Plutonium and Its Alloys From atoms to microstructure Siegfried S. Hecker 290 Los Alamos Science Number 26 2000 Plutonium and Its Alloys lutonium is an element at odds with itself—with little provocation, it can change its density by as much as 25 percent; it can be as brittle as glass or as malleable as alu- Pminum; it expands when it solidifies; and its freshly-machined silvery surface will tarnish in minutes, producing nearly every color in the rainbow. To make matters even more complex, plutonium ages from the outside in and from the inside out. It reacts vigor- ously with its environment—particularly with oxygen, hydrogen, and water—thereby, degrading its properties from the surface to the interior over time. In addition, plutonium’s continuous radioactive decay causes self-irradiation damage that can fundamentally change its properties over time. Only physicists would think of using such a material. In the periodic table, plutonium is element 94, and it fits near the middle of the actinide series (ranging from thorium to lawrencium, atomic numbers 90 to 103). Plutonium is of practical interest principally because the 239 isotope has attractive nuclear properties for energy production and nuclear explosives. Manhattan Project physicists managed to extract the more than millionfold advantage of plutonium over conventional explosives. It was the chemists and metallurgists who learned how to extract plutonium, fabricate it, and keep it sound until the time of detonation. The Manhattan Project history of plutonium metallurgy recently published by Edward Hammel (1998) is a remarkable tale of scientific and engi- neering achievement. These pioneers were working with a metal whose electronic structure and consequent engineering properties were even more puzzling than its nuclear properties. In a remarkably short period, they learned enough to accomplish their goal and left the rest for us to decipher. The end of the Cold War has signaled a dramatic change in the nuclear weapons pro- grams of the nuclear powers. The challenge now is to help reduce the size of the nuclear arsenals while ensuring that the nuclear weapons are safe and reliable into the indefinite future—without nuclear testing and without a continuous cycle of new nuclear weapons development and deployment. Therefore, extending the lifetimes of plutonium components is more important now than in the past. Similarly, remanufacturing plutonium components for existing weapons systems has become a greater challenge because no new plutonium components have been fabricated for almost 12 years. Moreover, the manufacturing facilities no longer exist, and most of the technical experts have now retired. The long- term behavior of plutonium is also important at the back end of the nuclear weapons cycle—the dismantlement and disposition phases. Because many thousands of nuclear weapons are being withdrawn from the nuclear arsenals of Russia and the United States, we must deal with excess plutonium recovered from these warheads. But the reactive and continuously changing nature of plutonium makes this task a serious challenge. Com- pounding this challenge is the fact that excess weapons plutonium must be carefully secured against diversion or theft. Burning as fuel in nuclear reactors and geologic disposition are the most likely methods for its eventual disposal. In either case, plutonium must be stored for decades or longer. It has therefore become imperative that we under- stand the aging of plutonium and of its alloys or compounds. And if we are to accomplish this goal, the next generation of scientists and engineers must become deeply involved in deciphering the complexities of plutonium. Number 26 2000 Los Alamos Science 291 Plutonium and Its Alloys Both this article and “Mechanical 8 Behavior of Plutonium and Its Alloys” δ (page 336) describe the fascinating Pure Pu δ′ mysteries of plutonium metallurgy in a ε forum open to the research community 6 with the hope of attracting those young L men and women into plutonium γ research. At Los Alamos, we are trying L to move from an empirical approach to β one based on fundamental principles. 4 Pu Crystal Density At the moment, however, our knowl- Structure (g/cm3) edge rests with the practitioners—and α Length change (%) Simple Monoclinic 19.86 most of our experienced plutonium β Body-Centered Monoclinic 17.70 practitioners have retired or are nearing γ Face-Centered Orthorhombic 17.14 2 Pure Al δ Face-Centered Cubic 15.92 retirement. To develop a solid funda- δ′ Body-Centered Tetragonal 16.00 mental understanding of plutonium, we ε Body-Centered Cubic 16.51 L Liquid 16.65 need the most modern ideas and tools α from the international scientific research community. We can then apply this un- 0 200 400 600 800 derstanding to our practical problems, Temperature (°C) many of which must naturally remain secret to the public. Figure 1. Anomalous Length Changes in Plutonium Plutonium is a unique element in exhibiting six different crystallographic phases at The Unusual Properties ambient pressure (it has a seventh phase under pressure). In addition, unlike most of Plutonium metals, plutonium contracts on melting. Transformations to different crystal structures occur readily and are accompanied by very large volume changes. By comparison, Here, I will describe how plutonium aluminum’s behavior is predictable and uneventful. It expands monotonically on heat- is unusual before tackling the question ing in the solid phase, and it also expands on melting. The dashed lines show that of why it is so. thermal contraction on cooling the liquid (L) phase of plutonium extrapolates to that Manhattan Project pioneers were of the β-phase; the thermal contraction on cooling the ε-phase extrapolates to that of puzzled by plutonium’s unusual behav- the γ-phase. ior right from the beginning. As soon as they received sufficient amounts of the new element to measure its density, significant changes in other properties elements such as aluminum or gallium. they found unexplained variations rang- (see Table I). In particular, the δ-phase, The benefits of adding gallium, and ing from 8 to 24 grams per cubic which is stable at high temperatures, is thereby retaining plutonium in the δ- centimeter (g/cm3)—see the article desirable because its highly symmetric phase, are easily derived from Figure 2. “The Taming of ‘49’” on page 48. face-centered-cubic (fcc) structure All alloying elements are “impurities” Also, some tiny samples were as mal- makes it very malleable (ductile) and in a nuclear chain reaction because they leable as aluminum, whereas others easily formed into desired shapes. In reduce the number of plutonium-239 were as brittle as glass. The list of contrast, the room temperature α-phase atoms per unit volume, but metallurgical remarkable properties is quite long (see is an engineering nightmare—its simple considerations strongly favor using the the box “The Unusual Properties of monoclinic, low-symmetry structure δ-phase alloys for weapons applications. Plutonium” on page 294), but it was makes it very brittle. (It has been the The amount of alloying elements, how- only after the war that those properties metallurgists’ tradition to designate ever, must be kept to a minimum, so were studied systematically. polymorphic phases of elements and plutonium-rich alloys are of greatest The most exasperating property alloys with symbols from the Greek interest. Because requirements for a con- from an engineering standpoint is the alphabet, beginning with α for the trolled chain reaction in a nuclear reactor extraordinary thermal instability of plu- lowest-temperature phase.) are very different, reactor alloys or com- tonium—that is, the large length (or The Manhattan Project pioneers soon pounds span a much broader range of volume) changes during heating or cool- discovered that they could prevent trans- plutonium concentrations. ing shown in Figure 1. These volume formation to the three low-temperature The mysteries of plutonium metal- (or phase) changes are accompanied by phases by intentionally adding chemical lurgy have been studied over the 292 Los Alamos Science Number 26 2000 Plutonium and Its Alloys years within the metallurgical and con- 8 δ densed-matter physics communities— δ′ unfortunately, with rather little collabo- ε ration between the two. These Pure Pu 6 L communities do not even share a com- γ mon language. For example, they cannot agree on what to call a phase change— β whereas physicists prefer transition, 4 metallurgists prefer transformation. Yet, the behavior of plutonium defies conventional metallurgical wisdom. 2 Understanding plutonium involves a Length change (%) Pu 4.5 at.% Ga close collaboration between physicists, metallurgists, and chemists. Metallurgists α δ ε 0 must learn to appreciate the intricacies δ ε L of electronic bonding, especially the L role of 5f electrons. Physicists must Pu 3 at.% Ga develop an appreciation for the role of –2 microstructure and crystal defects in 0 200 400 600 800 ° determining the engineering properties Temperature ( C) of plutonium. My intention in writing this article and the companion one on Figure 2. The Benefits of Alloying Plutonium mechanical properties was to bridge Both unalloyed plutonium and Pu-Ga alloys expand upon solidification to the bcc the gap between the two communities ε-phase, which expands when it transforms to the fcc δ-phase. Upon cooling, howev- and complement the very informative er, plutonium alloys do not exhibit the enormous shrinkage of unalloyed plutonium. articles on plutonium condensed-matter They contract only slightly as they cool to room temperature because they remain in physics found in Volume I of this issue the δ-phase, avoiding the transformation to γ, β, and α. Increases in gallium concen- of Los Alamos Science. tration shift the melting temperature and the δ to ε transformation to slightly higher temperatures. 5f Electrons for Metallurgists Table I. Comparison of Some Properties of α- and δ-Phase Plutonium On a fundamental level, the proper- Property α-Plutonium δ-Plutonium ties of solids are determined by their (unalloyed)(1.8 at.