Transuranium element

Abstract: The place of the very heaviest elements in the periodic system is of great scientific interest and is of crucial importance in understanding the physical and chemical properties of these elements. Ideas on this question have varied considerably over the years. Periodic charts have commonly placed thorium, protactinium, and uranium in positions immediately below the elements hafnium, tantalum, and tungsten. The latter group of elements are members of a transition series in which the 5d electron shell is being filled. Hence this earlier placement of the three heavy elements corresponded to the assumption that they are members of a 6d transition series. Indeed thorium, protactinium, and, to a much smaller extent, uranium do show considerable resemblance in chemical properties to the 4d and 5d transition series elements. The electronic configuration beyond the radon core on this basis would be written thus: thorium (6d 27s 2), protactinium (6d 37s 2), and uranium (6d 47s 2).

Abstract: The primary waste form resulting from nuclear energy production is spent nuclear fuel (SNF). There are a number of different types of fuel, but they are predominantly uranium based, mainly UO 2 or, in some cases, metallic U. The UO 2 in SNF is a redox-sensitive semiconductor consisting of a fine-grained (5-10 μm), polycrystalline aggregate containing fission-product and transuranium elements in concentrations of 4 to 6 atomic percent. The challenge is to predict the long-term behavior of UO 2 under a range of redox conditions. Experimental results and observations from natural systems, such as the Oklo natural reactors, have been used to assess the long-term performance of SNF.

Abstract: Monazite (Ln3+PO4) and related solid solutions are a well-known source of rare earth elements on earth. They may also accommodate large amounts of thorium and uranium without sustaining damage to the structure by self-irradiation. Such observations led to monazite-type structures being proposed as a potential host matrix for sequestering long-lived radionuclides produced during the nuclear fuel cycle and/or plutonium and americium from dismantled nuclear weapons. Monazite has two main advantages as a matrix for the containment of radioactive waste (or “radwaste”). The first is a highly flexible structure that permits accommodation of high concentrations of actinides. The incorporation of trivalent elements may be achieved by direct synthesis of An3+PO4 (An3+ = plutonium, Pu to einsteinium, Es), while tetravalent cation incorporation requires coupled substitutions, either on the anionic site (leading to monazite-huttonite solid solution) or on the cationic site (monazite-cheralite solid solution). Various methods developed for the preparation of such compounds are summarized here, as well as the experimental conditions required for the production of sintered pellets, with a particular focus on plutonium-bearing compositions. The second highly favorable property of monazite is its high chemical durability. Several experimental procedures developed to determine normalized leaching rates are reviewed, as well as results obtained from natural and synthetic monazite. Potential phases formed during dissolution were considered because they also partially control the concentration of actinides in the media. A preliminary list for such phases of interest, as well as corresponding thermodynamic data, is presented.