Production of Transuranium Elements

The most important method of production of the first transuranium elements is neutron irradiation of uranium. After the discovery of the neutron by Chadwick in 1932, this method was applied since 1934 by Fermi in Italy and by Hahn in Berlin. The method is based on the concept that absorption of neutrons by nuclides with atomic number Z leads to formation of neutron-rich nuclides that change by fi decay into nuclides with atomic numbers Z - -1. Unexpectedly, the experiments carried out by Hahn and Strassmann led to the discovery of nuclear fission in 1938. 

The production of transuranium elements by neutron irradiation can be described by 

After long irradiation times, elements with atomic numbers Z -I- 2, Z -I- 3 etc. are generated in amounts that increase with irradiation time. The formation of transuranium elements by neutron irradiation of is illustrated in Fig. 14.5. (n, y) reactions and radioactive decay compete with each other. Formation of heavier nuclides is favoured if 

At the extremely high fluxes of a nuclear explosion, fast multiple neutron capture leads to very neutron-rich isotopes of U or Pu, respectively, changing rapidly into elements of appreciably higher atomic numbers by a quick succession of transmutations. This method of formation of heavier elements is also indicated in Fig. 14.5. The elements can be found in the debris of nuclear underground explosions. 

By irradiation with deuterons, one proton is introduced into the nucleus and (dn), or (d, 2n) reactions lead to the production of elements with Z - - 1, for instance 

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Lying just below barium in the periodic table, radiums chemistry is essentially identical to bariums chemistry. In fact, there is a famous story in which confusion between the two elements played an important role. In the 1930s, Italian physicist Enrico Fermi (1901-54) and his coworkers were investigating the action of neutrons on samples of uranium. (The neutron had only been discovered in 1930. Its use in physics was still relatively new.) Their expectation was that the absorption of neutrons by uranium would lead to the production of transuranium elements (elements lying beyond uranium in the periodic table), as shown by the following equation ... 

There are two kinds of methods for production of transuranium elements as indicated in the previous O Sect. 18.1.1 neutron capture reactions in nuclear reactors and charged-particle-induced reactions at accelerators. 

Riedel, C., Norenberg, W. Theoretical estimates for the production of transuranium elements in heavy-ion collisions. Z. Phys. A290, 385-391 (1979)... 

The main source of transuranium elements is the high-flux reactor, in which or heavier nuclei get transformed into higher-Z elements by multiple neutron capture. In the USA, there is a national program for the production of transuranium elements utilizing the high-flux reactor (HFIR) at Oak Ridge. The heaviest nuclide produced in the reactor is Fm. Neutron-deficient nuclides are synthesized in charged-particle accelerators and very neutron-rich nuclides with short half-lives are produced in reactors. 

Hence, there are convincing reasons to expect strong chemical effects on the 5f2 system Pu(VI) known from PuOj2 and PuF6. Unfortunately, the electron transfer bands are lower (though broader) in the visible than the 5 f2 internal transitions. Two candidates for the lowest wave-number of such an electron transfer band in PuO 2 253 are situated at 17000 or 19200 cm"1. The spectroscopic difference that MOj does not seem to have electron transfer spectra in the visible is accompanied by a maximum chemical stability of NpOj. However, the rate of lsO exchange at 23 °C in 1M perchloric acid is 0.31 s"1 in NpOj (PuOj is much slower, UOj much more rapid) but below 6 10"7 s"1 in NpOj2231. Much of the reported chemistry of transuranium elements is influenced by redox reactions, due to products of the intense radioactivity. Thus, lg of the uranium... 

Actinides served already as targets, when neutron capture and subsequent P decay were used for the first synthesis of transuranium elements. Later, up to the synthesis of seaborgium, actinides were irradiated with light-ion beams from accelerators. At that time it was already known that cold fusion reactions yield higher cross sections for heavy element production. 

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. 

The actinide iodate system is one of considerable interest that has attracted chemists for more than 150 years (vide supra). In fact one of the first forms that was isolated in was as the iodate salt, presumably as 1 0(103)4 [63], The precipitation of iodate compounds of the actinides has been used for decades as a method of separated them from lanthanides and other fission products. The precipitation of thorium iodate is perhaps best known in this regard [64-66], but several patents exist describing selective precipitation of transuranium elements [67-72], Despite the key importance of iodate in actinide chemistry the structures of actinide iodates were not described in detail until approximately 2000. 

Recovery of transuranium elements is mainly of interest for solid waste from the refabrication of mixed-oxide fuel. Plutonium is the major element to be recovered, and Am may be recovered as a by-product. Other transuranium elements are usually present in minor quantities. The treated wastes are seldom decontaminated to levels of plutonium that would permit unrestricted release. 

Tc is available through the /l -decay of Mo (Fig. 2.1.B), which can be obtained by irradiation of natural molybdenum or enriched Mo with thermal neutrons in a nuclear reactor. The cross section of the reaction Mo(nih,v) Mo is 0.13 barn [1.5], Molybdenum trioxide, ammonium molybdate or molybdenum metal are used as targets. This so-called (n,7)-molybdenum-99 is obtained in high nuclidic purity. However, its specific activity amounts to only a few Ci per gram. In contrast, Mo with a specific activity of more than in Ci (3.7 10 MBq) per gram is obtainable by fission of with thermal neutrons in a fission yield of 6.1 atom % [16]. Natural or -enriched uranium, in the form of metal, uranium-aluminum alloys or uranium dioxide, is used for the fission. The isolation of Mo requires many separation steps, particularly for the separation of other fission products and transuranium elements that arc also produced. 

The use of the actinide elements fall into three categories (i) for imderstanding fundamental chemistry and the nature of the periodic system, (ii) as products, in the large scale use of nuclear energy, and (iii) miscellaneous applications, where the particular physical, chemical or nuclear properties are valuable. Only the last aspect is discussed here, the others are treated elsewhere in this book. The availability of transuranium element isotopes suitable for experiments is listed in Table 16.4. 

The production of new elements by nuclear transformation (the change of one element into another) is carried out by bombarding various nuclei with particles in accelerators. The transuranium elements have been synthesized in this way. 

Abstract This chapter reviews the historical perspective of transuranium elements and the recent progress in the production and study of nuclear properties of transuranium nuclei. Exotic decay properties of heavy nuclei are also introduced. Chemical properties of transuranium elements in aqueous and solid states are summarized based on the actinide concept. For new application of studying transuranium elements, an X-ray absorption fine structure (XAFS) method and computational chemistry are surveyed. 

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