Comparative actinide behavior

These quantities are both higher for experiment 4 and 5 than for the three experiments with unaltered repository components The loss of Pu through the core for experiments 1 and 2, for example, was 0 4 and 0 1 dpm/mL, respectively in experiments 4 and 5 the loss of Pu through the core appears to be 20 to 40 times greater This exercise is intended to show that there is no legitimate way to compare the behavior of altered and unaltered basalt from these data Experiment 6, yet to begin, should clarify this situation. What is clear from comparing the data in Table VI is that actinide behavior in altered and unaltered repository situations will be quite different ... 

Tanthanide chemistry is approaching its 200th Anniversary, but except for data on thorium and uranium the chemistry of the actinides is a comparative youngster of some 30 years. However, the two chemistries are intimately associated because their elements are of the f transition type and thus formally comparable with each other and different from other elements. Indeed, these parallels made it possible to unravel actinide behavior in the early days of transuranium element production. In addition to their chemical similarities, the two series also share the properties of magnetism and radiant energy absorption and emission characteristic of /-electron species. However, important differences exist also, particularly in oxidation states, in bonding, and in complex-ion formation. 

This volume of the Handbook covers a variety of topics with three chapters dealing with a range of lanthanide magnetic materials, and three individual chapters concerning equiatomic ternary 3hterbium intermetallic compoimds, rare-earth polysulfides, and lanthanide organic complexes. Two of the chapters (206 and 210) also include information on the actinides and the comparative lanthanide/actinide behaviors. 

Other aspects of the actinide oxides are discussed and the general properties and behavior of the actinide oxides compared to those of the lanthanide oxides in section 3. 

The purpose of this paper is to compare the biogeochemical behavior of the actinides which are important constituents of nuclear fuel cycles. To the extent possible, the environmental behavior of the essentially man-made elements Pu, Am, Cm and Np will be compared to Th and U, which are ubiquitous in the environment. By comparing the man-made actinides to naturally occurring actinides, a generic perspective of the potential hazard of the synthetic actinides to people is obtainable. 

Summary. Through an examination of the comparative behavior of the actinide elements in terrestrial and aquatic food chains, the anticipated accumulation behavior of the transuranium elements by people was described. The available data suggests that Pu, Am and Cm will not accumulate to a greater degree than U in the skeletons of individuals exposed to environmentally dispersed activity. The nature of the contamination event, the chemical and physical associations in soils and sediments, the proximity to the... 

The radioly tic behavior of a substituted picolinamide calix[6]arene, studied for the separation of actinides from lanthanides, was recently investigated by Mariani (263). For doses ranging from 0.014 to 0.055 MGy, the distribution ratios of both Am(III) and Ln(III) strongly increased, whereas after absorbed doses higher than 0.10 MGy, they decreased to values lower than those measured for nonirradiated samples. The selectivity for Am/Eu remained constant. Comparable experiments under a nitrogen atmosphere indicated the role of oxygen in the radiolysis, because the distribution ratios decreased by factors of 10 and 1.5-5 for Am-Eu and other lanthanides, respectively. The increase for lower doses was then explained by the formation of oxidized radiolytic products. No evidence of new products was obtained with the ESI-MS technique. 

K) -1690 10 kJ mol 1. The preparation and properties of this and other actinide (IV) complex oxides are described and are compared with other perovskites BaM03. The relative stabilities of tetravalent and hexavalent uranium in various environments are compared in terms of the oxidation-reduction behavior of uranium in geological nuclear waste storage media in perovskite, uranium(IV) is very unstable in comparison with uranium(VI). ... 

Comparing only the two lanthanides, a very similar behavior under pressure could be established. In the case of U3+ at least the qualitative behavior, except for Bp, also matched that of the lanthanides. Quantitatively, the absolute shifts for the actinide ion were larger, a result that could be expected from the much more expanded wavefunctions of the 5f shell. It should be noted, however, that the relative changes were also very similar compared to the lanthanides. 

The ionic radii for the commonest oxidation states (Table 20-1) are compared with those of the lanthanides in Fig. 20-1. There is clearly an actinide contraction, and the similarities in radii of both series correspond to similarities in their chemical behavior for properties that depend on the ionic radius, such as hydrolysis of halides. It is also generally the case that similar compounds in the same oxidation state have similar crystal structures that differ only metrically. 

In this section, results of studies of the geochemistry of fission products and actinides are summarized. The chemistry of the fission products is described as a group first because their behavior is relatively simple compared to the actinides. Next, general trends and then site-specific environmental chemistry of the actinides are summarized. 

A second reason for the wealth of chemical investigations of the early actinide elements is the relative diversity of their chemistry. While the chemistry of the later actinides is most often restricted to that of the tri- and tetravalent oxidation states, compounds of the early actinides can be isolated in all oxidation states from +3 to +7. The accessibility of a range of oxidation states is the impetus for signficant chemical interest in the early actinides, but also vastly complicates investigation of these elements under some circumstances, such as aqueous redox behavior. In the case of plutonium, ions in four different oxidation states (+3, +4, +5, and - -6) can exist simultaneously in comparable concentrations in the same solution. 

The properties of Fm metal and of its solid compounds are for the most part unknown because there are insufficient quantities to prepare even microsamples. In the numerous thermo-chromatographic studies by Zvara and coworkers, the evaporation of Fm and Md tracer from molten La at 1150°C was compared with the behavior of other selected lanthanides and actinides (7). 

The ionization of Lr would be expected to stop with the f- core intact because of the enhanced binding energy of possible valence electrons in the filled f shell. The stable valence state of Lr would then be the (III) state. Experiments to confirm this oxidation state of Lr were undertaken by Silva and coworkers (1). They compared the extraction behavior of Lr with several tri- and tetravalent actinides and with Ba +, Ra +, and No +. A chelating extractant, thenoyltrifluoroacetone dissolved in methyl isobutyl ketone, was employed to extract the tracer ions from aqueous solutions that had been buffered with acetate anions. Their results, shown in Figure 8, very clearly demonstrate that Lr is extracted within the same pH range as the trivalent actinides, and therefore, proves that Lr is trivalent. 

One difficulty is that there is no stable isotope of plutonium with which to compare its abundance. To really quantify its abundance, it is necessary to consider the amount of " " Pu relative to an isotope of a similar element. The definition of similar depends on the problem to be addressed. In studies of nucleosynthesis, the similar element used is usually uranium, another actinide, which is produced in the same stellar environments. In studies of the history of specific meteorite parent bodies, the similar element is more commonly a light rare earth element (TREE) like neodymium, since the geochemical behavior of plutonium is apparently most similar to that of the LREE. We will discuss the details of the experimental technique of each approach below. 

Speciation and reactivity of actinide compounds comprise an important area for quantum chemical research. Even more so than in the case of lanthanides, f-type atomic orbitals of actinides can affect the chemistry of these elements [185,186] the more diffuse 5f-orbitals [187] lead to a larger number of accessible oxidation states and to a richer chemistry [188]. The obvious importance of relativistic effects for a proper description of actinides is often stressed [189-192]. A major differences in chemical behavior predicted by relativistic models in comparison to nonrelativistic models are bond contraction and changes in valency. The relativistic contribution to the actinide contraction [189,190] is more pronounced than in the case of the lanthanides [191,192]. For the 5f elements, the stabilization of valence s and p orbitals and the destabilization of d and f orbitals due to relativity as well as the spin-orbit interaction are directly reflected in the different chemical properties of this family of elements as compared with their lighter 4f congeners. Aside from a fundamental interest, radioactivity and toxicity of actinide compounds as well as associated experimental difficulties motivate theoretical studies as an independent or complementary tool, capable of providing useful chemical information. 

The partitioning behavior of some metal ions depends greatly on the pH in the ABS and their distribution ratios may change markedly in acidic or basic media. The distribution ratios of actinides and lanthanides in the PEG-2000/(NH/i)2SO/i ABS are very low at low pH compared to those at high pH. Changing the pH can thus be used to strip metal ions back into a salt-rich phase [21. In Ref. 66, 70% Fe " was extracted into the PEG-rich phase at pH 4.5 and separated from Sc " and Cu " in a PEG-2000/ (NH4)2SO-i ABS with Chrome Azurol S. The extraction of Fe " at pH < 2 was much lower ( I0%) thus Fe " could be stripped by changing the pH. 

Heavy Fermion Systems are intermetallics which consist of rare earths or actinides together with other metal species. Examples are CeAl3 [43] and UPt3 [44]. These materials have partially filled 4/or 5/shells. At high temperatures the / electrons are localized. This behavior is comparable to conventional alloys with rare earths or actinides. With decreasing temperature the systems order in an antiferromagnetic state. Heavy Fermion Systems, however, behave like normal metals but the effective mass of the electrons is significantly enhanced (often by a factor of hundred). 

Soon after the discovery of element 104, Silva et al. (1970) studied its solution chemistry for the known isotope 75-s 261Rf. Positive identification was made by measuring its known half-life and decay characteristics. Several hundred elutions with a-hydroxyisobutyrate solutions from cation exchange resin columns were performed to compare the behavior of 261Rf with those of No2+, trivalent actinides, Hf4"1", and Zr4+. The behavior of Rf was similar to those of Zr and Hf, which did not sorb on the column and entirely different fromNo2+ and the trivalent actinides that did sorb on the column at pH 4.0. These studies showed unequivocally that Rf should be placed under Zr and Hf in the periodic table. 

---