Actinide elements from fission

The main problems in the separations are (a) separation of the actinides as a group from the lanthanide ions (which are formed as fission fragments in the bombardments which produce the actinides) and (b) separation of the actinide elements from one another. 

The lanthanide elements are very difficult to separate because of their highly similar chemistry, but the earlier actinide elements have sufficiently different redox chemistry to allow easy chemical separations. This is important in the nuclear power industry, where separations have to be made of the elements produced in fuel rods of nuclear power stations as fission products, and of the products Np and Pu, which arise from the neutron bombardment of the uranium fuel. 

In the chemistry of the fuel cycle and reactor operations, one must deal with the chemical properties of the actinide elements, particularly uranium and plutonium and those of the fission products. In this section, we focus on the fission products and then chemistry. In Figures 16.2 and 16.3, we show the chemical composition and associated fission product activities in irradiated fuel. The fission products include the elements from zinc to dysprosium, with all periodic table groups being represented. 

Most radioactive particles and vapours, once deposited, are held rather firmly on surfaces, but resuspension does occur. A radioactive particle may be blown off the surface, or, more probably, the fragment of soil or vegetation to which it is attached may become airborne. This occurs most readily where soils and vegetation are dry and friable. Most nuclear bomb tests and experimental dispersions of fissile material have taken place in arid regions, but there is also the possibility of resuspension from agricultural and urban land, as an aftermath of accidental dispersion. This is particularly relevant to plutonium and other actinide elements, which are very toxic, and are absorbed slowly from the lung, but are poorly absorbed from the digestive tract. Inhalation of resuspended activity may be the most important route of human uptake for actinide elements, whereas entry into food chains is critical for fission products such as strontium and caesium. 

One possible application in which large amounts of rare earths and actinides would be processed occurs in some schemes for nuclear waste management. If it should prove to be advantageous to remove transplutonium elements from nuclear waste, for example, the recovery of Am and Cm from the much larger amounts of rare earths would be required. This problem has been investigated by the author in tracer tests with rare earth mixtures typical of fission products, using a heavy rare earth such as holmium as a stand-in for Am and Cm (Fig. 5). It is clear that the bulk of the holmium can be recovered in reasonable purity, and that the bulk of the lighter rare earths is effectively separated from the very small amount of heavy rare earths, Am, and Cm. 

The FP-3 fission products are oxidized by MgCl2 or ZnCl2, are transferred to the salt phase, and are finally taken up by an acceptor alloy. The FP-4 elements are too inert to be oxidized by MgCl2 or ZnCl2 and remain with the donor alloy. The actinide elements are then separated from the FP-4 Fission products by salt transport to the acceptor alloy followed by vacuum retorting and conversion of metallic intermediates to suitable products. 

Molten-Tin Process for Reactor Fuels (16). Liquid tin is being evaluated as a reaction medium for the processing of thorium- and uranium-based oxide, carbide, and metal fuels. The process is based on the carbothermic reduction of UO2 > nitriding of uranium and fission product elements, and a mechanical separation of the actinide nitrides from the molten tin. Volatile fission products can be removed during the head-end steps and by distilling off a small portion of the tin. The heavier actinide nitrides are expected to sink to the bottom of the tin bath. Lighter fission product nitrides should float to the top. Other fission products may remain in solution or form compounds with... 

The mutual separation of actinide elements and the selective isolation of useful actinides from fission products are indispensable for the nuclear fuel cycle and have become important subjects of investigation for the development of advanced nuclear fuel reprocessing and TRU (TRans Uranium elements) waste management [1], A variety of research concerning the separation chemistry of actinides has so far been accumulated [2]. There are, however, only a few theoretical studies on actinides in solution[3-5]. Schreckenbach et a), discussed the stability of uranyl (VI) tetrahydroxide [UO,(OH) ] [3] and Spencer and co-workers calculated the optimized structures of some uranyl and plutonyl hydrates [AcO, nH,0 (Ac = U, Pu and n = 4,5,6)] [4],... 

Properties Solid. Mp 55C, min purity 95%. Use Reagent for extraction of metals from aqueous and nonaqueous solutions, including fissionable actinide elements. 

An extraction process for separating actinide elements (principally uranium, U, and plutonium, Pu) from fission products in an aqueous solution of spent fuel rods is illustrated in Figure 5.31. The extraction solvent is 30% tributyl phosphate (TBP) in kerosene. The most extractable of the fission products are zirconium, niobium and ruthenium. Zirconium, Zr, is used herein to represent the fission products. Determine the number of stages required in the wash section and in the extraction section. Determine the percentage of the Pu in the feed which is recovered in the extract product. V denotes the relative volumetric flowrate. 

Table 11.2 Amounts of fission-product elements and actinide elements in the waste from 1 MT LWR uranium fuel (30,000 MWd/MT bumup) at discharge from reprocessing (150 days cooled fuel elements) and 6 years after discharge (contributions of more than 0.1 percent) accmding to ORIGEN...

From the fission and capture cross-sections, and half-lives the radioactivity of each actinide element in one kg spent fuel (and radium) has been calculated and shown in Fig. 21.8 for a PWR UO2 fuel with a bumup of 33 MWd per kg IHM. EXue to the use of a log-log scale in Figure 21.8, the decay curve of any single nuclides is s smooth curve bending downwards. Inflexion points indicate the existence of several radioactive isotopes of the same element with different half-lives. 

---