Phase Transitions in Pure Plutonium
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Phase Transitions in Pure Plutonium Jeremy N. Mitchell, Franz, J. Freibert, and Daniel S. Schwartz Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545 ABSTRACT The five solid-solid phase transformations of pure Pu are typically represented in idealized thermal expansion plots as having sharp onsets and finishes with linear expansion behavior between the transitions. These behaviors are in reality less common, and the various transitions may have bursting behavior, curved onsets and finishes, and non-linear thermal expansion. In this presentation we will review the transformation behavior of diverse set of pure Pu types. These types include zone-refined pure Pu, electro-refined pure Pu, pure Pu doped with 1000 appm Ga, and alpha-phase Pu within an as-cast 1.9 atomic. % Ga alloy. INTRODUCTION The first reported dilatometry data set on plutonium [1], collected during the Manhattan Project, revealed four solid-solid phase transitions and five stable phases (α, β, γ, δ, ε) between room temperature and 600 °C. Other noteworthy behaviors in this data set include the large volume changes associated with these transitions, negative coefficient of thermal expansion (CTE) for face-centered cubic delta phase, the large temperature hysteresis of the forward and reverse transitions, and step-like features during some of the transitions. A sixth superambient phase, δ´, was first reported in detail by Ellinger [2] and is generally seen in metal with low total impurity contents [3]. Detailed phase transition studies conducted in the 1960’s by American, British, and French scientists further explored the details of these transitions and their mechanisms using various thermophysical properties measurements (Fig. 1), but there has been limited data published since these classic papers from 40-50 years ago. In fact, the transformation behavior of plutonium is now often represented by simplified schematic dilatometry representations that leave out many of the details that reveal their nature [e.g. 4, 5] (Fig. 1). In this paper, we revisit some of these details using dilatometry collected on a suite of samples with varying impurity contents. EXPERIMENT Four types of plutonium samples were used in this study: zone-refined (ZR) α-phase Pu; electrorefined (ER) α-phase Pu; α-phase Pu doped with 1000 atomic ppm (appm) Ga, hereafter referred to as 3T alpha; and an as-cast Pu-1.9 at. % Ga alloy. The bulk chemistry and processing history of the ZR metal has been reported previously in detail [6]. The ER plutonium was produced using standard Los Alamos electrorefining methods [5]. The total impurity contents for these two types of metal, comparing only those elements measured for both types, is 493 appm for zone-refined and 335 appm for electro-refined. The total impurity content for all elements measured is 826 and 1407 appm for ZR and ER, respectively. The impurity contents of the 3T
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alpha and the as-cast Pu-Ga alloy are not currently available. The Pu-Ga alloy was included in this study because
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