Uranium feed materials processing

Constraints of Plant Design. The potential hazards encountered in the uranium feed materials processing industry include many that are common to the heavy chemicals industry. However special problems present themselves owing to direct radiation, the possibility of inhalation and ingestion of radioactive dusts and gases, nuclear safety, and more unusual chemical hazards. 

At various facilities that process uranium for defense programs, uranium is released to the atmosphere under controlled conditions, resulting in deposition on the soil and surface waters. Monitoring data from the area surrounding the Fernald Environmental Management Project (formerly the Fernald Feed Materials Production Center) showed that soil contained uranium released from the facility (Stevenson and Hardy 1993). 

The present process still depends on the production of UF4 as a pure intermediate which may be reduced to metal for fueling the Magnox reactors or further fluorinated with fluorine gas to produce UFg, the essential feed material for all of the uranium isotopic-enriching processes. 

Typically, the input flow of UFs is adjusted so that roughly half of the feed diffuses through the walls of the barrier tubes into interstitial space, becoming enriched (product), while the rest remains in the tubes, becoming depleted (tails). To obtain significant enrichments, this process must be repeated multiple times (Krass et al. 1983). In a cascade of diffusion cells, the product of one enrichment cell is used as feedstock for another. The cascade is simple if the tails are discarded the cascade is countercurrent if the tails are reintroduced as feed in a lower emichment stage. Simple cascades are not used because of the potentially profligate waste of uranium. After the initial start up of a countercurrent cascade, feed material is introduced only in the amount necessary to balance the withdrawal of product and tails. 

In Chapter 2, we take a more detailed look at the analytical chemistry pertaining to key commercial activities, that is, uranium mining and its utilization in the nuclear fuel cycle (NFC) first, in the milling process, uranium-containing deposits are processed to form uranium ore concentrates (UOC) that are then shipped to uranium conversion facilities (UCF), where the uranium is transformed into high-purity nuclear grade compounds. These can serve as fuel for nuclear power plants or as feed material for isotope enrichment. Then we discuss the analytical aspects of compliance with the strict specifications of the materials used in enrichment plants and in fuel fabrication facilities. Finally, we deal with the analytical procedures to characterize irradiated fuel and waste disposal of spent fuel. 

The product, called uranium ore concentrate (and sometimes yellow cake), contains 65%-85% UjOg, is then shipped to the UCF where the uranium is dissolved and concentrated, and then pnrifled and converted either to the proper form needed for fnel elements (usnally nraninm oxide for graphite type or heavy water reactors) or to the feed material reqnired for isotope enrichment (usually uranium hexafluoride) (Figure 1.10). Following is either fabrication of fuel elements or enrichment to LEU for fneling light water reactors or to HEU for special reactors or nuclear weapons (special nnclear materials (SNM)). The product of the enrichment process, either LEU or HEU, mnst then be converted into the suitable form for the applicatiou— once again nsnally an oxide or metal. 

In natural uranium ores, the fraction of the atoms of the fissile isotope is about 0.72%. For many commercial applications, like production of fuel for light water reactors or several types of research reactors and other nuclear functions, its fraction must be increased, that is, isotope enrichment is carried ont. The main isotope separation methods, or isotope enrichment processes, ntilize the small differences in between the mass of U-235 and U-238. The two major commercial methods that have supplied most of the enriched uranium to date, gaseous diffusion and gas centrifuges, use the only gaseous compound of nraninm, nranium hexafluoride (UFg), as the feed material. Both methods utilize the difference between the mass of UFg (349 Da) and UFg (352 Da) where the mass ratio difference that is 0.86%. The product and tails of the enrichment process are also with the same chemical form, but the isotope composition of the material is altered in the enrichment process. Schematic diagrams of the principle of operation of these methods can be found on the web and in many textbooks, so will not be shown here. 

There remains the question of how to come by the first core loading without separation of Pu. One possibility [XX-8, XX-33] is to use LWR spent fuel as the feed material and to remove from it only part of the uranium and part or all of the FP. For example, if the LWR spent fuel contains 1% Pu and minor activities (MA), it is necessary to remove approximately 90% of the uranium to make a fuel with 11 to 12 % of Pu and MA by weight. This could hopefully be done using a highly proliferation-resistant process, possibly a combination of an AIROX process and a fluoride volatilization process or a simplified version of the UREX process. Another feed option that could be considered is the spent fuel from MOX fuelled LWRs. The transuranium isotopes (TRU) content in such spent fuel can be approximately half of that needed for ENHS like reactors. Hence, only -50% of the uranium need be extracted along with FP to make fuel for ENHS like reactor. The latter is likely to offer a more economical fuel cycle. 

Table 5.27 lists the principal uranium refineries of the Western world and their feed and products. In all these refineries except Allied Chemical s, the sequence of operations follows some or aU of the steps shown in Fig. 5.21, in which uranium ore concentrates are first purified by solvent extraction and then converted to the materials of principal practical importance, uranium dioxide, uranium metal, or uranium hexafluoride. The steps in these refining operations will be described in process sequence in Secs. 9.2 through 9.6. 

Figure 10.1 is a material flow sheet for the first cycle of one form of the Redox process [F3]. Rutonium in the feed was converted to hexavalent plutonyl nitrate Pu 02(N0s)j, by oxidation with dichromate ion Cr2 07 ", as this is the plutonium valence with highest distribution coefficient into hexone. In the decontamination contactor, hexavalent uranium and plutonium nitrates were extracted into hexone solvent, and fission-product nitrates were removed from the solvent by a scrub solution containing aluminum nitrate, sodium nitrate, and sodium dichromate. 

Thermal diffusion of UF . The thermal diffusion process makes use of the small difference in 23su/Js u ratio that is established when heat flows through a mixture of UFj and UF. The principle of the process is described in Chap. 14. The process was used [Al] in 1945 in the United States by the Manhattan Prqect to enrich uranium to 0.86 percent U. This slightly enriched material was used as feed for an electromagnetic separation plant. Although the process could be put into production quickly because of the simplicity of the equipment, it... 

While other materials have been used as feed to uranium-enrichment processes, the most widely used volatile compound of uranium is the hexafluoride. At room temperature, UFe is a colorless solid with a density of 5.1 g/cm. It sublimes at atmospheric pressure, and at room temperature has a vapor pressure of 100 torr. The main disadvantage of working with UFe is its high chemical reactivity. It reacts vigorously with water, but is not very reactive with dry air. UF5 reacts with most metals however, nickel, copper, and aluminum are resistant. This holds only for pure UFg the presence of even small amounts of HF increases the rate of attack on even the resistant metals. 

Several feed metal enrichments (0.86, 0.95, 1.25, and 2.1% U) are processed routinely throu the NLO digestion systems. To the casual observer, the differences in enrichment mi t seem to be Of iittle concern, yet the amount of material that may be processed at the lowest enrichment can be several orders of magnitude more than that which may be processed at the highest enrichment. Thus, special consideration has to be taken of the process equipment involved, particularly since much of it was originally designed for natural uranium. 

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