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Magnetic Levitation Results in High-Purity Plutonium Metal

Plutonium, a member of the actinide series of elements, exhibits seven unique crystal structures (phases) in the solid state that range in symmetry from simple monoclinic (sm) to face-centered-cubic (fcc). One phase, the easily worked fcc phase, denoted by delta, is thermodynamically stable in pure plutonium from 319°C to 451°C and can be stabilized down to room temperature by small additions of gallium. The actinide series is characterized by the presence of 5f-electrons forming a narrow energy band at the Fermi energy (Ef), with an increasing complexity in the electronic properties that culminates in plutonium.

Figure 1 . Levitation zone refining apparatus illustrated with a stainless steel rod loaded in the cold boat. The hot zone on the rod can be clearly seen from the red glow caused by the induction from the coil.

The complex electronic structure coupled with small perturbations such as the presence of trace elemental impurities, has effects on both microscopic and macroscopic behavior. Overall, the primary metallic and metalloid impurities in plutonium are iron, uranium, magnesium, calcium, nickel, aluminum, potassium, and silicon. Oxides and hydrides, formed from surface reactions between the metal and oxygen and water vapor are equally undesirable. Additionally, small amounts of some common trace elemental impurities (thorium, aluminum, zirconium, and cerium) stabilize the delta phase, while others affect the melting point. Iron and nickel, if present in sufficient amounts, form eutectics with plutonium that melt slightly above 400°C. Furthermore, plutonium decays over time into its daughter products, which contribute to the total impurity content. The primary plutonium decay products are isotopes of uranium, neptunium, and americium. One, Americium-241, is a d stabilizer and also interferes with a determination of nuclear cross sections.

A comprehensive knowledge is lacking for plutonium, compared to the level of knowledge for other elemental metals. The high degree of complexity in plutonium's intrinsic properties severely restricts extrapolation of knowledge from surrogate metallic systems. Consequently, fundamental measurements on well-characterized, research-grade (100 to 200 ppm total impurities) plutonium are required. Therefore, our efforts are directed at the preparation and characterization of large quantities (200 to 300 g) of research-grade plutonium, in support of fundamental experimentation.



Figure 2. Molten plutonium metal in a levitated state in the middle of the crucible. Enough gallium has been added to stabilize the delta phase upon solidification (see Figure 3).

Our approach to the preparation of research-grade plutonium metal is to start with doubly electrorefined, vacuum-cast plutonium metal and further purify it using levitation zone refining in concert with levitation distillation at reduced pressure. In particular, levitation zone refining targets metals and metalloids while levitation vacuum distillation targets decay products and gases. Double-electrorefined plutonium that has been chill-cast contains 500 to 600 ppm total impurities. Impurity totals are derived from mass spectrometry and atomic emission data in which 75 to 80 trace elements are analyzed for and then quantified. Historically, 20 to 40 elements represent the typical range of elements documented in the early literature.

Magnetic Levitation of Metallic Plutonium

Obtaining research-grade plutonium requires minimizing the contact of large amounts of plutonium with other materials at reduced pressure. Presently, levitation furnaces provide a tool for accomplishing this task. Radio-frequency-power-induced electric current flows into a crucible while the crucible acts as a transformer inducing a current in the direction opposite to the current in the induction coil. Magnetic fields in the crucible and the plutonium are opposed, causing repulsion and levitation between molten plutonium and the crucible walls. Magnetic levitation of plutonium metal at elevated temperatures (700°C to 1000°C) enables purification; eliminating plutonium/crucible interactions further minimizes the contact with other elements. Magnetic levitation is the fundamental operating basis for both the zone refining apparatus and the distillation apparatus.

The effectiveness of zone refining in reducing the concentration of impurities depends on the manner in which the impurity partitions itself in the liquid and solid states during the melting and solidification processes. The zone refining process involves casting a rod of unalloyed plutonium (usually 240 g) and then serially passing a molten zone through the rod in one direction at a slow rate (Figure 1). Impurities travel with, or opposite to, the direction of motion of the zone, depending on whether it lowers or raises the melting point of the rod metal, respectively. Consequently, impurities are swept and become concentrated in the ends of the rod, thereby leaving the remainder purified. The degree of separation approaches an infinite-simal limit as the number of passes increases.

Results to date have shown the reduction of impurities in double-electrorefined and vacuum-cast unalloyed plutonium from a total of 522.9 ppm ( 78.4 ppm) impurities to 184 ppm (27.6 ppm) through levitation zone refining; uranium accounts for 121 ppm of the measured 184 ppm. One important event, previously undocumented, is the absence of plutonium/crucible interaction; no crucible material has been detected in the plutonium by trace elemental analysis. In addition, in all cases, trace analysis showed impurities to have moved to the ends of the rod in accord with their predicted partitioning behavior. It was also determined that the slower rates of molten zone movement increased the purification efficiency. Additionally, reducing the atmosphere and a surface cleaning of the rod have proved beneficial.

Levitation Vacuum Distillation

Removing Americium-241 from plutonium is possible because americium exhibits a high vapor pressure relative to plutonium. The Americium-241 is separated when plutonium metal is heated to the liquid state under reduced pressure (10-7 torr). The molten plutonium is levitated while Americium-241 is driven off and condensed onto a cold surface (Figure 2). The plutonium is cooled and solidifies to the shape of a button (Figure 3).


Figure 3. Plutonium metal (delta-phase alloy) half-sphere solid from a levitation distillation run.

Recent results from levitation distillation show that the lowest americium level achieved was 1.50 ppm (0.30 ppm). In addition, no crucible/plutonium interactions were evident. Plutonium in the liquid state appears as a viscous boiling liquid. Figure 2 shows a top view looking through the glove box window into the distillation apparatus vacuum chamber view port at the plutonium metal after 1 hour with the power supply at 40 kW. The effect of the levitation force that can be seen in the illustration is that the top surface of the plutonium is spherical instead of flat. Bubbles appear on the surface while the impurities are driven off in the vapor state. After 30 minutes the bubbling rate slows down to the occasional formation of a large bubble, but no splatter occurs as a result.

Summary

Large batches (100 g300 g) of unalloyed plutonium were purified of trace elements to a level of 184 ppm, and uranium accounted for 66% of the total impurity level. In all batches the impurities traveled to the ends of the rod as predicted from their partitioning behavior. Trace analysis detected no plutonium/crucible interaction in either purification method. The lowest concentration of Americium-241 measured from the levitation distillation process was 1.50 ppm (0.30 ppm). The purification hinges on levitating plutonium metal at elevated temperatures.

Principal investigators on this project were Michael S. Blau (NMT-6) and Jason C. Lashley (MST-8).

We thank Floyd Rodriguez of the Advanced Technology Group for his assistance in the laboratory. We are also grateful to Tom Yoshida, Amy Wong, and Debbie Figg of the Analytical Chemistry Group for the trace analysis. We are grateful to John Quagliano of the Analytical Chemistry Group for development of data analysis. We thank Galen Straub and John Wills of the Equation of State Group, James L. Smith of the Superconductivity Technology Center, Albert Migliori of the Condensed Matter Physics Group, Sig Hecker of the Materials Science Division Office, Mike Stevens, Roger Moment, and Mike Stout of the Structure/Property Relations Group, David Olivas, Karl Staudhammer, and Ramiro Pereyra of the Physical Metallurgy Group, and Dave Embury of the Center for Materials Science. We are grateful for the joint funding support from Stephen Sterbenz of the Los Alamos Neutron Scattering Center, Eugene Farnum of the Structure/Property Relations Group; Alan Picklesimer, Dean Preston, and Miles Baron of the Nuclear and Hydrodynamic Applications Group, and Don Wolkerstorfer of the Nuclear Weapons Technology Program Office.


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