Plutonium

Typical wastewaters

• nuclear fuel processing and weapons production waste (legacy site surface water and groundwater) Pu(IV) sorbed and on colloids, with PuO₂⁺ and the hydroxo carbonate Pu(OH)₂(CO₃)₂²⁻ dissolved in oxic carbonate water; plutonium-238, -239 and -240 dissolved in surface water and groundwater at 9 of 45 Superfund NPL sites measured values are in the Hanford, Savannah River and Rocky Flats entries below
• drinking water treatment residuals (floc, carbon, resin, reverse osmosis concentrate) Pu(IV) on iron or aluminium hydroxide floc and carbon, charged and colloidal forms on resin and in the RO concentrate every process in WHO Table 9.4 that works concentrates plutonium into a transuranic waste
• spent fuel reprocessing liquid effluent to sea (Sellafield, Irish Sea) plutonium alpha (Pu-238, Pu-239, Pu-240) and beta emitting Pu-241 in treated effluent discharged by pipeline 2022: plutonium alpha 0.07 TBq and Pu-241 0.73 TBq against permit limits of 0.29 and 6 TBq per year (2018: 0.14 and 1.9 TBq); the marine critical group dose, 72 µSv in 2022, is dominated by plutonium alpha and Am-241 from historic discharges
• spent fuel reprocessing liquid effluent to sea (La Hague, English Channel; OSPAR reporting) Pu-239+240 and Pu-241 within the reported total alpha; OSPAR states total alpha from nuclear sites is mainly Pu-239, Pu-240 and Am-241 (OSPAR 2010) 2023, TBq per year: La Hague Pu-239+240 0.00245, Pu-241 0.244, total alpha 0.0322; Sellafield, reported under decommissioning in 2023, Pu-239+240 0.0645, Pu-241 0.591, total alpha 0.0648
• plutonium finishing plant acidic processing waste discharged to ground (Hanford 216-Z-9 trench) plutonium in about pH 2.5, 5 M nitrate waste with organic processing solvents; in the organic phase it moves almost unhindered through sediment at pH below 4, in aqueous fluid only at pH below 4 and mostly below 2 over 4 million litres released; plutonium has migrated 37 m below the trench; columns: 14 percent breakthrough in aqueous fluid at pH below 2, 94 and 86 percent in organic solvent at pH 1 and 3
• legacy site groundwater (Hanford 200 West, well 299-W₁₅-8 beside the Z-9 trench) Pu-239/240 in unfiltered groundwater 8.3 pCi/L on 7 May 1990 and 1.9 pCi/L on 13 November 1991, with Am-241 0.14 and 5.9 pCi/L; the only two plutonium detections in the 200-ZP-1 operable unit before the well went dry
• reactor cooling reservoir at a weapons site (Pond B, Savannah River Site) reactor derived plutonium at µBq/L in the water column, peaking in shallow water with Fe(III) particulate organic matter at the onset of stratification; most of the inventory retained in shallow sediment isotope ratios show reactor plutonium overwhelms Northern Hemisphere fallout
• storm runoff and pond discharge from plutonium contaminated soil (Rocky Flats 903 Pad) Pu-239/240 with Am-241, 40 to 90 percent in particles above 0.45 µm and 10 to 60 percent colloidal; the colloidal plutonium is Pu(IV) bound to a 10 to 15 kDa negatively charged organic macromolecule rather than iron oxide or clay spring and summer samples 1998 to 2000; humic and fulvic acids enhance remobilisation during erosion

1 · Identity

Symbol, number

Pu, 94

Oxidation states in water

+3, +4, +5 and +6 can all exist in environmental water; Pu(III) dominates under reducing conditions up to about pH 8.5, above which Pu(IV) takes over; under oxidising conditions above pH 4 the +4, +5 and +6 states coexist, many workers holding Pu(V) as PuO₂⁺ to be the dominant dissolved state, while EPA's own calculation for river water gives the Pu(IV) hydroxo carbonate Pu(OH)₂(CO₃)₂²⁻ as about 90 percent of dissolved plutonium above pH 6.5 (EPA Kd volume II). Plutonium-238 (half life 86 years), -239 (24,400), -240 (6,580) and -241 (13.2, beta) are the isotopes of concern.

Note

The element entry covers the discovery, the allotropes and the metal's reactions. This chapter is about the four dissolved states and the sorption that keeps plutonium in the sediment.

2 · Occurrence in water

Natural sources

None at measurable levels; the traces in uranium ores are below any water relevance.

Anthropogenic sources

Accidental releases and disposal of wastes from fuel processing and from the production and detonation of nuclear weapons; contamination by plutonium-238, -239 or -240 was identified at 9 of 45 Superfund NPL sites as airborne particulate, soil and plutonium dissolved in surface water and groundwater (EPA Kd volume II). Global fallout plutonium in German soils 30 to 40 years after deposition was less than 1 percent readily exchangeable, more than 57 percent bound to organic matter and much of the rest in the oxide and mineral fractions (Bunzl 1995 via EPA).

3 · Speciation

Dissolved plutonium complexes with hydroxide, carbonate, nitrate, sulfate, phosphate, the halides, natural organic acids and humics, and with EDTA; Pu(IV) has the highest ionic potential and so hydrolyses most readily and forms the strongest complexes, in the order Pu(IV), Pu(III), Pu(VI), Pu(V). Chloride and nitrate complexes are weak (log K 1 to 2), fluoride, sulfate, phosphate, citrate and oxalate strong (log K 6 to 30), and the mixed hydroxo carbonate complexes such as Pu(OH)₂(CO₃)₂²⁻ are among the strongest; humic complexes may dominate below pH 5 to 6 (EPA Kd volume II). In EPA's river water calculation PuF₂²⁺ and PuO₂⁺ dominate at pH 3 to 3.5, PuO₂⁺ at pH 4 to 5, PuO₂⁺ with Pu(OH)₂(CO₃)₂²⁻ at 5.5 to 6.5, and Pu(OH)₂(CO₃)₂²⁻ with minor Pu(OH)₄ (aq) above 6.5. Polymeric plutonium does not form at environmental concentrations, which are at least seven orders of magnitude too low. Amorphous hydrous PuO₂ controls dissolved plutonium near 10⁻8 mol/L under oxidising conditions, aged partly crystalline PuO₂ about two orders lower, about 10⁻10 mol/L without carbonate, and alkaline high carbonate water can raise it to micromolar; under reducing conditions PuO₂ limits it above pH 8 and Pu₂(CO₃)₃ below. Disproportionation is not significant at trace concentrations in oxidising water. Plutonium sorbs to clays, oxides, oxyhydroxides, aluminosilicates and organic matter with Kd from 11 to 300,000 mL/g; iron hydroxides adsorb Pu(V) and Pu(VI) and reduce them to Pu(IV) at the surface, manganese dioxide oxidises sorbed Pu(IV) and Pu(V) to Pu(VI), light speeds these surface redox changes, and carbonate lowers sorption of Pu(IV) and Pu(V) on goethite, so plutonium is most mobile in high pH carbonate rich groundwater (EPA Kd volume II).

Solubility

Amorphous hydrous PuO₂ about 10⁻8 mol/L; aged PuO₂ about 10⁻10 mol/L without carbonate; micromolar in alkaline high carbonate water; Pu₂(CO₃)₃ under reducing acid to neutral conditions (EPA Kd volume II).

Hydrolysis

Pu(IV) hydrolyses more readily than any other state, up to four hydroxyls; Pu(V) least.

Complexation

Carbonate and hydroxo carbonate strongest among inorganic ligands, then phosphate, fluoride, sulfate; humate, citrate, oxalate, EDTA strong; chloride and nitrate weak.

Precipitates

Hydrous PuO₂, Pu₂(CO₃)₃; plutonium carried in iron and manganese oxide and in organic rich sludge.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Fix the oxidation state at collection or the fractionation between dissolved, colloidal and sorbed plutonium changes in the bottle; filter and ultrafilter in the field if the colloid fraction matters, and keep samples out of light, which speeds surface redox changes (EPA).

7 · Regulatory limits

Limits change, and many are set locally. Treat these as the published values to start from, not as your compliance target: check the standard in force at your site and the numbers written into your own permit.

8 · Health and environmental effects

Toxicity

Extremely toxic through its radioactivity and accumulates in bone (element entry); ingestion dose coefficient 2.5 x 10⁻7 Sv/Bq for plutonium-239 (WHO Table 9.2).

Bioaccumulation

Not addressed for aquatic organisms in the sources read; in soil it binds to organic matter and oxides and becomes non exchangeable over decades (EPA).

Ecotoxicity

Not addressed in the sources read.

Flags

• The speciation distribution is EPA's MINTEQA₂ calculation for a mean river water at one Eh to pH relation and 1.36 x 10⁻15 mol/L; it is illustrative, and the field consensus that Pu(V) dominates oxic water is noted in the same source.
• The hydroxo carbonate formation equation is written from the species named, not from printed constants.
• The only concentration is one alkaline lake maximum from 1984.
• The Euratom values were read from the retained UK copy on legislation.gov.uk; its plutonium detection limit was not returned by the page read.

Gaps

• Colloid facilitated transport (the Nevada Test Site kilometre scale migration) was not sourced.
• No stability constants or solubility products were read as numbers beyond the order of magnitude solubility limits quoted.
• Isotopic plutonium method numbers and detection limits were not read.
• The CWW BAT conclusions have no radioactivity parameter; no discharge row is written.
• No exchange or surface complexation constants for plutonium were read; the resin and iron surface equations are electron and charge balances written from the mechanisms EPA describes in words.

Sources

Identity

Name and symbol

Plutonium, Pu

Atomic number

94 protons

Position

no group (f-block) · period 7 · f-block · actinide

CAS number

7440-07-5

Atomic structure

Atomic mass

244 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f⁶
[Rn] 7s²⁵f⁶

Electrons per shell

2, 8, 18, 32, 24, 8, 2

Valence electrons

8 ns, (n-1)d and (n-2)f

Physical properties

State at room temperature

Solid

Melting point

913 K (639.85 °C)

Boiling point

3,501 K (3,227.85 °C)

Density

19.84 g/cm3

Appearance

silvery white, tarnishing to dark gray in air

Thermal conductivity

6.74 W/(m·K)

Electrical resistivity

1.460 µΩ·m at 0 °C

Electrical conductivity

684,931.507 S/m

Crystal structure

monoclinic

Molar heat capacity

35.5 J/(mol·K)

Chemical properties

Oxidation states

+6, +5, +4, +3

Electronegativity

1.28 (Pauling Scale)

Ionisation energy

6.06 eV
1st 584.7 kJ/mol

Electron affinity

not in sources

Atomic radius

empirical 187, covalent 187, van der Waals 243 pm

Ionic radius

Pu³⁺ 100; Pu⁴⁺ 86; Pu⁵⁺ 74; Pu⁶⁺ 71 pm

Reactivity

A reactive, strongly reducing actinide whose 5f electrons sit at the border between localized and delocalized behavior; it has six allotropes, tarnishes in air, and shows four coexisting oxidation states in solution, Pu3+, Pu4+, PuO2+ and PuO2^2+, with Pu(VII) rare.

with water

Attacked by steam and moist air, oxidizing rapidly to a mixture of PuO2 and plutonium hydride; with excess water vapor only the powdery PuO2 coating forms, and Pu4+ hydrolyzes near neutral pH to a green colloid.

with oxygen, air

Tarnishes in air to a dull gray, yellow or olive-green film and reacts readily with oxygen to PuO and PuO2 plus intermediate oxides; bulk metal ignites only above 400 C, but the powders, hydrides and Pu2O3 are pyrophoric.

with acids

Dissolves readily in concentrated mineral acids, especially hydrochloric, hydroiodic and perchloric acid, giving blue-lavender Pu3+ and salmon Pu4+; in concentrated nitric or hydrochloric acid Pu4+ forms anionic complexes such as Pu(NO3)6^2-; unaffected by alkalis.

with halogens

Reacts with all four halogens to the trihalides PuX3 (X = F, Cl, Br, I) and with fluorine also to PuF4; oxyhalides PuOCl, PuOBr and PuOI are known, and the metal explodes in carbon tetrachloride.

Typical compounds

• PuO₂ plutonium dioxide the stable oxide, the form in MOX fuel and heat sources
• PuF₄ plutonium tetrafluoride reduced with calcium or barium to make the metal
• PuF₃ plutonium trifluoride the trihalide type PuX3 formed with every halogen
• PuCl₃ plutonium trichloride trivalent chloride from the metal and chlorine
• PuN plutonium nitride from the metal and nitrogen, a candidate fuel
• PuC plutonium carbide from the metal and carbon

Occurrence, production and use

Crustal abundance

Not Applicable

Oceanic abundance

Not Applicable

Occurrence and sources

• plutonium-239 made by neutron irradiation of uranium in reactors the greatest source; separated from spent fuel

Extraction, production

Irradiation of uranium-238 in a reactor; neutron capture and beta decay give plutonium-239

a nuclear transformation stated in words by the source; the chemical separation from spent fuel is not described by the sources in hand

Plutonium metal by reducing plutonium tetrafluoride with calcium

$\ce{PuF4 + 2 Ca -> Pu + 2 CaF2}$

no balanced equation printed by the source

Uses

Only two of plutonium's isotopes, plutonium-238 and plutonium-239, have found uses outside of basic research. Plutonium-238 is used in radioisotope thermoelectric generators to provide electricity for space probes that venture too far from the sun to use solar power, such as the Cassini and Galileo probes. Plutonium-239 will undergo a fission chain reaction if enough of it is concentrated in one place, so it is used at the heart of modern day nuclear weapons and in some nuclear reactors.

Plutonium has assumed the position of dominant importance among the transuranium elements because of its use as an explosive ingredient in nuclear weapons and the place which it holds as a key material in the development of industrial use of nuclear power. During fission, a fraction of the binding energy, which holds a nucleus together, is released as a large amount of electromagnetic and kinetic energy which is quickly converted to thermal energy. Fission of a kilogram of plutonium-239 can produce an explosion equivalent to 21,000 tons of TNT which is equivalent to about 22 million kilowatt hours of heat energy. In 1982 it was estimated that about 300,000 kg had accumulated. The most common chemical process, PUREX (Plutonium, URanium EXtraction) reprocesses spent nuclear fuel to extract plutonium and uranium which can be used to form a mixed U/Pu oxide or "MOX" fuel for reuse in nuclear power reactors. MOX fuel production is also a good mechanism to reduce excessive defense plutonium stockpiles for peaceful purposes, which in effect is forging "swords into plowshares."

Plutonium isotopes undergo radioactive decay, which produces decay heat. Different isotopes produce different amounts of heat per mass. Pu-238 with a half-life of 88 years has a relatively high heat production rate which makes it useful as a power source with a long service life. The decay heat is usually listed as watt/kilogram, or milliwatt/gram. Pu-238 is a heat source in radioisotope thermoelectric generators, which are used to power spacecraft and extra-terrestrial rovers. As a power and heat source, Pu-238 has also been used to power instruments left on the Moon by Apollo astronauts, weather satellites and interplanetary probes and powers the Cassini Saturn mission and the Mars rovers.

Plutonium-238 was at one time used successfully to power artificial heart pacemakers but has been replaced by lithium-based primary cells. Plutonium-238 was studied as a way to provide supplemental heat to scuba divers. Pu-238 mixed with beryllium is a convenient method to generate neutrons.

• Defence: fissile material in nuclear weapons
• Nuclear power: key material in the development of nuclear power; bred from uranium-238 in reactors
• Space power: radioisotope power source on the Mars Curiosity rover and the New Horizons spacecraft

Safety, toxicity

not in sources

Discovery and name

Discovered by

Glenn T. Seaborg, Arthur Wahl, Joseph W. Kennedy, Edwin McMillan

Discovered

1940 to 1941

First isolated

not in sources

Named by

not in sources

Origin of the name

after dwarf planet Pluto, itself named after classical god of the underworld Pluto

Plutonium is unique among the elements in its physicochemical complexities by virtue of its position at a transitional location in the periodic table where the 5f electrons are at the border between delocalized (not associated with a single atom) and localized (associated with a single atom) behavior and it is considered one of the most complex of the elements. Plutonium also sits near the juncture where the actinide series transitions from main d-block element chemistry to rare earth like behavior as a result of the actinide contraction. Because of its defense and commercial importance, plutonium is one of the most intensely investigated of elements.

Plutonium metal has a bright silvery appearance at first and takes on a dull gray, yellow or olive green tarnish when oxidized in air. A relatively large piece of plutonium is warm to the touch because of the energy given off by alpha decay. Larger pieces will produce enough heat to boil water. The metal readily dissolves in concentrated mineral acids. Plutonium metal normally has six allotropes or crystal structures; alpha (α), beta (β), gamma (γ), delta (δ), delta prime (δ') and epsilon (ε). It forms a seventh phase (zeta, ζ) under high temperature and a limited pressure range. These allotropes have very similar energy levels but significantly varying densities (from 16.00 to 19.86 grams/cm3) and crystal structures. This makes plutonium very sensitive to changes in temperature, pressure, or chemistry, and allows for dramatic volume changes following phase transitions. At room temperature plutonium is in its alpha (α) form, the most common structural form of the element. It is as hard and brittle as cast iron unless alloyed with other metals to form the room-temperature stabilized delta (δ) phase which makes it soft and ductile. Unlike most metals, it is not a good conductor of heat or electricity. It has a low melting point (640 °C) and an unusually high boiling point (3,228 °C).

Plutonium can form alloys and intermediate compounds with most other metals. Gallium, aluminum, americium, scandium and cerium can stabilize the δ phase of plutonium metal. Nuclear fuel pellets can be formed by alloying plutonium with various metals such as: aluminum; zirconium; cerium; cerium-cobalt; uranium-titanium, uranium-zirconium and uranium-molybdenum. Thorium-plutonium-uranium alloys were investigated as a nuclear fuel for fast breeder reactors. A plutonium-gallium-cobalt alloy (PuCoGa5) was found to be an unconventional superconductor, showing superconductivity below 18.5 Kelvin, an order of magnitude higher than the highest between heavy fermion systems known.

Plutonium forms compounds with a variety of other elements. Plutonium reacts with pure hydrogen, forming plutonium hydrides. It also reacts readily with oxygen, forming PuO and PuO2 as well as intermediate and sub-stoichiometric oxides. The metal reacts with the halogens, giving rise to trivalent Pu compounds with the general formula PuX3 where X can be F, Cl, Br or I and tetravalent plutonium compounds such as PuF4. The following oxyhalides are observed: PuOCl, PuOBr and PuOI. Plutonium reacts with carbon to form PuC, nitrogen to form PuN and silicon to form PuSi2. Pu3+ and Pu4+ oxalates are important intermediates that are calcined to form oxides as a step in plutonium processing. Other important compounds in reprocessing are fluoride, peroxide, acetylacetone, carbonate and hydroxide.

The color displayed by plutonium solutions depends on both the oxidation state and the extent of complexation by various ligands. In aqueous solution plutonium exhibits five ionic valence states: Pu+3 (blue lavender), Pu+4 (salmon-colored, when uncomplexed), PuO+ (lavender), PuO+2 (orange-brown) and PuOxOHy (dark green in basic solution). The pentavalent ion, PuO+ is unstable in aqueous solutions and it disproportionates into Pu+4 and PuO+2. However, PuO2+ can be stabilized in aqueous solution in a narrow pH range around 4.5. By virtue of the close proximity of the electrode potentials of the various plutonium redox couples (~ 1 Volt/NHE), four oxidation states can co-exist in solution simultaneously: Pu3+, Pu4+, PuO2+ and PuO22+.

Pu4+ is a "hard" (ionic) cation with the largest electronic charge of plutonium ions and it forms complexes with a variety of inorganic and organic ligands. In dilute perchloric acid, Pu4+ is un-complexed and is salmon-colored. However in concentrated acids, Pu4+ forms anionic complexes such as: Pu(NO3)62- (dark green) and Pu(Cl)62- (brick red). Pu4+, having a high ionic charge readily hydrolyzes (combines with hydroxide ion) at near-neutral pH values forming a green colloidal suspension that behaves like a solution but is actually a solid precipitate that can be separated by ultra-centrifugation.

Plutonium-organic complexes are very important for separation, reprocessing, and purification and include: Tributyl phosphate (TBP); Di-(2-ethylhexyl)phosphoric acid (DEHPA or HDEHP); octyl(phenyl)-N,N-diisobutyl-carbamoylmethylphosphine oxide (CMPO); crown-ethers; and many others.