An Overview of the Nuclear Separations Course Presentations Raymond G. Wymer Vanderbilt University Introduction This paper is an overview of the remainder of the course presentations. Its purpose is to put the remainder of the presentations into the context of the nuclear fuel cycle. Extensive use has been made of excerpts from the individual presentations (usually with some modifications). Separations are at the heart of all nuclear fuel cycles, whether the cycle is the uranium- plutonium or the thorium-uranium cycle and whether the cycle is the once-through cycle or complete spent fuel recycle. The various parts of the uranium fuel cycle, with the exception of waste management and reprocessing are shown in figure 1. At present in the U.S. only interim storage is practiced. Figure 1. The Uranium Fuel cycle A very wide variety of separations processes that support the uranium fuel cycle have been studied since the beginning of the nuclear era in the 1940s in order to meet the requirements of nuclear energy production and military applications, i.e., naval vessel propulsion and nuclear weapons production. Many separations have been studied at the very small scale, e.g., the very earliest separation of plutonium; others have reached the laboratory bench scale; and others have been carried to engineering-scale demonstration. A few have reached very large industrial-scale application throughout the world. The fissioning of uranium and plutonium produces elements and their isotopes of nearly all the elements in the periodic table as well as neutron capture reactions in uranium and plutonium to produce elements not found in nature1. The extremely large number of elements produced during fission has required chemical separations studies that are very extensive in order to complete the fuel cycle. To develop, demonstrate, and implement the necessary fuel cycle separations processes it has been necessary to study many aspects of fundamental chemistry as well as sophisticated chemical engineering unit operations. Chemistry of Fuel Cycle Elements A necessary first step in the development of separations processes is an understanding of the chemistry of chemical elements whose separation is sought. Understanding the chemistry includes a knowledge of the solid, liquid and gaseous (or vapor) chemical species of the elements. The earliest separations of plutonium, for example, were based on co-precipitation of plutonium species; purification of uranium has been based almost entirely on liquids, i.e., on solution chemistry2; uranium enrichment has been based entirely on the gaseous compound UF6. Additionally, there are important subdivisions of chemical behavior within the three material phases noted above. Especially important subdivisions are the formation of complex ions and compounds in the liquid phase, and both co-precipitation and sorption on surfaces of ions and compounds in the solid phase. The paper by Jarvinen discusses the important roles played in nuclear separations by precipitation and crystallization; Wymer discusses the key roles played by complexation reactions. Precipitation and Crystallization The distinction between precipitation and crystallization may be based on the speeds of the process and the size of the solid particles produced. The term precipitation commonly refers to a process resulting in rapid solid formation that can give small crystals that may not appear crystalline to the eye, but still may give very distinct x-ray diffraction peaks. Amorphous solids (as indicated by x-ray diffraction) may also be produced. Agglomeration describes the tendency of small particles in a liquid suspension to coalesce into larger aggregates. Other terms used in the literature include aggregation, coagulation, and flocculation. Crystallization often involves “aging.” The term aging refers to a variety of other processes that change a precipitate after it forms. Aging usually results in larger particle sizes and may be referred to with terms such as digestion or “ripening” of the precipitate. The details of performing the precipitation or crystallization process can be very important to produce a pure product and one that separates well from the liquid phase. For actinide metal ions limiting the amount of added precipitating agent such as oxalic acid (e.g., to form Pu2(oxalate)3) is used both to control initial super-saturation and to get larger particle sizes that are readily filtered and to limit the formation of soluble anionic complexes of the actinide element that may form in the presence of excess precipitant and reduce the yield of the product. 1 Actually, trace amounts of many of the so-called “new” elements have been found in the natural environment, possibly produced by cosmic radiation. 2 A noteworthy exception is purification of uranium by distillation of UF6 prior to enrichment. Co-precipitation refers to the variety of ways that other solutes in a multi-component solution may associate with a precipitate or crystal. This includes surface adsorption, incorporation of other anions or cations in the lattice of a growing crystal as part of a stable solid solution, or by entrapment. There may also be physical inclusion of pockets of “mother liquor.” Co- precipitation is a very important method for recovering small amounts of a solute that may be far below its solubility limit. In this case the solute sought precipitates with a major solute component (sometimes referred to as the carrier). Co-precipitation has been a crucial separation process to isolate traces of radionuclide, e.g., plutonium and some transplutonium elements, and to investigate their chemical behavior. The bismuth phosphate process used for the first large-scale purification of plutonium from neutron-irradiated uranium is an example of co-precipitation. Precipitated BiPO4 from acid solutions carries the trivalent and tetravalent actinides (especially Pu(IV)), but not the pentavalent and hexavalent ions. The BiPO4 solid carries only small amounts of the fission products. The different oxidation states of the actinide ions (particularly U, Np, and Pu) in aqueous solution show large differences in precipitation chemistry that facilitates their separation and purification. Table 1 lists examples of the decontamination factors3 achieved by precipitation for common contaminants of plutonium in oxidation states III-IV. Because of the relatively low decontamination factors, precipitation is generally not selective enough to be used as the primary process for separation and purification of plutonium or other actinides from fission products in irradiated fuel or targets4. Multiple precipitations may be used to achieve greater separation, but the nature of the precipitates and the handling difficulties often makes this approach infeasible. Table 1. Decontamination factors for plutonium precipitated from an irradiated plutonium alloy dissolved in nitric acid Pu(III) Pu(IV) Pu(IV) Pu(III) Element oxalate oxalate peroxide fluoride Fe 33 10 50 1.4 Co 47 > 95 30 8.6 Zr 3.5 > 44 1 1.1 Mo > 13 > 15 > 140 1.1 Ru > 38 33 > 14 36 Ce 1 1 6 1.1 Many solid materials have been used as sorbents for final purification of products of separation. Activated charcoal, silica, alumina, clays, and iron hydroxides are among the many materials used to sorb ions from solutions. The negatively charged surfaces of oxide materials can sorb cations from solution sometimes with surprisingly good selectivity. 3 Decontamination factor is defined as the ratio of concentration of the substance of interest in a phase before separation to its concentration in that phase after separation. 4 Decontamination factors of 106 or higher are typically required to produce a U or Pu product of adequate purity. Complexation reactions Complexation reactions are chemical reactions wherein a chemical species, usually a metallic cation, combines chemically with another species, usually an anion of an acid, to form a chemical entity having chemical properties unlike either of the initial reactants. Complexation of elements is very strongly dependent on the valence states of the elements. Changing the valence of an element dramatically changes its chemistry and consequently changes its complexation reactions and its separations chemistry. Table 2 lists common valence states of the elements of interest here as well as some of their important features. The most common valence states are in bold face font. Table 2. Important features of Some Common Valence States Element Valences Features - Tc +4, +5, +6, Environmentally mobile as TcO4 ; Tc2O7 is volatile at relatively low +7 temperatures 2+ +4 U +3, +4, +5, +6 UO2 forms extractable species; U is used in oxide fuels; UF6 is volatile and is used in enrichment processes - Np +3, +4, +5, Environmentally mobile as NpO2 ; forms extractable species +6, +7 Pu +3, +4, +5, Environmentally Mobile as Pu+4 colloid; Pu+4 is extractable; Pu+4 is +6, +7 used in oxide fuels Am +3, +4,+5, +6 Am+3 is very stable in aqueous media; Am6+ is potentially useful in separations from other actinides Cm +3, +4 Cm+3 is the only common valence state in aqueous solution; it behaves much like rare earths Uranium Uranium is at the heart of commercial nuclear power and is vital to the entire nuclear enterprise. Its presence and use worldwide has resulted in a vast literature, not only on uranium complexation and separations reactions, but also on all aspects of the uranium fuel cycle: mining and, milling; isotope enrichment; reactor fuel manufacturing; spent reactor fuel reprocessing; and weapons