Ab-initio Based Diffusive Studies of Plutonium with Relevance to Nuclear Waste Management
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Ab-initio Based Diffusive Studies of Plutonium with Relevance to Nuclear Waste Management A. K. Setty, West Virginia University, Physics Department, Morgantown, West Virginia 26506-6315, [email protected] ABSTRACT An approach to study the diffusion of plutonium (Pu) waste into the material of the storage containers is presented. The delta phase of Pu has been examined, in this article, using a spin and orbitally polarized ab-initio (full potential LMTO) approach [1, 2]. These computations are relatively inexpensive, and it is the intention to see if the results obtained would be satisfactory (compared to the accurate SIC-LDA computations [1-2]) for diffusive studies of Pu into the material of storage containers (for example, into FCC iron as a surrogate for steel [2]). The combination of LDA and spin and orbitally polarized LMTO calculations that we use successfully predict a delocalized 5f, (i.e. bonding) monoclinic ground state and a localized FCC excited state with a density about 8% greater than experiment (compared to the 30% error from pure LDA calculations by various groups [3-6]). The density of states is computed and compared to experimental photoemission studies; there is qualitative agreement. The spin, orbital and total magnetic moment for the FCC phase is computed; the results are qualitatively in agreement with those of other studies. An analytical method for studying the time-dependent diffusion of Pu across an interface with another atomic species is presented. INTRODUCTION Elemental plutonium is one of the most unusual physical systems studied in that it has a very complex phase diagram with six crystallographic allotropes (α,β,γ,δ,δ’,ε) [3]. The low temperature phase is a low symmetry monoclinic (α) structure, with low atomic volume, which gives way, in a series of crystallographic phase transitions, to a room temperature δ phase with many unusual properties. The α−δ phase transition occurs with an unusually large (24%) atomic volume change [4-5]. The volume for the latter phase is grossly underestimated (25-35%) in density functional studies [6-9] with LDA/GGA approximations. This is perhaps the largest LDA contraction observed in such studies for any material. Furthermore, the negative thermal expansion in the delta phase [10] is not understood. These unusual properties of plutonium (and also of U and Np) are attributed to the role of f electrons (the predominantly f contribution to the density of states is evidence of this), and in particular, to their localized/itinerant behavior. Consequently the abrupt α−δ volume change could be due to decreased f bonding (i.e. f-electron localization). The position of plutonium in the periodic table illustrates the role of localization/itinerancy of the 5f electrons. To the left the light actinides (Th-Pu) show itinerant, bonding 5f electronic behavior [11-13] while to the right one has localized, non-bonding 5f electronic behavior [12]. This gives rise to the possibility of partial delocalization in some of the phases of plutonium, and as shall
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