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)
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Table I. Physical Properties of Plutonium Compared with Other Metals
Thermal Electrical Metal Conductivity Resistivity Compressibility Young’s Modulus (cal/cmásáK) (Ω cm × 10Ð6) (GPaÐ1) (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 δ-phase to room temperature, making plutonium malleable and easily pressed into different shapes. Yet, this most useful and familiar phase is the least understood theoretically. It has a close-packed structure, but the lowest density. Also, the δ-phase alloys exhibit phase instability, 40 µm transforming readily between the δ- and α-phase (see micrographs) as temperature is changed or stress is applied. And the electronic structure of δ-plutonium, which Grains of Pu–2 at. % Ga δ-phase alloy are may be unique in the periodic table, remains unexplained. shown in this micrograph.
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 are the best indicators of what these electrons are doing, and the graph on the µ 20 m opposite page clearly demonstrates the similarity between the early actinides Illustrated here is the formation of through plutonium and the transition metals. Each additional 5f electron in the α′-phase platelets in δ-phase grains. 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
2 together. In contrast, at americium, the 5f electrons He start behaving like the 4f electrons of the rare earths, 9 10 7 8 localizing at each lattice site and becoming chemi- 5 6 O F Ne 1 C N B 17 18 cally inert. H 15 16 13 14 S Cl Ar 4 P With no 5f contribution to bonding, the atomic 3 Al Si 36 Be 34 35 Li 32 33 Kr 30 31 Br volume suddenly increases at americium and con- 12 29 As Se 11 27 28 Ga Ge 54 25 26 Cu Zn 52 53 Na Mg 24 Ni 50 51 tracts only slightly with increasing atomic number 22 23 Fe Co 49 I Xe 20 21 Cr Mn 47 48 Sb Te 19 V 46 Sn 86 Sc Ti 44 45 Cd In 85 because the 5f electrons remain localized in Ca 43 Ag 83 84 K 41 42 Rh Pd 82 Rn 39 40 Tc Ru 80 81 Po At 37 38 Nb Mo 78 79 Pb Bi the remainder of the series. The pattern of local Y Zr 76 77 Hg Tl 117 118 Sr 75 Au 115 116 Rb 73 74 Ir Pt 114 118 magnetic moments confirms this picture. The light 57 * 72 Re Os 112 113 116 117 55 56 Ta W 110 111 114 115 La Hf 108 109 112 113 Cs Ba 106 107 110 111 actinides show no local moments (as expected 104 105 88 89 ** Sg Bh Hs Mt 87 Db 70 71 Rf 67 68 69 if all the valence electrons are in the conduction Ra Ac 64 65 66 Fr 61 62 63 Yb Lu 58 59 60 Tb Dy Ho Er Tm band), whereas the heavy actinides and the rare Ce Pr Nd Pm Sm Eu Gd 97 98 99 100 101 102 103 Lr 90 91 92 93 95 96 Fm Md No earths generally have local moments that are 89 Bk Cf Es 14 14 Am Cm 11 12 13 5f 5f U Np 7 9 10 5f 5f 5f Ac Th Pa 4 7 5f 5f 5f 2 3 5f 5f Ð Ð 6d produced by their localized 5f and 4f electrons, Ð 5f 5f Ð Ð Ð 2 Ð Ð 6d Ð 2 2 7s 2 6d 6d 2 2 2 7s 7s 6d 6d 2 2 2 7s 7s 7s 6d 2 2 7s 7s 7s respectively. 2 2 7s 7s 7s2 7s 7s We now have rigorous first-principles calculations 94 Pu that reproduce the general trends in atomic volumes, local moments, 5f6 and ground-state structures. These calculations also hint at the origin of instability Ð in plutonium and its lighter neighbors. We can therefore begin to make sense 2 7s of the intriguing temperature-composition phase diagram connecting all the actinides that was drawn almost 20 years ago (see the connected phase Periodic table showing the valence diagram on the opposite page). electrons in isolated atoms of the The most important insight from calculations is that, in the early actinides, actinides. the 5f electrons from different atoms overlap—but just barely. They therefore
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2.3
2.2
2.1
2.0 Rare earths (4f) The atomic radii of the actinide metals 1.9 (Å) are compared with those of the rare δ-Pu ws Actinides (5f) earths and transition metals. R 1.8
1.7 Transition metals (5d) 1.6
1.5
Rare earths: Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Actinides: Th Pa U Np Pu Am Cm Bk Cf Transition metals:La Hf Ta W Re Os Ir Pt Au occupy a very narrow conduction (or energy) band, which has a very high density of states near the Fermi energy. As the number of 5f electrons populating that band increases, the specific properties of the band begin to dominate the bonding proper- ties of the metal. For example, lattice distortions, or low-symmetry structures, are favored in ° narrow-band materials because they split the band Temperature ( C) in certain regions and thereby lower the total energy. Thus, the connected phase diagram shows that uranium, neptunium, and plutonium exhibit 1500 bcc low-symmetry ground-state structures rather than high-symmetry structures, which are found in Liquid most metals. Before this new insight, the low- symmetry ground state of plutonium was attrib- 1000 uted to directional or covalent-like bonding resulting from the angular characteristics of fcc Tetragonal fcc f electrons. Also, because the narrow 5f band
Orthorhombic ε T C e overlaps the s, p, and d bands, a number of elec- t o r 500 a δ′ m M g O o o r p t δ tronic configurations have nearly equal energy, n n h l dhcp a o e o l c . x l in leading to the multiple allotropic forms of c M ic γ u O o b n
i r o β uranium, neptunium, and plutonium and their c t h c lin o i α great sensitivity to external influences. . c Thus, many of the peculiarities of plutonium Ac Th Pa U Np Pu Am Cm (anomalously low melting point, astonishingly high number of allotropes, and an unusual preference for low-symmetry crystal The connected binary-phase diagram structures) are not part of a single anomaly but the culmination of a systematic of the actinides illustrates the transition trend in the early actinides resulting from the narrow 5f band. After plutonium, the from typical metallic behavior at thorium 5f electrons are completely localized, and typical metallic structures and behaviors to complex behavior at plutonium and return. Right at plutonium, however, between the monoclinic α-phase, which is sta- back to typical metallic behavior past ble at room temperature, and the fcc δ-phase, which is stable at elevated tempera- americium. tures, there appears to be a major transition in the 5f electronic structure from bonding to partial localization. It is that tendency toward partial localization that seems to contribute to plutonium’s dramatic variability and sensitivity to external influences. The fundamental nature of the transition is at the research frontier of condensed matter physics. The consequences of the transition are critically impor- tant to the structural properties of plutonium—particularly to phase stability.
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Complex Chemistry of Plutonium
The chemical behavior of plutonium, like its solid-state behavior, is controlled by its electronic structure. The lowest energy configuration of the valence electrons is nominally 5f67s2, but the energy levels of the 6d and 5f orbitals are similar, and the 5f67s2 and 5f56d7s2 electronic configurations compete. In fact, as one moves from thorium to plutonium in the periodic table, the orbital energies invert—the 5f orbitals become lower in energy than the 6d orbitals. Unlike the 4f orbitals in the lanthanides (rare earths), the 5f and 6d orbitals in the light actinides up to ameri- cium extend far enough to be accessible for chemical bonding. Consequently, the light actinides exhibit complex chemical behavior reminiscent of the d–transition elements. In the heavy actinides beyond americium, the 5f orbitals are pulled in and not involved in chemical bonding, and therefore the chemical behavior of those heavy actinides and the lanthanides are quite similar. Each plutonium oxidation state has a In the metal, multiple electronic configurations of nearly equal energy lead to characteristic color in solution. multiple solid-state phases. In the atom, they produce a tendency to supply more bonding electrons in chemical reactions and thus to exhibit a very complex chem- istry. A plutonium atom in aqueous solution will readily lose between three and seven of its outer electrons to form positively charged cations in five formal oxida- tion states ranging from Pu(III) to Pu(VII). Many different oxidation states, however, make solution chemistry very complex because each of PuO CO (aq) PuO (OH ) (aq) them forms different compounds or complexes. The plutoni- 2 3 2− 2 2 2 PuO2(CO3)2 um complexes span a broad range of structural motifs and 1.0 + PuO2F typically have high coordination numbers (that is, the pluto- 4− 2+ PuO (CO ) nium cation surrounds itself with many anions). Coordina- PuF PuO + 2 3 3 2 2 tion numbers from 3 to 12 have been observed for various oxidation states. 0.5 − Moreover, under acidic conditions, the energy needed Rain/streams PuO2CO3 to add or subtract electrons (the reduction-oxidation, or Ocean redox potential) and thereby change oxidation states is very 3+ Pu similar (approximately 1 volt). Thus, plutonium will often PuO2OH(aq) Groundwater change oxidation states in solution and follow several differ- 0 ent chemical pathways simultaneously. It can also exist + PuOH2 in several oxidation states within the same solution and, Redox potential, Eh (volts) Pu(OH)4(aq) at very low pH, can even exhibit four oxidation states Pu(III) Pu(IV) simultaneously. This last feature makes plutonium unique −0.5 Pu(V) among the elements. Pu(VI) The shapes of 5f and 6d orbitals, in addition to their radial Range of natural waters extents, affect molecular (not metallic) bonding. The f- and 02468101214d-electron wave functions with three and two quanta of pH angular momentum, respectively, are nonspherical and have highly directional lobes, producing certain unique covalent The range of plutonium complexes and motifs. For example, in water, actinides in the (V) and (VI) oxidation state immedi- oxidation states in water containing car- bonate, hydroxyl, and fluoride ions is evi- ately bond with two oxygens to form the linear “actinyl” motif. Plutonyl O=Pu=O, dent in the redox potential vs pH diagram. uranyl O=U=O, and neptunyl O=Np=O are examples. Once formed, other ligands In many natural waters, plutonium will be bond in the equatorial plane bisecting the actinyl motif (see figure). in the relatively insoluble IV state. Redox Each oxidation state can form various molecular complexes, each with a charac- reactions, however, will allow plutonium teristic solubility and chemical reactivity. The instabilities of plutonium’s oxidation to assume any of the oxidation states III, states, coupled with the creation of free radicals through its radioactive decay, yield V, and VI. Overall therefore, the environ- solution chemistry that is constantly changing and affecting chemical processing mental behavior of plutonium is extremely operations, nuclear waste storage and treatment, and the reactivity and mobility complicated. The red dots mark condi- of plutonium in the environment. Over the years, the redox states of plutonium have tions under which plutonium can coexist in three different oxidation states been controlled by complexation with various ligands. Some of these, such as simultaneously.
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the nitrate anion, have played a historically critical role in the processing and purifi- cation of plutonium. Other ligands, such as the carbonate and hydroxyl anions, are ubiquitous in natural groundwaters and play an influential role in the transport of plutonium in the natural environment. Pu When actinides enter the environment, the chemical interactions are inordinately The molecule
complex because, in addition to natural waters, hundreds of chemically active com- PuO2(NO3)2(H2O)2 pounds and minerals exist in earth formations. Precipitation and dissolution of contains the lin- actinide-bearing solids limit the upper actinide concentration in solution, while com- ear plutonyl motif plexation and redox reactions determine the species’ distribution and stability. O=Pu=O at its The interaction of a dissolved species center. with mineral and rock surfaces O and/or colloids determines if and how those species will migrate through the environ- ment. The prevalent oxidation state in groundwater is Pu(IV), whereas Pu(III), Pu(V), and Pu(VI) are also common in streams, brines, or bogs. The light actinides, including Np(V) carbonate complex 5– plutonium, form the strongest, NpO2(CO3)3
most stable complexes in the IV oxidation state. (See the article “The Chemical Interac- tions of Actinides in the Envi-
ronment” on page 392.) Np(IV) aquo ion