Plutonium
Plutonium is a radioactive chemical element with atomic number 94 and chemical symbol Pu.[1][2] Classified as an actinide in the periodic table, it is a transuranic element occurring naturally only in trace quantities but primarily produced artificially through neutron irradiation of uranium-238 in nuclear reactors.[3][4] First synthesized in December 1940 at the University of California, Berkeley, by Glenn T. Seaborg and his team via deuteron bombardment of uranium, plutonium was isolated in February 1941.[5][6] The element exhibits complex allotropic behavior with six phases at ambient pressure, a silvery-white metallic appearance that rapidly tarnishes in air due to oxidation, and high reactivity including pyrophoricity in finely divided forms.[7] Plutonium-239, the fissile isotope bred in reactors, powers implosion-type nuclear weapons and serves as fuel in mixed-oxide reactor fuel, while plutonium-238 generates heat for space missions via radioisotope decay.[2][8] Its extreme toxicity, alpha-emitting radioactivity, and proliferation risks underscore stringent handling protocols and international safeguards on production.[9][3]Properties
Physical properties
Plutonium is a silvery-white metal that tarnishes rapidly upon exposure to air, forming a dull gray, yellow, or olive-green oxide layer.[1][10] In its pure form, it exhibits a bright metallic luster initially.[11] The element exists as a solid at standard temperature and pressure, with a density of 19.816 g/cm³ in its room-temperature alpha phase, though this varies across its six allotropic forms due to differences in crystal packing.[12][13] Its melting point is 640 °C and boiling point is 3230 °C, reflecting relatively low melting but high boiling characteristics for a heavy actinide.[12][14] Plutonium has an empirical atomic radius of approximately 151 pm and a van der Waals radius of 200 pm.[11] Unlike typical metals, plutonium displays poor thermal and electrical conductivity, with room-temperature electrical resistivity around 1.5 × 10⁻⁶ Ω·m—high for a metal—and thermal conductivity that remains low even at elevated temperatures up to 822 K.[1][12] These properties arise from its complex electronic structure and phase instability, contributing to anisotropic expansion and contraction during thermal cycling.[15]
| Property | Value |
|---|---|
| Density (α-phase, 25°C) | 19.816 g/cm³ |
| Melting point | 640 °C |
| Boiling point | 3230 °C |
| Electrical resistivity (RT) | 1.5 × 10⁻⁶ Ω·m |
The density-temperature relationship exhibits sharp changes at phase transition points, as plutonium undergoes transformations between its allotropes, with the alpha phase being the densest.[15] This behavior necessitates careful handling to avoid mechanical stresses from expansion coefficients varying by phase, such as a linear thermal expansion coefficient of about 55 × 10⁻⁶ K⁻¹ in certain modifications.[16]
Allotropes
Plutonium exhibits six distinct allotropic phases under ambient pressure, each characterized by unique crystal structures, densities, and temperature stability ranges, resulting from the complex bonding involving its 5f electrons. These phases lead to substantial volume contractions and expansions during transitions, with density variations spanning approximately 15.92 to 19.86 g/cm³, which poses significant challenges for machining and structural integrity in applications.[1][17] The α phase, stable below approximately 122 °C, adopts a monoclinic crystal structure with a high density of 19.86 g/cm³, rendering it hard and brittle, akin to ceramic materials.[18][4] Upon heating, it transforms to the β phase around 125–211 °C, which has a body-centered monoclinic structure and lower density of 17.70 g/cm³.[18] The γ phase follows between roughly 211–310 °C, featuring a face-centered orthorhombic structure at 17.14 g/cm³.[18] Higher-temperature phases include the δ phase, stable from about 310–452 °C, with a face-centered cubic structure and the lowest density of 15.92 g/cm³; this phase is notably ductile when stabilized at room temperature via alloying with elements like gallium (typically 0.5–1 atomic percent), enabling easier fabrication for nuclear components.[1][19] The δ' phase, body-centered tetragonal with 16.00 g/cm³ density, exists narrowly from 452–475 °C, transitioning to the ε phase (body-centered cubic, 16.51 g/cm³) up to the melting point at 640 °C.[18]| Phase | Crystal Structure | Density (g/cm³) | Approximate Stability Range (°C) |
|---|---|---|---|
| α | Monoclinic | 19.86 | < 122 |
| β | Body-centered monoclinic | 17.70 | 125–211 |
| γ | Face-centered orthorhombic | 17.14 | 211–310 |
| δ | Face-centered cubic | 15.92 | 310–452 |
| δ' | Body-centered tetragonal | 16.00 | 452–475 |
| ε | Body-centered cubic | 16.51 | 475–640 |
Chemical properties
Plutonium is a chemically reactive metal that exhibits oxidation states from +3 to +6 under typical conditions, with +4 being the most stable and common in its compounds.[21] Oxidation states +2 and +7 have been achieved through specialized synthetic methods, such as organometallic complexes for +2 and strong oxidants for +7.[22] In aqueous solutions, these states display distinct colors: Pu(III) ranges from green to violet, Pu(IV) from pink to brown, Pu(V) light purple, Pu(VI) light brown, and Pu(VII) dark green.[19] The metal tarnishes rapidly in air, forming a passive oxide layer of PuO₂ that protects the bulk from further oxidation under dry conditions, though finely divided plutonium is pyrophoric and ignites spontaneously.[19] Exposure to moist air or water vapor accelerates oxidation, with water reacting directly to form plutonium hydroxide and hydrogen gas; the reaction rate for massive metal is slow compared to alkali metals but increases with surface area.[18] Plutonium does not react with alkalies but dissolves readily in concentrated hydrochloric, hydroiodic, and perchloric acids; it resists dilute nitric acid due to the stable oxide coating.[19] Plutonium forms a variety of compounds, including oxides like PuO₂ (used in nuclear fuel), hydrides such as PuH₂ (which expand the metal volume), and halides like PuF₄ and PuCl₃.[13] These compounds reflect its actinide chemistry, where f-orbital involvement leads to variable coordination geometries and complex redox behavior, enabling disproportionation reactions in solution, such as 3Pu(IV) → Pu(III) + 2Pu(V).[21]| Oxidation State | Typical Ion | Color in Aqueous Solution (approx.) | Notes |
|---|---|---|---|
| +3 | Pu³⁺ | Green-violet | Reducing conditions |
| +4 | Pu⁴⁺ | Pink-brown | Most stable, hydrolyzes easily |
| +5 | PuO₂⁺ | Light purple | Unstable, disproportionates |
| +6 | PuO₂²⁺ | Yellow-brown | Stable in acidic media |
Electronic structure and compounds
Plutonium, atomic number 94, has a ground-state electron configuration of [Rn] 5f⁶ 7s², with the 5f orbitals partially filled and contributing to its chemical behavior as a member of the actinide series.[23][14] The 5f electrons in plutonium exhibit intermediate localization, distinguishing it from lighter actinides where 5f orbitals are more itinerant and heavier ones where they are more localized, leading to unique bonding properties that blend d- and f-electron characteristics.[24] In compounds, plutonium displays a wide range of oxidation states from +2 to +7, with +3, +4, +5, and +6 being the most prevalent and stable under typical conditions; the +2 state was first synthesized in a cyclopentadienyl complex in 2017, while +7 occurs transiently in strong oxidants.[1][25] These states arise from the accessibility of 5f and 6d electrons, enabling facile redox changes, and plutonium ions in solution—such as Pu³⁺ (green to violet), Pu⁴⁺ (brown), PuO₂⁺ (+5, pink to blue), and PuO₂²⁺ (+6, yellow)—often coexist, complicating speciation due to hydrolysis and complexation.[21][26] Key binary compounds include plutonium(IV) oxide (PuO₂), a refractory fluorite-structured solid stable to high temperatures and widely used in nuclear fuels for its low solubility and resistance to radiation damage.[10] Plutonium tetrafluoride (PuF₄) forms green crystals and serves as an intermediate in purification, while plutonium trihalides like PuCl₃ and PuBr₃ exhibit Pu(III) states with layered structures.[27] Plutonium reacts with hydrogen to form hydrides such as PuH₂ and PuH₃, which are pyrophoric and decompose above 400°C, and with nitrogen to yield plutonium nitride (PuN), a potential fast-reactor fuel with high thermal conductivity.[27] Organoplutonium compounds, though less common due to reactivity, include alkyl derivatives like (C₅H₅)₃Pu demonstrating covalent f-orbital involvement.[10]Nuclear Characteristics
Isotopes and abundance
Plutonium has no stable isotopes, with 15 known radioactive isotopes ranging from mass number 228 to 247. The primary decay modes are alpha decay for most isotopes, supplemented by spontaneous fission in even-mass heavier isotopes and beta minus decay for odd-neutron variants like plutonium-241. Plutonium-244 is the longest-lived isotope, with a half-life of approximately 82 million years, decaying via alpha emission to uranium-240.[1] The table below lists selected plutonium isotopes, their half-lives, and primary decay modes:| Isotope | Half-life | Primary decay mode(s) |
|---|---|---|
| ^{238}Pu | 87.7 years | α |
| ^{239}Pu | 24,000 years | α |
| ^{240}Pu | 6,560 years | α, spontaneous fission |
| ^{241}Pu | 14 years | β⁻ |
| ^{242}Pu | 375,000 years | α |
| ^{244}Pu | 82 million years | α |
Nucleosynthesis and natural occurrence
Plutonium isotopes are primarily synthesized via the rapid neutron-capture process (r-process) in astrophysical events such as core-collapse supernovae and neutron star mergers, where extreme neutron fluxes enable the formation of heavy, neutron-rich nuclei beyond the iron peak.[32][33] This process involves successive neutron captures on seed nuclei followed by beta decays, producing transuranic elements like plutonium, which cannot form through slower stellar nucleosynthesis pathways such as the s-process due to insufficient neutron densities.[34] The r-process yields favor even-even isotopes, with plutonium-244 (half-life 81.3 million years) being among the more stable, though all plutonium isotopes decay relatively rapidly on geological timescales.[35] On Earth, plutonium does not occur in primordial quantities, as any synthesized during solar system formation would have decayed given half-lives ranging from 14.35 years (Pu-238) to 80.8 million years (Pu-244).[36] Instead, trace natural occurrence arises from secondary processes: neutron capture on uranium-238 in ore deposits, where U-238 absorbs neutrons (from spontaneous fission of U-238 or cosmic rays) to form U-239, which beta-decays to neptunium-239 and then plutonium-239.[37][38] Concentrations remain exceedingly low, typically on the order of parts per trillion in uranium-rich minerals, far below levels from anthropogenic sources.[36] Additional traces stem from ancient natural nuclear reactors, such as those at Oklo, Gabon, operating about 1.9 billion years ago, where fission chains produced minor plutonium alongside other actinides under self-sustaining conditions.[39] More recently, deep-sea sediment cores reveal influxes of extraterrestrial plutonium-244 from nearby supernovae within the last 10 million years, with detections of approximately 181 atoms per sample indicating r-process ejecta deposition.[32][40] These cosmic contributions, while confirming ongoing stellar nucleosynthesis, contribute negligibly to Earth's inventory compared to reactor-produced plutonium.[34]Fission properties
Plutonium-239 is fissile, undergoing induced fission upon capture of low-energy thermal neutrons to release approximately 2.89 neutrons on average per fission event, enabling a self-sustaining chain reaction in suitable configurations.[41] Its thermal neutron fission cross-section measures 747 barns, substantially higher than the 582 barns for uranium-235, contributing to superior neutron economy in reactors.[42] The isotope's prompt neutron multiplicity supports effective multiplication factors (k) exceeding 1 in critical assemblies, with k_infinity values around 2.9 in infinite media under thermal conditions due to low parasitic capture.[43] The bare-sphere critical mass for weapons-grade plutonium-239 (delta phase, density 19.84 g/cm³) is approximately 10 kilograms, corresponding to a radius of about 6.4 cm, far lower than the 52 kilograms for uranium-235 owing to plutonium's higher density and cross-sections.[41] This compactness facilitated implosion-type designs in nuclear weapons, as gun-type assemblies prove impractical for plutonium due to predetonation risks from spontaneous fission.[41] Reflectors and tampers reduce the required mass to 4-6 kg in practical devices by minimizing neutron leakage. Spontaneous fission rates distinguish plutonium isotopes: Pu-239 exhibits negligible rates (half-life ~10^16 years), but Pu-240's rate yields ~415,000 neutrons per second per kilogram, necessitating high-purity plutonium (<7% Pu-240) for weapons to avoid fizzle yields from premature criticality.[29] Pu-241, also fissile with a thermal cross-section of 1010 barns, contributes to reactivity but decays rapidly (half-life 14.3 years), while even isotopes like Pu-242 have low fissionability with thermal neutrons.[42] In fast-spectrum environments, plutonium isotopes show enhanced fission probabilities, with Pu-239's fast fission cross-section enabling breeding ratios >1 in breeder reactors.[43]Decay heat and radiological behavior
Plutonium isotopes predominantly undergo alpha decay, emitting alpha particles with energies typically exceeding 5 MeV, accompanied by low-energy gamma rays and x-rays below 20 keV, which convert to heat through interaction with surrounding material.[44] This decay mode dominates for isotopes like plutonium-239 (half-life 24,110 years) and plutonium-238 (half-life 87.7 years), with the alpha particles depositing nearly all their kinetic energy locally as thermal energy, typically yielding about 5-6 MeV per decay after accounting for gamma contributions.[28] Plutonium-241, however, decays primarily by beta emission to americium-241 (half-life 14.35 years for the beta process), introducing additional radiological complexity through subsequent alpha decay of the daughter product.[45] Decay heat generation varies significantly by isotope due to differences in half-life and decay energy. Plutonium-238 exhibits the highest specific power, approximately 0.57 watts per gram, arising from its relatively short half-life and efficient alpha decay, making it suitable for radioisotope thermoelectric generators where steady thermal output powers spacecraft.[29] In contrast, plutonium-239 produces far lower heat, on the order of milliwatts per gram, reflecting its longer half-life, while admixtures like plutonium-240 (half-life 6,561 years) contribute modestly to overall heat in reactor-grade or weapons-grade material through both alpha decay and spontaneous fission.[28] This intrinsic heat necessitates cooling in stored plutonium stockpiles to prevent thermal runaway or structural degradation, with total decay heat in mixed-isotope forms scaling roughly with isotopic composition— for instance, weapon-grade plutonium (typically <7% Pu-240) generates about 1-2 watts per kilogram initially.[46] Radiologically, plutonium poses minimal external hazard due to alpha particles' short range (stopped by skin or paper), but its behavior shifts dramatically upon internalization via inhalation or ingestion, where alpha emissions cause dense ionization tracks leading to cellular damage, fibrosis, and elevated cancer risk in organs like lungs or bones.[37] Even-mass isotopes such as plutonium-238, -240, and -242 undergo spontaneous fission at low rates (e.g., 415 neutrons per kilogram per second for pure Pu-240), emitting prompt neutrons that enable criticality assessments but complicate handling by inducing neutron activation in nearby materials.[29] Plutonium-239, while primarily alpha-decaying, contributes fewer neutrons (~2,600 fissions per kilogram per second in pure form), yet its fissionability amplifies radiological output under neutron irradiation.[45] Gamma emissions remain low across isotopes, often <1% of total energy, but daughter products like americium-241 from Pu-241 decay introduce penetrating 60 keV gammas over time.[44] In environmental or dispersal scenarios, plutonium's radiological behavior is governed by its particulate form and solubility; insoluble oxides dominate releases (e.g., from fuel fabrication), lodging in respiratory tissues with prolonged retention (biological half-life >100 days in lungs), whereas soluble forms like Pu(IV) distribute systemically via blood to liver and skeleton.[37] Spontaneous neutron emission from isotopes like Pu-240 limits safe storage densities, as accumulated neutrons can drive subcritical multiplication or material activation, requiring neutron-absorbing diluents in packaging.[2] Overall, these properties—high internal alpha dose potential coupled with manageable external fields—dictate stringent handling protocols, emphasizing containment over shielding for routine operations.[2]Production
Laboratory synthesis
Plutonium was first synthesized on December 14, 1940, at the Radiation Laboratory of the University of California, Berkeley, through deuteron bombardment of uranium-238 using the 60-inch cyclotron.[47] [48] A team led by Glenn T. Seaborg, including Edwin M. McMillan, Joseph W. Kennedy, and Arthur C. Wahl, induced the nuclear reaction ^{238}U + ^2H → ^{239}U + ^1H, where the resulting uranium-239 underwent successive beta decays: ^{239}U (half-life 23.5 minutes) to ^{239}Np (half-life 2.3 days), and then to ^{239}Pu (half-life 24,110 years).[48] [8] The initial identification relied on tracer-level quantities, with chemical separation of the new element from uranium and fission products via precipitation and solvent extraction techniques, confirming its distinct properties akin to other actinides rather than rare earths.[8] On February 23–24, 1941, the team performed the first chemical identification, verifying plutonium as element 94 through its characteristic oxidation states and precipitation behaviors.[8] This cyclotron method produced microgram quantities insufficient for macroscopic study, limiting yields due to the low cross-section of the (d,p) reaction and beam intensity constraints of the era's accelerator technology.[49] To obtain weighable amounts, Seaborg's group at the Met Lab in Chicago developed carrier-assisted isolation in 1942, using lanthanum fluoride to coprecipitate plutonium from neutron-irradiated uranium samples, yielding the first visible sample on August 20, 1942, and the first weighing (2.77 micrograms of plutonium oxide) the following month.[50] [5] These laboratory techniques, reliant on radiochemical purification and ion-exchange separation, enabled initial characterization of plutonium's metallic properties and reactivity, paving the way for scale-up in production reactors.[50] Laboratory synthesis remained confined to research settings post-discovery, supplanted by reactor-based neutron capture on uranium-238 for bulk production due to vastly higher efficiency.[4]Industrial-scale production
Industrial-scale production of plutonium relies on the neutron capture by uranium-238 in nuclear reactors to form plutonium-239, followed by chemical separation from irradiated fuel.[51] This process was first achieved at the Hanford Site in Washington state, where the B Reactor began operations on September 26, 1944, producing the first industrial quantities of plutonium for the Manhattan Project.[52] Hanford's nine production reactors and four reprocessing facilities generated nearly two-thirds of all plutonium used by the United States for national security purposes, totaling approximately 67 metric tons over four decades.[53] The core method involves loading aluminum-clad uranium metal "slugs" into reactor channels, irradiating them for periods ranging from days to months to optimize Pu-239 yield while minimizing unwanted isotopes like Pu-240, then discharging and dissolving the fuel in nitric acid.[52] Plutonium is then separated via the PUREX (Plutonium Uranium Redox Extraction) process, a solvent extraction technique using tributyl phosphate in kerosene to isolate plutonium and uranium from fission products and other actinides in aqueous nitric acid solutions.[54] At Hanford, initial separations used bismuth phosphate coprecipitation before transitioning to PUREX in the 1950s for higher efficiency and purity, enabling production rates that peaked at over 10 metric tons per year across multiple sites by the late 1950s.[52] The Savannah River Site in South Carolina complemented Hanford with five heavy-water moderated reactors (R, P, L, K, and C) operational from 1953 onward, producing about 30 metric tons of plutonium, primarily weapons-grade, alongside tritium.[55] Combined U.S. production from 1944 to 2009 reached 111.7 metric tons, with the majority dedicated to nuclear weapons pits—each containing 3 to 6 kilograms of plutonium—fabricated at facilities like Rocky Flats, which processed material at rates exceeding 1,000 pits annually during peak Cold War demand.[56][57] These operations emphasized weapons-grade plutonium (less than 7% Pu-240) by limiting irradiation times to reduce neutron captures on Pu-239.[58] Post-Cold War, industrial-scale production ceased in the U.S., with no dedicated reactors operating after 1988 at Hanford's N Reactor and 1988 at Savannah River's K Reactor, shifting focus to stockpile stewardship and limited Pu-238 production for space applications at sites like Oak Ridge.[53][59] Reprocessing of civilian spent fuel via PUREX variants occurs abroad, such as in France's La Hague facility, yielding reactor-grade plutonium (19-23% Pu-240) for mixed-oxide fuel, but U.S. policy prohibits commercial reprocessing to avoid proliferation risks.[60]Current global production and stockpiles
As of early 2024, the global stockpile of separated plutonium totals approximately 565 metric tons, with about 140 metric tons classified as weapons-usable and the remainder primarily civilian or military-obligated material unsuitable for direct weapons use due to isotopic composition or international safeguards.[61] This inventory reflects cumulative production from nuclear reactors and reprocessing facilities since the mid-20th century, with limited transparency in military programs complicating precise accounting.[62] Major holders include Russia (193 tons total, including 88 tons weapons-usable), the United States (87.6 tons total, 38.4 tons weapons-usable), the United Kingdom (120 tons total, 3.2 tons weapons-usable), and France (102 tons total, 6 tons weapons-usable).[61] China holds about 3 tons, nearly all weapons-usable, while Japan possesses around 45 tons of civilian plutonium stored domestically and abroad.[61] [63]| Country | Total Stockpile (metric tons) | Weapons-Usable (metric tons) |
|---|---|---|
| Russia | 193 | 88 |
| United States | 87.6 | 38.4 |
| United Kingdom | 120 | 3.2 |
| France | 102 | 6 |
| China | 3 | 2.9 |
| Japan | ~45 (civilian) | 0 |