Occurrence actinides

Actinides occurrence and preparation. With the exception of U and Th, the availability of the actinides of the first half of the series ranges from the g to kg scale that of the elements of the second half of the series from the mg scale for Cf to the sub-mg scale for Es. Isotopes of Np, Pu, Am, Cm can be available as byproducts of nuclear fuel processing other elements such as Ac, Cf, Bk, Es can be obtained by irradiation of selected isotopes in high flux reactors, or by reprocessing large quantities of ore (Pa). 

The Mott-like transition, a central concept for the description of the actinide metal series, causes the sudden increase of the atomic volumes, encountered when between Pu and Am (Fig. 3). All other properties indicate the onset of a 5f localized behaviour at Am (see Part V) the 5 f pressure, which had contained to smaller values the equilibrium interactinide distance, suddenly gives in, with the withdrawal of the 5f s within the atomic core. The occurrence of such a transition within a series characterized by an unsaturated shell, is a unique phenomenon of the actinide series. In lanthanides, it does not occur except perhaps under pressure in cerium metal the approaching of cerium atoms induces suddenly the itineracy of 4f orbitals and a sudden volume collapse - see Chap. C. Neither it occurs in d-transition metal series, where the atomic volumes have an almost parabolic behaviour when plotted vs. Z (see Fig. 3 and Chap. C). The current... 

Minimize changes in the highly active feed composition to avoid forming precipitates, the occurrence of which could trap minor actinides and thus decrease their recovery yields... 

However, the conditions are often far from those of industrial situations. In order to better simulate solvent degradation during the PUREX process, a test loop was created in the 1990s in a CEA laboratory (Fontenay-aux-Roses, France), with the EDIT loop (Extraction Desextraction Irradiation Traitement) (21, 22). The laboratory simulation of industrial conditions consisted of a succession of representative physical and chemical treatments after the irradiation of the solvent (i.e., alkali and acid treatments, distillation). Indeed, these treatments can modify the final solvent composition because of the elimination of some compounds or the occurrence of secondary reactions. A few years later, the MARCEL (Module Avance de Radiolyse dans les Cycles d Extraction Lavage) test loop was built at Marcoule to follow the regeneration efficiencies of degraded solvents involved in actinide separation processes (4, 5). 

The lanthanides (Ln) include lanthanum (La) and the following fourteen elements—Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu— in which the 4f orbitals are progressively filled. These fifteen elements together with scandium (Sc) and yttrium (Y) are termed the rare-earth metals. The designation of rare earths arises from the fact that these elements were first found in rare minerals and were isolated as oxides (called earths in the early literature). In fact, their occurrence in nature is quite abundant, especially in China, as reserves have been estimated to exceed 84 x 106 tons. In a broader sense, even the actinides (the 5f elements) are sometimes included in the rare-earth family. 

This effect originates in the various f electron configurations of lanthanides and actinides. This effect is based on the correlation observed between the full pattern of the effect and the sequence of values of the L quantum number [52]. The correlation consists of the occurrence of the same double symmetry in (a) the series of L quantum number values of the ground terms of f element ions and (b) the sequence of relatively stabilized or destabilized f electron configurations (i.e.) the double-double effect. Accordingly... 

The occurrence of a heavy-electron state in metals is most distinctly observed in compounds where one of the chemical constituents is an element of the rare-earth (4f) or actinide (5f) series. Within these series, it is the elements at the beginning or the end of the res-Sective row of the periodic system that are most likely involved in this effect (Ce, Yb, U, Np). 

The known structures of the lanthanide and actinide metals are indicated in table 5.01, from which it will be seen that the structures characteristic of the true metals, and particularly the hexagonal close-packed arrangement, are common. Polymorphism, however, is of frequent occurrence among these elements, and plutonium, for example, crystallizes in no fewer than six modifications—the A1 and A2 structures indicated and four others of greater complexity. Praseodymium, neodymium and samarium are of interest in that they possess close-packed structures in which the sequence of layers is... 

The early work on phosphate glasses led to the idea that crystalline phosphates might make extremely durable waste forms, particularly for actinides. The earliest suggestion was for the use of monazite (Boatner 1978, Boatner et al. 1980, McCarthy et al. 1978, 1980). The attractive qualities of monazite as a nuclear waste form are (1) a high solubility for actinides and rare earths (10 to 20 wt %) (2) evidence from natural occurrences of good chemical durability (3) an apparent resistance to radiation damage, as natural monazites are seldom found in the metamict state, despite very high alpha-decay event doses (Boatner and Sales 1988). There have been extensive studies of monazite and apatite as potential waste form phases, and a considerable amount of work on a number of synthetic phosphate phases has been completed. 

The main advantages of phosphate phases as nuclear waste forms are the high capacity for actinides (up to 20 wt %), a relatively high chemical durability (Donald et al. 1997, Trocellier 2001), and the ability to anneal radiation damage effects at relatively low temperatures (300 to 400°C). Of particular importance for monazite and apatite is the fact that they occur naturally thus, models that are used to extrapolate the physical and chemical behavior of the waste form over long periods can be confirmed by comparison to natural occurrences. 

I. Discovery, Occurrence, and Synthesis of the Actinides ii. Radioactivity and Nuclear Reactions of Actinides... 

In 1972 a hybridized non-degenerate 6d and 5f virtual-bound-states model was used to describe the properties of the actinide metals, including berkelium [138]. It accounted for the occurrence of localized magnetism in Bk metal. In 1974 a review of the understanding of the electronic properties of berkelium metal as derived from electronic band theory was published [139]. Included was the relativistic energy band structure of face-centered cubic Bk metal (5f 6d 7s ), and the conclusion was that berkelium is a rare-earth-like metal with localized (ionic) 5f electrons resulting from less hybridization with the 6d and 7s itinerant bands than occurs in the lighter actinides. 

---