Chemical Properties
silvery white metal; highly reactive; αform: monoclinic, a=0.6183 nm, b=0.4822 nm, c=1.0963 nm; ionic radius of Pu++++ is 0.0887nm; stable form from room temp to 115°C; enthalpy of vaporization 333.5kJ/mol; enthalpy of fusion 2.82kJ/mol; discovered in 1940–1941; prepared in ton quantities in nuclear reactors; 238Pu produced in kg amounts from 237Np; important fuel for producing power for terrestrial and extraterrestrial applications [MER06] [KIR78] [CRC10]
Description
Plutonium was first isolated and produced in 1941 at the
University of California-Berkeley, by nuclear chemist Glenn T.
Seaborg and his colleagues, Joseph W. Kennedy, Edwin M.
McMillan, and Arthur C. Wahl. Minute amounts of plutonium
exist naturally, but large amounts are produced in nuclear
reactors when uranium absorbs an atomic particle such as a
neutron.
Natural occurrences of plutonium are very rare, but it can
occur in a reaction called spontaneous fission. This type of
reaction occurs when ores of uranium with a high localized
concentration decay in the right conditions and produce small
amounts of plutonium. Synthetic plutonium is produced in
a controlled nuclear reactor when uranium-238 absorbs
a neutron and becomes uranium-239, ultimately decaying to
plutonium-239. Plutonium has at least 15 different isotopes.
Different isotopes of uranium and different combinations of
neutron absorption and radioactive decay create the different
isotopes of plutonium. Plutonium was discovered during
wartime; therefore, the majority of plutonium production was
for nuclear weapons. Other plutonium applications range from
being energy sources on deep space probes to small amounts
providing power to heart pacemakers.
Environmental Fate
Plutonium was dispersed in the environment by fallout from
aboveground weapons testing that occurred from the 1940s
through the 1960s. Approximately one-fifth of the plutonium
fallout from a nuclear weapons test remained on the test site.
The remaining plutonium fallout was released into the atmosphere,
absorbed to particulate matter, and transported back to
the surface by either dry or wet deposition. Additional releases
can be traced to accidental reactor effluent releases, improper
disposal of radioactive waste, and military accidents. Each
release directly introduced plutonium into the ecosystem,
where it has stayed.
Plutonium and its isotopes have relatively long half-lives.
The most common plutonium isotopes are 238Pu, 239Pu,
and 240Pu. The decay process for each of these varies, and all
are extremely long in duration. 238Pu (the principal isotope
for satellites) has a half-life of 87.7 years, 239Pu (a principal
isotope for nuclear weapons) has a half-life of 24 100 years,
and 240Pu has a half-life of 6560 years. Plutonium undergoes
a change in form through radioactive decay. As each of these
isotopes decay, they release energy and form a new product.
The energy being released is referred to as radiation. Plutonium
reactions in the environment are either oxidative or
reductive reactions. Plutonium can be found in five different
oxidations states: plutonium(III), plutonium(IV), plutonium(
V), plutonium(VI), and plutonium(VII).
Most atmospheric and underwater nuclear weapons
testing were stopped by the Partial Test Ban Treaty of 1963.
The treaty did not cease the testing of nuclear weapons, it
only banned testing aboveground, and the testing continued
underground until the 1980s. Although the move to test
underground was to reduce the release of plutonium into
the environment, releases still occurred via test venting.
Plutonium can migrate vertically at various rates depending
on meteorological conditions, the form of plutonium as it
enters the environment, and human activity. However,
almost all plutonium introduced into the environment can
be found in the surface soil. Soil particles are the primary
mode of distribution. Plutonium compounds are ionic and
will not volatilize from the soil. Surface soils contaminated
with plutonium have been known to resuspend in the
atmosphere via fugitive dust emissions, causing it to be
rereleased into the environment. Parameters such as particle
size, presence of organic substances, and soil pH can
influence the distribution of the plutonium isotopes in
the soil and sediment. It has been documented in arctic
surface sediment studies that the partition coefficients (Kd) for 239,240Pu range from 8 × 10+4 to 1.5 × 10+5. Plutonium
fallout from the atmosphere can be deposited as insoluble
dioxide. This is an important factor since microorganisms
can change the oxidation state of plutonium, potentially
causing it to increase or decrease in solubility. But microorganism
changes occur only in a small fraction of the
plutonium released into the environment and this is based
on the released plutonium form. If the plutonium does
enter the soluble phase, it can become available for plant
uptake. Studies have shown that the plutonium(IV) oxidation
state is able to hydrolyze in the environment, and
plants can readily uptake the contaminant.
Atmospheric plutonium fallout can reach surface water
and settle in the sediment under the water. The long halflives
of plutonium isotopes allow for contaminated sediment
to act as a repository, and eventually the contaminants
can resuspend in the water column, reintroducing them into
the environment. How plutonium in surface water acts is
dependent on the oxidation state of the released plutonium
and the nature of the suspended particulates in the environment.
It has been reported that plutonium in water with
suspended solids have Kd values ranging from 1 × 10+5 to
7 ×10+5. It was also reported that plutonium with colloidal
materials can be mobile in groundwater systems over long
distances. Plutonium(III) and plutonium(IV) are considered to
be reduced forms of plutonium, while plutonium(V) and
plutonium(VI) are oxidized forms. The primary oxidative state
of plutonium in the environment is plutonium(IV).
Toxicity evaluation
The toxicity of plutonium is based on the radiation emitted
during the exposure and radiological decay of the plutonium
isotope. Radiation emitted by plutonium can have many
different mechanistic impacts on cells, including ionization
and destruction of cell constituents that can result in a variety of
effects ranging from cell death with/without regeneration to
cells growing out of control if the DNA damage resulting is not
arrested by normal healthy body repair mechanisms. Additional
factors of toxicity include the radiosensitivity of the
tissue exposed and the retention time. Tissues that undergo
rapid cell regeneration are more sensitive than slower or nonregenerating
cells.