Uranium chemical properties

Uranium hexa-/ f/-butoxide is an exception and does not react with water (55). References 3 and 5 discuss chemical properties of alkoxides. In some cases hydrolysis is reversible, but usually it is not (23,56). 

The same chemical separation research was done on thorium ores, leading to the discovery of a completely different set of radioactivities. Although the chemists made fundamental distinctions among the radioactivities based on chemical properties, it was often simpler to distinguish the radiation by the rate at which the radioactivity decayed. For uranium and thorium the level of radioactivity was independent of time. For most of the radioactivities separated from these elements, however, the activity showed an observable decrease with time and it was found that the rate of decrease was characteristic of each radioactive species. Each species had a unique half-life, ie, the time during which the activity was reduced to half of its initial value. 

By this time, the Periodic Table of elements was well developed, although it was considered a function of the atomic mass rather than atomic number. Before the discovery of radioactivity, it had been estabUshed that each natural element had a unique mass thus it was assumed that each element was made up of only one type of atom. Some of the radioactivities found in both the uranium and thorium decays had similar chemical properties, but because these had different half-Hves it was assumed that there were different elements. It became clear, however, that if all the different radioactivities from uranium and thorium were separate elements, there would be too many to fit into the Periodic Table. 

The geological term, uranium occurrence, implies a naturally occurring anomalous concentration of uranium. The term, uranium deposit, implies a mass of naturally occurring mineral material from which uranium could be exploited at present or in the future. An often-encountered term in uranium mineralogy is uranium ore mineral. It is a mineral having such physical and chemical properties and occurring in a deposit in such concentra-... 

The four isotopes, as those of any element, have the same chemical properties. The four are not, however, uniformly distributed in the earth s crust the occurrence of three of them, in minerals and rocks, is associated with the radioactive decay of isotopes of thorium and uranium. In most minerals and rocks the relative amounts (or the isotopic ratios) of the isotopes of lead (often expressed relative to the amount of stable lead-204) are generally within well-known ranges, which are independent of the composition of the mineral or rock they are, however, directly related to the amounts of radioactive thorium and uranium isotope impurities in them. 

A primary goal of chemical separation processes in the nuclear industry is to recover actinide isotopes contained in mixtures of fission products. To separate the actinide cations, advantage can be taken of their general chemical properties [18]. The different oxidation states of the actinide ions lead to ions of charges from +1 (e.g., NpOj) to +4 (e.g., Pu" " ) (see Fig. 12.1), which allows the design of processes based on oxidation reduction reactions. In the Purex process, for example, uranium is separated from plutonium by reducing extractable Pu(IV) to nonextractable Pu(III). Under these conditions, U(VI) (as U02 ) and also U(IV) (as if present, remain in the... 

Following the characterization of U(COT)2, the analogous thorium complex was synthesized 12). Its physical and chemical properties were so different from those of the uranium compound that initially there was question as to whether the complex had the same -sandwich structure. The X-ray structure anal5 is however showed it to be isostructural with U(COT)2 11). Subsequently, Pu(COT)2 13), Np(COT) 13), and Pa(COT)2 14, 15) have been prepared and their X-ray powder patterns show them all to be isostructural with U(COT)2. 

The most studied non-stoichiometric system in actinide CaF2-structured compounds is the An-0 system all actinide dioxides (with the exception of Th02) present large departures from stoichiometry. Since uranium and plutonium dioxides (and their solid solutions) are employed as fuels in nuclear reactors, a very large effort has been dedicated to the study of their physical and physico-chemical properties. All these properties are affected by the oxygen composition of the compound. 

These two kinds of lead are now known to be isotopes, or inseparable elements which belong in the same space in the periodic table and yet differ in atomic weight and in radioactive properties. According to Frederick Soddy, the first clear recognition of isotopes as chemically inseparable substances was that of H. N. McCoy and W. H. Ross in 1907 (75,107). Strictly speaking, the science of radioactivity has revealed only five naturally occurring new elements with distinctive physical and chemical properties polonium, thoron, radium, actinium, and uranium X2. All the other natural radioactive elements share previously occupied places in the periodic table. 

All the preliminary studies on the chemical properties of element 43 were conducted with unweighable amounts of material. Segre estimated that the amount they used was about 10-10 gram (8). In 1940 Segre and C. S. Wu (9) found element 43 among the fission products of uranium. Much larger amounts were obtained from this source. 

All the early work on plutonium was done with unweighable amounts on a tracer scale. When it became apparent that large amounts would be needed for the atomic bomb, it was necessary to have a more detailed knowledge of the chemical properties of this element. Intensive bombardment of hundreds of pounds of uranium was therefore begun in the cyclotrons at Berkeley and at Washington University in St. Louis. Sepa-ration of plutonium from neptunium was based on the fact that neptunium is oxidized by bromate while plutonium is not, and that reduced fluorides of the two metals are carried down by precipitation of rare earth fluorides, while the fluorides of the oxidized states of the two elements are not. Therefore a separation results by repeated bromate oxidations and precipitations with rare earth fluorides. 

Since this is so, it was inevitable that as soon as Seaborg and his collaborators had clearly established the identity and properties of neptunium and plutonium, they would look for the next higher elements, numbers 95 and 96. The general similarity in chemical properties of uranium, neptunium, and plutonium led Seaborg to believe that these new elements could be isolated by methods similar to those already used. 

The first genuine transuranic element was discovered at Berkeley, where Edwin McMillan used Lawrence s cyclotron in 1939 to bombard uranium with slow neutrons. He saw beta decay from what he predicted was element 93, and set about trying to isolate it. McMillan saw that the element sits beneath the transition metal rhenium in the Periodic Table, and so he assumed it should share some of rhenium s chemical properties. But when he and Fermi s one-time collaborator Emilio Segre performed a chemical analysis, they found that eka-rhenium (in Mendeleyev s terminology) behaved instead like a lanthanide, the series of fourteen elements that loops out of the table after lanthanum (see page 152). Disappointed, they figured that all they had found was one of these known elements. 

B) R. DeWitt, Uranium Hexafluoride A Survey of the Physico-Chemical Properties , GAT-280, Goodyear Atomic Corp, Portsmouth, Contract AT<33-2>1 (1960) C) L.S. Allen, A Parametric Survey of Criticality-Limited Fast Reactors Employing Uranium Fluoride Fuels , TR32-198, Jet Propulsion Lab, Contract NAS-7-100 (1962) D) C.A. Geffen et al, Assessment of the Risk of Transporting Uranium Hexafluoride by Truck and Train , Battelle, Richland, Contract EY-76-C-06-1830 (1978)... 

For the purposes of photophysics and photochemistry it is therefore sufficient to keep in mind the simple picture of an atom as a heavy, positively charged nucleus around which move light, negatively charged electrons. In the smallest atom, that of hydrogen, there is a single electron, whereas in the uranium atom, which is the heaviest natural element known on Earth, there are 92 electrons. It is the motion of these electrons which determines the chemical properties of an atom or a molecule so that it is now necessary to consider in a qualitative way the structure of these elementary particles of matter. 

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. 

Especially interesting in a discussion of radionuclide speciation is the behaviour of the transuranium elements neptunium, plutonium, americium and curium. These form part of the actinide series of elements which resemble the lanthanides in that electrons are progressively added to the 5f instead of the 4f orbital electron shell. The effective shielding of these 5f electrons is less than for the 4f electrons of the lanthanides and the differences in energy between adjacent shells is also smaller, with the result that the actinide elements tend to display more complex chemical properties than the lanthanides, especially in relation to their oxidation-reduction behaviour (Bagnall, 1972). The effect is especially noticeable in the case of uranium, neptunium and plutonium, the last of which has the unique feature that four oxidation states Pum, Pu, Puv and Pu are... 

The atoms of the chemical elements, are, as I have already said, extremely complex, but their structure is not yet completely understood. To some part of each kind of atom its chemical properties and its spectrum are probably due. It is conceivable that this part may be the earliest to form, with its surrounding rings or envelopes at first not quite adjusted to permanent stability. With the final adjustment the isotopes as such should disappear, and the normal element be completed. This is speculation, and its legitimacy remains to be established. A careful comparison of the spectra of the elements from thallium up to uranium might furnish some evidence as to its validity. The spectrum of uranium, for example, may contain lines which really belong to some of its derivatives. 

These results illustrate the importance of the chemical species of the element present in the deposit with regard to ion emission (and gives insight into the effect of the oxidizing/reducing nature of the ion emitter) but tell little about the actual mechanisms active in the ion emitting process. As an example, the ions could be emitted either from the deposit itself or from an intermediate material that formed as a consequence of the chemical properties, or it could be entirely an interface phenomenon in which the deposit only served as a repository for the uranium species and the supporting filament served as the ionization surface. 

---