Ab-initio Studies of Plutonium with Relevance to Nuclear Waste Management
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Ab-initio Studies of Plutonium with Relevance to Nuclear Waste Management Arun K. Setty1 , B. R. Cooper1, and D.L. Price2 1 Department of Physics, West Virginia University, Morgantown, WV 2 University of Memphis, Memphis, Tennessee (deceased) ABSTRACT Plutonium, well known for its unusual nuclear and material properties, has remained an unsolved problem for over a half century. Due to the continuing issue of safe, longterm nuclear waste storage, gaining an understanding of plutonium and its compounds cannot be overstated. Self-irradiation in plutonium leads to vacancy formation [1], and our computations indicate that the electronic structure we predict for pure delta plutonium is preserved in the presence of vacancies, and that vacancies do stabilize the delta phase. A preliminary study of self-diffusive properties of plutonium, and of diffusion at the interface of plutonium and iron indicates that plutonium atoms readily diffuse across the interface with steel. This has relevance in nuclear waste storage in steel containers for assessing the depth of penetration to be dealt with in surface treatment for decontamination. The effect of thermomigration (Soret effect) in the plutonium-steel system appears to facilitate the movement of plutonium atoms into the bulk steel.
I. INTRODUCTION The problem being addressed is to study the diffusive processes by which plutonium from nuclear wastes permeates steel, with which it is in contact. The focus is on delta face-centered-cubic plutonium. Knowledge of the diffusion constants of plutonium in steel and technologically relevant alloys is important in determining the depth of penetration and in estimating the period for which the waste is safely contained in the containers before entering the surroundings. The diffusion occurs via a variety of mechanisms including vacancy mediation (diffusion of plutonium atoms in bulk steel due to vacancies present in steel) and grain boundary diffusion. We will concentrate on the former in this article. Associated processes including vacancy formation and self-diffusion are also studied. Studies of diffusion in plutonium-related wastes has been hindered by the lack of a complete understanding of the electronic structure of plutonium. This element has one of the most complex phase diagrams known, with multiple phase transitions occurring with increasing temperature. These phase transitions are accompanied by abrupt changes in volume and changes from metallic to insulating behavior. The ground state (the low temperature phase persisting down to T=0) is monoclinic, which is unusual in itself, and the technologically relevant high temperature phase (delta) is face-centered-cubic, and less dense by about 27%. A number of other strange properties are present, including a negative thermal expansion coefficient in the delta phase, and an anomalous melting temperature. All of these unusual phenomena can be explained on the basis of an unusual electronic state [2]. We have developed a rigorous ab-initio (SIC-LDA, FP-LMTO) based theory of th
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