters to form the fuel assemblies, which are placed into the reactor core (Figure 4). The individual rods for a pressurized water reactor (PWR) are about 1 inch in diameter and 4 meters in length. Fuel assemblies for PWRs contain from 179 to 264 rods, and a fully fueled PRW will contain from 121 to 193 assemblies. A PWR must be shut down for refueling. This occurs at intervals of 1 to 2 years, when about a third of the fuel assemblies are replaced. The spent fuel assemblies are removed to cooling pools at the reactor site.
Figure 4- Components of a Nuclear Fuel Assembly
Courtesy of Areva NP Inc.
Complete Bibliography on Uranium from the Alsos Digital Library for Nuclear Issues.
|2005-2009 Kennesaw State University
Principal Investigator Laurence Peterson
Project Director Matthew Hermes