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Plutonium Is a Physicist's Dream but an Engineer's Nightmare. with Little

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Plutonium Is a Physicist's Dream but an Engineer's Nightmare. with Little Plutonium An element at odds with itself lutonium is a physicist’s dream but an engineer’s nightmare. With little provocation, the metal changes its density by as much as 25 percent. It can Pbe as brittle as glass or as malleable as aluminum; it expands when it solidi- fies—much like water freezing to ice; and its shiny, silvery, freshly machined surface will tarnish in minutes. It is highly reactive in air and strongly reducing in solution, forming multiple compounds and complexes in the environment and dur- ing chemical processing. It transmutes by radioactive decay, causing damage to its crystalline lattice and leaving behind helium, americium, uranium, neptunium, and other impurities. Plutonium damages materials on contact and is therefore difficult to handle, store, or transport. Only physicists would ever dream of making and using such a material. And they did make it—in order to take advantage of the extraordinary nuclear properties of plutonium-239. Plutonium, the Most Complex Metal Plutonium, the sixth member of the actinide series, is a metal, and like other metals, it conducts electricity (albeit quite poorly), is electropositive, and dissolves in mineral acids. It is extremely dense—more than twice as dense as iron—and as it is heated, it begins to show its incredible sensitivity to temperature, undergoing dramatic length changes equivalent to density changes of more than 20 percent. Six Distinct Solid-State Phases of Plutonium Body-centered orthorhombic, 8 atoms per unit cell Face-centered cubic, 4 atoms per unit cell Body-centered monoclinic, Body-centered cubic, 8 34 atoms per unit cell 2 atoms per unit cell δ δ′ ε Contraction on melting 6 γ L L 4 β Monoclinic, 16 atoms per unit cell Anomalous Thermal Expansion and Phase Instability of Plutonium Length change (%) 2 Pure Pu Pure Al α 0 200 400 600 800 Temperature (°C) 16 Los Alamos Science Number 26 2000 Plutonium Overview Table I. Physical Properties of Plutonium Compared with Other Metals Thermal Electrical Metal Conductivity Resistivity Compressibility Young’s Modulus (cal/cm·s·K) (Ω cm × 10–6) (GPa–1) (GPa) Aluminum 0.530 2.9 0.015 70 Stainless Steel 0.036 70 0.0007 180 α-Plutonium 0.010 145 0.020 100 δ-Plutonium (Pu-Ga) 0.022 100 0.033 42 At certain temperatures, the density changes are discontinuous as plutonium suddenly transforms into a new phase, or crystal structure. As it is being heated to its melting point at atmospheric pressure, plutonium will take on six distinct crystal structures; at higher pressures, it will take on a seventh structure. At room temperature and below, it is in the brittle α-phase, which has an unusual low- symmetry monoclinic structure typical of minerals. At 583 kelvins, it takes on the highly symmetric face-centered-cubic structure of the δ-phase, a close-packed structure typical of metals such as aluminum and copper. Even in a single phase, plutonium demonstrates unusual behavior—expanding when heated in the α-phase at a rate almost 5 times the rate in iron and contract- ing while being heated in the δ-phase. Then, at an unusually low 913 kelvins, plu- tonium melts. While melting, it contracts because the liquid is denser than the previous solid phase. In the liquid state, plutonium has a very high surface tension and the greatest viscosity of any element. The table above compares plutonium with other metals. Like stainless steel, plutonium is a poor electrical and thermal conductor, and it is very soft, or compressible—even more so than aluminum. In addition, recent measurements have shown that the softening increases much more rapidly than expected with increasing temperature. The α-phase has undergone brittle frac- Cooling below room temperature brings out other atypical behaviors. Plutoni- ture under torsion. um’s electrical resistivity, already very high at room temperature, increases as the temperature is lowered to 100 kelvins. The energy required to heat plutonium (its specific heat) is 10 times higher than normal at temperatures close to absolute zero. Its magnetic susceptibility, also atypically high, remains constant with perhaps a slight increase as the temperature is lowered, indicating a tendency toward magnet- 0.004 α-Pu 150 0.003 UBe13 Np .cm) µΩ 100 0.002 Am Most metals— Nb, Mo, Ta, Hf, 50 Pt, Na, K, Rb 0.001 Electrical resistivity ( Pa U α Magnetic susceptibility (emu/mole) -Pu δ-Pu Th 0 50 100 150 200 250 300 200 400 600 800 Temperature (K) Temperature (K) Anomalous Resistivity High Magnetic Susceptibility Number 26 2000 Los Alamos Science 17 Plutonium Overview ism. But even at the lowest temperatures, plutonium never settles down to a state of long-range order (either magnetic or superconducting) as other metals do. Plutonium is sensitive not only to temperature and pressure but also to chemi- cal additions. In fact, much of its density and structural instability can be circum- vented by additions of a few atomic percent of aluminum or gallium. These additions retain the face-centered-cubic plutonium malleable and easily pressed into different shapes. Yet, this most useful δ and familiar phase is the least understood theoretically.-phase to room It hastemperature, a close-packed making structure, but the lowest density. Also, the 40 µ Grains of Pu–2 at. % Ga m transforming readily between the δ δ shown in this micrograph. -phase alloy are is changed or stress is applied. And the electronic-phase structure alloys exhibitof phase instability, δ- and α may be unique in the periodic table, remains-phase unexplained. (see micrographs) as temperature δ-plutonium, which It’s the 5f Electrons Why does plutonium metal behave so strangely? Knowing that electronic structure determines nonnuclear properties, we turn to the periodic table and recent insights from modern calculations. The actinides mark the filling of the 5f atomic subshell much like the rare earths mark the filling of the 4f sub- shell. Yet, the 5f electrons of the light actinides behave more like the 5d electrons of the transition metals than the 4f electrons of the rare earths. Atomic volumes Illustrated here is the formation of20 µm are the best indicators of what these electrons are doing, and the graph on the α′-phase platelets in opposite page clearly demonstrates the similarity between the early actinides δ-phase grains. through plutonium and the transition metals. Each additional 5f electron in the early actinides leads to a decrease in atomic volume. The reason was guessed decades ago. Like the 5d electrons, the 5f electrons go into the conduction band, where they increase the chemical bonding forces, pulling the atoms closer 1 2 together. In contrast, at americium, the 5f electrons H He 3 4 5 6 7 8 9 10 start behaving like the 4f electrons of the rare earths, Li Be B C N O F Ne localizing at each lattice site and becoming chemi- 11 12 13 14 15 16 17 18 Na Mg Al Si P S Cl Ar cally inert. 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 With no 5f contribution to bonding, the atomic K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr 37 38 volume suddenly increases at americium and con- 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe tracts only slightly with increasing atomic number 55 56 57 * 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn because the 5f electrons remain localized in 87 88 89 ** 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 the remainder of the series. The pattern of local Fr Ra Ac Rf Db Sg Bh Hs Mt 110 111 112 113 114 115 116 117 118 magnetic moments confirms this picture. The light 58 59 60 61 62 63 64 65 Ce 66 actinides show no local moments (as expected Pr Nd 67 68 69 Pm Sm Eu Gd 70 89 90 71 91 92 93 95 96 Tb Dy Ho Er Tm Yb Lu if all the valence electrons are in the conduction Ac Th Pa U Np 97 98 99 100 101 102 103 2 Am – – 5f 3 4 Cm Bk band), whereas the heavy actinides and the rare 5f 5f 7 Cf Es 6d 2 5f 7 9 Fm 6d 6d 5f 5f 10 11 Md No Lr 6d 6d 5f 5f 12 13 7s2 2 2 – 5f 5f 14 14 earths generally have local moments that are 7s 7s 2 2 6d – 5f 5f 7s 7s 2 – – 7s 2 2 – – 7s 7s 7s2 2 – 6d 7s 7s2 2 produced by their localized 5f and 4f electrons, 7s 7s2 7s2 94 respectively. Pu that reproduce the generalWe now trends have in atomicrigorous volumes, first-principles local moments, calculations 5f6 and ground-state structures. These calculations also hint at the origin of instability – in plutonium and its lighter neighbors. We can therefore begin to make sense 7s2 Periodic table showing the valence of the intriguing temperature-composition phase diagram connecting all electrons in isolated atoms of the the actinides that was drawn almost 20 years ago (see the connected phase actinides. diagram on the opposite page). The most important insight from calculations is that, in the early actinides, the 5f electrons from different atoms overlap—but just barely.
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