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= Plutonium =
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Introduction
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Plutonium is a chemical element; it has symbol Pu and atomic number
94. It is a silvery-gray actinide metal that tarnishes when exposed to
air, and forms a dull coating when oxidized. The element normally
exhibits six allotropes and four oxidation states. It reacts with
carbon, halogens, nitrogen, silicon, and hydrogen. When exposed to
moist air, it forms oxides and hydrides that can expand the sample up
to 70% in volume, which in turn flake off as a powder that is
pyrophoric. It is radioactive and can accumulate in bones, which makes
the handling of plutonium dangerous.
Plutonium was first synthesized and isolated in late 1940 and early
1941, by deuteron bombardment of uranium-238 in the 1.5 m cyclotron at
the University of California, Berkeley. First, neptunium-238
(half-life 2.1 days) was synthesized, which then beta-decayed to form
the new element with atomic number 94 and atomic weight 238 (half-life
88 years). Since uranium had been named after the planet Uranus and
neptunium after the planet Neptune, element 94 was named after Pluto,
which at the time was also considered a planet. Wartime secrecy
prevented the University of California team from publishing its
discovery until 1948.
Plutonium is the element with the highest atomic number known to occur
in nature. Trace quantities arise in natural uranium deposits when
uranium-238 captures neutrons emitted by decay of other uranium-238
atoms. The heavy isotope plutonium-244 has a half-life long enough
that extreme trace quantities should have survived primordially (from
the Earth's formation) to the present, but so far experiments have not
yet been sensitive enough to detect it.
Both plutonium-239 and plutonium-241 are fissile, meaning they can
sustain a nuclear chain reaction, leading to applications in nuclear
weapons and nuclear reactors. Plutonium-240 has a high rate of
spontaneous fission, raising the neutron flux of any sample containing
it. The presence of plutonium-240 limits a plutonium sample's
usability for weapons or its quality as reactor fuel, and the
percentage of plutonium-240 determines its grade (weapons-grade,
fuel-grade, or reactor-grade). Plutonium-238 has a half-life of 87.7
years and emits alpha particles. It is a heat source in radioisotope
thermoelectric generators, which are used to power some spacecraft.
Plutonium isotopes are expensive and inconvenient to separate, so
particular isotopes are usually manufactured in specialized reactors.
Producing plutonium in useful quantities for the first time was a
major part of the Manhattan Project during World War II that developed
the first atomic bombs. The Fat Man bombs used in the Trinity nuclear
test in July 1945, and in the bombing of Nagasaki in August 1945, had
plutonium cores. Human radiation experiments studying plutonium were
conducted without informed consent, and several criticality accidents,
some lethal, occurred after the war. Disposal of plutonium waste from
nuclear power plants and dismantled nuclear weapons built during the
Cold War is a nuclear-proliferation and environmental concern. Other
sources of plutonium in the environment are fallout from many
above-ground nuclear tests, which are now banned.
Physical properties
=====================
Plutonium, like most metals, has a bright silvery appearance at first,
much like nickel, but it oxidizes very quickly to a dull gray, though
yellow and olive green are also reported. At room temperature
plutonium is in its α ('alpha') form. This allotrope is about as hard
and brittle as gray cast iron. When plutonium is alloyed with other
metals, the high-temperature δ allotrope is stabilized at room
temperature, making 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 (3228 °C). This gives a
large range of temperatures (over 2,500 kelvin wide) at which
plutonium is liquid, but this range is neither the greatest among all
actinides nor among all metals, with neptunium theorized to have the
greatest range in both instances. The low melting point as well as the
reactivity of the native metal compared to the oxide leads to
plutonium oxides being a preferred form for applications such as
nuclear fission reactor fuel (MOX-fuel).
Alpha decay, the release of a high-energy helium nucleus, is the most
common form of radioactive decay for plutonium. A 5 kg mass of (239)Pu
contains about atoms. With a half-life of 24,100 years, about of its
atoms decay each second by emitting a 5.157 MeV alpha particle. This
amounts to 9.68 watts of power. Heat produced by the deceleration of
these alpha particles makes it warm to the touch. due to its much
shorter half life heats up to much higher temperatures and glows red
hot with blackbody radiation if left without external heating or
cooling. This heat has been used in radioisotope thermoelectric
generators (see below).
The resistivity of plutonium at room temperature is very high for a
metal, and it gets even higher with lower temperatures, which is
unusual for metals. This trend continues down to 100 K, below which
resistivity rapidly decreases for fresh samples. Resistivity then
begins to increase with time at around 20 K due to radiation damage,
with the rate dictated by the isotopic composition of the sample.
Because of self-irradiation, a sample of plutonium fatigues throughout
its crystal structure, meaning the ordered arrangement of its atoms
becomes disrupted by radiation with time. Self-irradiation can also
lead to annealing which counteracts some of the fatigue effects as
temperature increases above 100 K.
Unlike most materials, plutonium increases in density when it melts,
by 2.5%, but the liquid metal exhibits a linear decrease in density
with temperature. Near the melting point, the liquid plutonium has
very high viscosity and surface tension compared to other metals.
Allotropes
============
Plutonium normally has six allotropes and forms a seventh (zeta, ζ) at
high temperature within a limited pressure range. These allotropes,
which are different structural modifications or forms of an element,
have very similar internal energies but significantly varying
densities and crystal structures. This makes plutonium very sensitive
to changes in temperature, pressure, or chemistry, and allows for
dramatic volume changes following phase transitions from one
allotropic form to another. The densities of the different allotropes
vary from 16.00 g/cm(3) to 19.86 g/cm(3).
The presence of these many allotropes makes machining plutonium very
difficult, as it changes state very readily. For example, the α form
exists at room temperature in unalloyed plutonium. It has machining
characteristics similar to cast iron but changes to the plastic and
malleable β ('beta') form at slightly higher temperatures. The reasons
for the complicated phase diagram are not entirely understood. The α
form has a low-symmetry monoclinic structure, hence its brittleness,
strength, compressibility, and poor thermal conductivity.
Plutonium in the δ ('delta') form normally exists in the 310 °C to 452
°C range but is stable at room temperature when alloyed with a small
percentage of gallium, aluminium, or cerium, enhancing workability and
allowing it to be welded. The δ form has more typical metallic
character, and is roughly as strong and malleable as aluminium. In
fission weapons, the explosive shock waves used to compress a
plutonium core will also cause a transition from the usual δ phase
plutonium to the denser α form, significantly helping to achieve
supercriticality. The ε phase, the highest temperature solid
allotrope, exhibits anomalously high atomic self-diffusion compared to
other elements.
Nuclear fission
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Plutonium is a radioactive actinide metal whose isotope,
plutonium-239, is one of the three primary fissile isotopes
(uranium-233 and uranium-235 are the other two); plutonium-241 is also
highly fissile. To be considered fissile, an isotope's atomic nucleus
must be able to break apart or fission when struck by a slow moving
neutron and to release enough additional neutrons to sustain the
nuclear chain reaction by splitting further nuclei.
Pure plutonium-239 may have a multiplication factor (keff) larger than
one, which means that if the metal is present in sufficient quantity
and with an appropriate geometry (e.g., a sphere of sufficient size),
it can form a critical mass. During fission, a fraction of the nuclear
binding energy, which holds a nucleus together, is released as a large
amount of electromagnetic and kinetic energy (much of the latter being
quickly converted to thermal energy). Fission of a kilogram of
plutonium-239 can produce an explosion equivalent to 21000 tonTNT. It
is this energy that makes plutonium-239 useful in nuclear weapons and
reactors.
The presence of the isotope plutonium-240 in a sample limits its
nuclear bomb potential, as (240)Pu has a relatively high spontaneous
fission rate (~440 fissions per second per gram; over 1,000 neutrons
per second per gram), raising the background neutron levels and thus
increasing the risk of predetonation. Plutonium is identified as
either weapons-grade, fuel-grade, or reactor-grade based on the
percentage of (240)Pu that it contains. Weapons-grade plutonium
contains less than 7% (240)Pu. Fuel-grade plutonium contains 7%-19%,
and power reactor-grade contains 19% or more (240)Pu. Supergrade
plutonium, with less than 4% of (240)Pu, is used in United States Navy
weapons stored near ship and submarine crews, due to its lower
radioactivity. Plutonium-238 is not fissile but can undergo nuclear
fission easily with fast neutrons as well as alpha decay. All
plutonium isotopes can be "bred" into fissile material with one or
more neutron absorptions, whether followed by beta decay or not. This
makes non-fissile isotopes of plutonium a fertile material.
Isotopes and nucleosynthesis
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Twenty-two radioisotopes of plutonium have been characterized, from
226Pu to 247Pu. The longest-lived are (244)Pu, with a half-life of
80.8 million years; (242)Pu, with a half-life of 373,300 years; and
(239)Pu, with a half-life of 24,110 years. All other isotopes have
half-lives of less than 7,000 years. This element also has eight
metastable states, though all have half-lives less than a second.
(244)Pu has been found in interstellar space and it has the longest
half-life of any non-primordial radioisotope. The main decay modes of
isotopes with mass numbers lower than the most stable isotope,
(244)Pu, are spontaneous fission and alpha emission, mostly forming
uranium (92 protons) and neptunium (93 protons) isotopes as decay
products (neglecting the wide range of daughter nuclei created by
fission processes). The main decay mode for isotopes heavier than
(244)Pu, along with (241)Pu and (243)Pu, is beta emission, forming
americium isotopes (95 protons). Plutonium-241 is the parent isotope
of the neptunium series, decaying to americium-241 via beta emission.
Plutonium-238 and 239 are the most widely synthesized isotopes.
(239)Pu is synthesized via the following reaction using uranium (U)
and neutrons (n) via beta decay (β(−)) with neptunium (Np) as an
intermediate:
:
{^{238}_{92}U} + {^{1}_{0}n} -> {^{239}_{92}U} ->[\beta^-] [23.5
\ \ce{min}] {^{239}_{93}Np} ->[\beta^-] [2.3565 \ \ce d]
{^{239}_{94}Pu}
Neutrons from the fission of uranium-235 are captured by uranium-238
nuclei to form uranium-239; a beta decay converts a neutron into a
proton to form neptunium-239 (half-life 2.36 days) and another beta
decay forms plutonium-239. Egon Bretscher working on the British Tube
Alloys project predicted this reaction theoretically in 1940.
Plutonium-238 is synthesized by bombarding uranium-238 with deuterons
(D or (2)H, the nuclei of heavy hydrogen) in the following reaction:
:
where a deuteron hitting uranium-238 produces two neutrons and
neptunium-238, which decays by emitting negative beta particles to
form plutonium-238. Plutonium-238 can also be produced by neutron
irradiation of neptunium-237.
Decay heat and fission properties
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Plutonium isotopes undergo radioactive decay, which produces decay
heat. Different isotopes produce different amounts of heat per mass.
The decay heat is usually listed as watt/kilogram, or milliwatt/gram.
In larger pieces of plutonium (e.g. a weapon pit) and inadequate heat
removal the resulting self-heating may be significant.
Decay heat of plutonium isotopes
Isotope !! Decay mode !! Half-life (years) !! Decay heat (W/kg) !!
Spontaneous fission (SF) neutrons (1/(g·s)) !! Comment
(238)Pu alpha (α) to (234)U 87.74 560 2600 Very high decay heat. Even
small amounts can cause significant self-heating. Used on its own in
radioisotope thermoelectric generators.
(239)Pu α to (235)U 24100 1.9 0.022 The main fissile isotope in use.
(240)Pu α to (236)U, SF 6560 6.8 910 The main impurity in samples of
(239)Pu. The plutonium grade is usually listed as percentage of
(240)Pu. High rate of SF hinders use in nuclear weapons.
(241)Pu beta-minus, to (241)Am 14.4 4.2 0.049 Decays to
americium-241; its buildup presents a radiation hazard in older
samples.
(242)Pu α to (238)U 376000 0.1 1700 (242)Pu α-decays to (238)U; also
decays by SF.
Compounds and chemistry
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At room temperature, pure plutonium is silvery in color but gains a
tarnish when oxidized. The element displays four common ionic
oxidation states in aqueous solution and one rare one:
* Pu(III), as Pu3+ (blue lavender)
* Pu(IV), as Pu4+ (yellow brown)
* Pu(V), as (light pink)
* Pu(VI), as (pink orange)
* Pu(VII), as (green)--the heptavalent ion is rare.
The color shown by plutonium solutions depends on both the oxidation
state and the nature of the acid anion. It is the acid anion that
influences the degree of complexing--how atoms connect to a central
atom--of the plutonium species. Additionally, the formal +2 oxidation
state of plutonium is known in the complex [K(2.2.2-cryptand)]
[PuIICp″3], Cp″ = C5H3(SiMe3)2.
Preparation of plutonium(VIII) compounds such as the volatile
tetroxide has also been claimed, but their existence remains
disputed.
Metallic plutonium is produced by reacting plutonium tetrafluoride
with barium, calcium or lithium at 1200 °C. Metallic plutonium is
attacked by acids, oxygen, and steam but not by alkalis and dissolves
easily in concentrated hydrochloric, hydroiodic and perchloric acids.
Molten metal must be kept in a vacuum or an inert atmosphere to avoid
reaction with air. At 135 °C the metal will ignite in air and will
explode if placed in carbon tetrachloride.
Plutonium is a reactive metal. In moist air or moist argon, the metal
oxidizes rapidly, producing a mixture of oxides and hydrides. If the
metal is exposed long enough to a limited amount of water vapor, a
powdery surface coating of PuO2 is formed. Also formed is plutonium
hydride but an excess of water vapor forms only PuO2.
Plutonium shows enormous, and reversible, reaction rates with pure
hydrogen, forming plutonium hydride. It also reacts readily with
oxygen, forming PuO and PuO2 as well as intermediate oxides; plutonium
oxide fills 40% more volume than plutonium metal. The metal reacts
with the halogens, giving rise to compounds with the general formula
PuX3 where X can be F, Cl, Br or I and PuF4 is also seen. The
following oxyhalides are observed: PuOCl, PuOBr and PuOI. It will
react with carbon to form PuC, nitrogen to form PuN and silicon to
form PuSi2.
The organometallic chemistry of plutonium complexes is typical for
organoactinide species; a characteristic example of an organoplutonium
compound is plutonocene. Computational chemistry methods indicate an
enhanced covalent character in the plutonium-ligand bonding.
Powders of plutonium, its hydrides and certain oxides like Pu2O3
are pyrophoric, meaning they can ignite spontaneously at ambient
temperature and are therefore handled in an inert, dry atmosphere of
nitrogen or argon. Bulk plutonium ignites only when heated above 400
°C. Pu2O3 spontaneously heats up and transforms into PuO2, which is
stable in dry air, but reacts with water vapor when heated.
Crucibles used to contain plutonium need to be able to withstand its
strongly reducing properties. Refractory metals such as tantalum and
tungsten along with the more stable oxides, borides, carbides,
nitrides and silicides can tolerate this. Melting in an electric arc
furnace can be used to produce small ingots of the metal without the
need for a crucible.
Cerium is used as a chemical simulant of plutonium for development of
containment, extraction, and other technologies.
Electronic structure
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Plutonium is an element in which the 5f electrons are the transition
border between delocalized and localized; it is therefore considered
one of the most complex elements. The anomalous behavior of plutonium
is caused by its electronic structure. The energy difference between
the 6d and 5f subshells is very low. The size of the 5f shell is just
enough to allow the electrons to form bonds within the lattice, on the
very boundary between localized and bonding behavior. The proximity of
energy levels leads to multiple low-energy electron configurations
with near equal energy levels. This leads to competing 5fn7s2 and
5fn−16d17s2 configurations, which causes the complexity of its
chemical behavior. The highly directional nature of 5f orbitals is
responsible for directional covalent bonds in molecules and complexes
of plutonium.
Alloys
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Plutonium can form alloys and intermediate compounds with most other
metals. Exceptions include lithium, sodium, potassium, rubidium and
caesium of the alkali metals; and magnesium, calcium, strontium, and
barium of the alkaline earth metals; and europium and ytterbium of the
rare earth metals. Partial exceptions include the refractory metals
chromium, molybdenum, niobium, tantalum, and tungsten, which are
soluble in liquid plutonium, but insoluble or only slightly soluble in
solid plutonium. Gallium, aluminium, americium, scandium and cerium
can stabilize δ-phase plutonium for room temperature. Silicon, indium,
zinc and zirconium allow formation of metastable δ state when rapidly
cooled. High amounts of hafnium, holmium and thallium also allows some
retention of the δ phase at room temperature. Neptunium is the only
element that can stabilize the α phase at higher temperatures.
Plutonium alloys can be produced by adding a metal to molten
plutonium. If the alloying metal is reductive enough, plutonium can be
added in the form of oxides or halides. The δ phase plutonium-gallium
alloy (PGA) and plutonium-aluminium alloy are produced by adding
Pu(III) fluoride to molten gallium or aluminium, which has the
advantage of avoiding dealing directly with the highly reactive
plutonium metal.
* PGA is used for stabilizing the δ phase of plutonium, avoiding the
α-phase and α-δ related issues. Its main use is in pits of implosion
bombs.
* Plutonium-aluminium is an alternative to PGA. It was the original
element considered for δ phase stabilization, but its tendency to
react with the alpha particles and release neutrons reduces its
usability for nuclear weapons. Plutonium-aluminium alloy can be also
used as a component of nuclear fuel.
* Plutonium-gallium-cobalt alloy (PuCoGa) is an unconventional
superconductor, showing superconductivity below 18.5 K, an order of
magnitude higher than the highest between heavy fermion systems, and
has large critical current.
* Plutonium-zirconium alloy can be used as nuclear fuel.
* Plutonium-cerium and plutonium-cerium-cobalt alloys are used as
nuclear fuels.
* Plutonium-uranium, with about 15-30 mol.% plutonium, can be used as
a nuclear fuel for fast breeder reactors. Its pyrophoric nature and
high susceptibility to corrosion to the point of self-igniting or
disintegrating after exposure to air require alloying with other
components. Addition of aluminium, carbon or copper does not improve
disintegration rates markedly, zirconium and iron alloys have better
corrosion resistance but they disintegrate in several months in air as
well. Addition of titanium and/or zirconium significantly increases
the melting point of the alloy.
* Plutonium-uranium-titanium and plutonium-uranium-zirconium were
investigated for use as nuclear fuels. The addition of the third
element increases corrosion resistance, reduces flammability, and
improves ductility, fabricability, strength, and thermal expansion.
Plutonium-uranium-molybdenum has the best corrosion resistance,
forming a protective film of oxides, but titanium and zirconium are
preferred for physics reasons.
* Thorium-uranium-plutonium was investigated as a nuclear fuel for
fast breeder reactors.
Occurrence
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Trace amounts of plutonium-238, plutonium-239, plutonium-240, and
plutonium-244 can be found in nature. Small traces of plutonium-239, a
few parts per trillion, and its decay products are naturally found in
some concentrated ores of uranium, such as the natural nuclear fission
reactor in Oklo, Gabon. The ratio of plutonium-239 to uranium at the
Cigar Lake Mine uranium deposit ranges from to . These trace amounts
of 239Pu originate in the following fashion: on rare occasions, 238U
undergoes spontaneous fission, and in the process, the nucleus emits
one or two free neutrons with some kinetic energy. When one of these
neutrons strikes the nucleus of another 238U atom, it is absorbed by
the atom, which becomes 239U. With a relatively short half-life, 239U
decays to 239Np, which decays into 239Pu. Finally, exceedingly small
amounts of plutonium-238, attributed to the extremely rare double beta
decay of uranium-238, have been found in natural uranium samples.
Due to its relatively long half-life of about 80 million years, it was
suggested that plutonium-244 occurs naturally as a primordial nuclide,
but early reports of its detection could not be confirmed. Based on
its likely initial abundance in the Solar System, present experiments
as of 2022 are likely about an order of magnitude away from detecting
live primordial 244Pu. However, its long half-life ensured its
circulation across the solar system before its extinction, and indeed,
evidence of the spontaneous fission of extinct 244Pu has been found in
meteorites. The former presence of 244Pu in the early Solar System has
been confirmed, since it manifests itself today as an excess of its
daughters, either 232Th (from the alpha decay pathway) or xenon
isotopes (from its spontaneous fission). The latter are generally more
useful, because the chemistries of thorium and plutonium are rather
similar (both are predominantly tetravalent) and hence an excess of
thorium would not be strong evidence that some of it was formed as a
plutonium daughter. 244Pu has the longest half-life of all transuranic
nuclides and is produced only in the r-process in supernovae and
colliding neutron stars; when nuclei are ejected from these events at
high speed to reach Earth, 244Pu alone among transuranic nuclides has
a long enough half-life to survive the journey, and hence tiny traces
of live interstellar 244Pu have been found in the deep sea floor.
Because 240Pu also occurs in the decay chain of 244Pu, it must thus
also be present in secular equilibrium, albeit in even tinier
quantities.
Astrophysical detection of plutonium is extremely limited, but is
found in the spectrum of the extremely chemically peculiar
Przybylski's Star.
Minute traces of plutonium are usually found in the human body due to
the 550 atmospheric and underwater nuclear tests that have been
carried out, and to a small number of major nuclear accidents. Most
atmospheric and underwater nuclear testing was stopped by the Limited
Test Ban Treaty in 1963, which of the nuclear powers was signed and
ratified by the United States, United Kingdom and Soviet Union. France
would continue atmospheric nuclear testing until 1974 and China would
continue atmospheric nuclear testing until 1980. All subsequent
nuclear testing was conducted underground.
Discovery
===========
Enrico Fermi and a team of scientists at the University of Rome
reported that they had discovered element 94 in 1934. Fermi called the
element 'hesperium' and mentioned it in his Nobel Lecture in 1938. The
sample actually contained products of nuclear fission, primarily
barium and krypton. Nuclear fission, discovered in Germany in 1938 by
Otto Hahn and Fritz Strassmann, was unknown at the time.
Plutonium (specifically, plutonium-238) was first produced, isolated,
and then chemically identified between December 1940 and February 1941
by Glenn T. Seaborg, Edwin McMillan, Emilio Segrè, Joseph W. Kennedy,
and Arthur Wahl by deuteron bombardment of uranium in the 60 in
cyclotron at the Berkeley Radiation Laboratory at the University of
California, Berkeley.
Neptunium-238 was created directly by the bombardment but decayed by
beta emission with a half-life of a little over two days, which
indicated the formation of element 94. The first bombardment took
place on December 14, 1940, and the new element was first identified
through oxidation on the night of February 23-24, 1941.
A paper documenting the discovery was prepared by the team and sent to
the journal 'Physical Review' in March 1941, but publication was
delayed until a year after the end of World War II due to security
concerns. At the Cavendish Laboratory in Cambridge, Egon Bretscher and
Norman Feather realized that a slow neutron reactor fuelled with
uranium would theoretically produce substantial amounts of
plutonium-239 as a by-product. They calculated that element 94 would
be fissile, and had the added advantage of being chemically different
from uranium, and could easily be separated from it.
McMillan had recently named the first transuranic element neptunium
after the planet Neptune, and suggested that element 94, being the
next element in the series, be named for what was then considered the
next planet, Pluto. Nicholas Kemmer of the Cambridge team
independently proposed the same name, based on the same reasoning as
the Berkeley team. Seaborg originally considered the name "plutium",
but later thought that it did not sound as good as "plutonium". He
chose the letters "Pu" as a joke, in reference to the interjection "P
U" to indicate an especially disgusting smell, which passed without
notice into the periodic table. Alternative names considered by
Seaborg and others were "ultimium" or "extremium" because of the
erroneous belief that they had found the last possible element on the
periodic table.
Hahn and Strassmann, and independently Kurt Starke, were at this point
also working on transuranic elements in Berlin. It is likely that Hahn
and Strassmann were aware that plutonium-239 should be fissile.
However, they did not have a strong neutron source. Element 93 was
reported by Hahn and Strassmann, as well as Starke, in 1942. Hahn's
group did not pursue element 94, likely because they were discouraged
by McMillan and Abelson's lack of success in isolating it when they
had first found element 93. However, since Hahn's group had access to
the stronger cyclotron at Paris at this point, they would likely have
been able to detect plutonium had they tried, albeit in tiny
quantities (a few becquerels).
Early research
================
The chemistry of plutonium was found to resemble uranium after a few
months of initial study. Early research was continued at the secret
Metallurgical Laboratory of the University of Chicago. On August 20,
1942, a trace quantity of this element was isolated and measured for
the first time. About 50 micrograms of plutonium-239 combined with
uranium and fission products was produced and only about 1 microgram
was isolated. This procedure enabled chemists to determine the new
element's atomic weight. On December 2, 1942, on a racket court under
the west grandstand at the University of Chicago's Stagg Field,
researchers headed by Enrico Fermi achieved the first self-sustaining
chain reaction in a graphite and uranium pile known as CP-1. Using
theoretical information garnered from the operation of CP-1, DuPont
constructed an air-cooled experimental production reactor, known as
X-10, and a pilot chemical separation facility at Oak Ridge. The
separation facility, using methods developed by Glenn T. Seaborg and a
team of researchers at the Met Lab, removed plutonium from uranium
irradiated in the X-10 reactor. Information from CP-1 was also useful
to Met Lab scientists designing the water-cooled plutonium production
reactors for Hanford. Construction at the site began in mid-1943.
In November 1943 some plutonium trifluoride was reduced to create the
first sample of plutonium metal: a few micrograms of metallic beads.
Enough plutonium was produced to make it the first synthetically made
element to be visible with the unaided eye.
The nuclear properties of plutonium-239 were also studied; researchers
found that when it is hit by a neutron it breaks apart (fissions) by
releasing more neutrons and energy. These neutrons can hit other atoms
of plutonium-239 and so on in an exponentially fast chain reaction.
This can result in an explosion large enough to destroy a city if
enough of the isotope is concentrated to form a critical mass.
During the early stages of research, animals were used to study the
effects of radioactive substances on health. These studies began in
1944 at the University of California at Berkeley's Radiation
Laboratory and were conducted by Joseph G. Hamilton. Hamilton was
looking to answer questions about how plutonium would vary in the body
depending on exposure mode (oral ingestion, inhalation, absorption
through skin), retention rates, and how plutonium would be fixed in
tissues and distributed among the various organs. Hamilton started
administering soluble microgram portions of plutonium-239 compounds to
rats using different valence states and different methods of
introducing the plutonium (oral, intravenous, etc.). Eventually, the
lab at Chicago also conducted its own plutonium injection experiments
using different animals such as mice, rabbits, fish, and even dogs.
The results of the studies at Berkeley and Chicago showed that
plutonium's physiological behavior differed significantly from that of
radium. The most alarming result was that there was significant
deposition of plutonium in the liver and in the "actively
metabolizing" portion of bone. Furthermore, the rate of plutonium
elimination in the excreta differed between species of animals by as
much as a factor of five. Such variation made it extremely difficult
to estimate what the rate would be for human beings.
Production during the Manhattan Project
=========================================
During World War II the U.S. government established the Manhattan
Project, for developing an atomic bomb. The three primary research and
production sites of the project were the plutonium production facility
at what is now the Hanford Site, the uranium enrichment facilities at
Oak Ridge, Tennessee, and the weapons research and design lab, now
known as Los Alamos National Laboratory, LANL.
The first production reactor that made (239)Pu was the X-10 Graphite
Reactor. It went online in 1943 and was built at a facility in Oak
Ridge that later became the Oak Ridge National Laboratory.
In January 1944, workers laid the foundations for the first chemical
separation building, T Plant located in 200-West. Both the T Plant and
its sister facility in 200-West, the U Plant, were completed by
October. (U Plant was used only for training during the Manhattan
Project.) The separation building in 200-East, B Plant, was completed
in February 1945. The second facility planned for 200-East was
canceled. Nicknamed Queen Marys by the workers who built them, the
separation buildings were awesome canyon-like structures 800 feet
long, 65 feet wide, and 80 feet high containing forty process pools.
The interior had an eerie quality as operators behind seven feet of
concrete shielding manipulated remote control equipment by looking
through television monitors and periscopes from an upper gallery. Even
with massive concrete lids on the process pools, precautions against
radiation exposure were necessary and influenced all aspects of plant
design.
On April 5, 1944, Emilio Segrè at Los Alamos received the first sample
of reactor-produced plutonium from Oak Ridge. Within ten days, he
discovered that reactor-bred plutonium had a higher concentration of
(240)Pu than cyclotron-produced plutonium. (240)Pu has a high
spontaneous fission rate, raising the overall background neutron level
of the plutonium sample. The original gun-type plutonium weapon,
code-named "Thin Man", had to be abandoned as a result--the increased
number of spontaneous neutrons meant that nuclear pre-detonation
(fizzle) was likely.
The entire plutonium weapon design effort at Los Alamos was soon
changed to the more complicated implosion device, code-named "Fat
Man". In an implosion bomb, plutonium is compressed to high density
with explosive lenses--a technically more daunting task than the
simple gun-type bomb, but necessary for a plutonium bomb. Uranium, by
contrast, can be used with either method.
Construction of the Hanford B Reactor, the first industrial-sized
nuclear reactor for the purposes of material production, was completed
in March 1945. B Reactor produced the fissile material for the
plutonium weapons used during World War II. B, D and F were the
initial reactors built at Hanford, and six additional
plutonium-producing reactors were built later at the site.
By the end of January 1945, the highly purified plutonium underwent
further concentration in the completed chemical isolation building,
where remaining impurities were removed successfully. Los Alamos
received its first plutonium from Hanford on February 2. While it was
still by no means clear that enough plutonium could be produced for
use in bombs by the war's end, Hanford was by early 1945 in operation.
Only two years had passed since Col. Franklin Matthias first set up
his temporary headquarters on the banks of the Columbia River.
According to Kate Brown, the plutonium production plants at Hanford
and Mayak in Russia, over a period of four decades, "both released
more than 200 million curies of radioactive isotopes into the
surrounding environment--twice the amount expelled in the Chernobyl
disaster in each instance". Most of this radioactive contamination
over the years were part of normal operations, but unforeseen
accidents did occur and plant management kept this secret, as the
pollution continued unabated.
In 2004, a safe was discovered during excavations of a burial trench
at the Hanford nuclear site. Inside the safe were various items,
including a large glass bottle containing a whitish slurry which was
subsequently identified as the oldest sample of weapons-grade
plutonium known to exist. Isotope analysis by Pacific Northwest
National Laboratory indicated that the plutonium in the bottle was
manufactured in the X-10 Graphite Reactor at Oak Ridge during 1944.
Trinity and Fat Man atomic bombs
==================================
The first atomic bomb test, codenamed "Trinity "and detonated on July
16, 1945, near Alamogordo, New Mexico, used plutonium as its fissile
material. The implosion design of "Gadget", as the Trinity device was
codenamed, used conventional explosive lenses to compress a sphere of
plutonium into a supercritical mass, which was simultaneously showered
with neutrons from "Urchin", an initiator made of polonium and
beryllium (neutron source: (α, n) reaction). Together, these ensured a
runaway chain reaction and explosion. The weapon weighed over 4
tonnes, though it had just 6 kg of plutonium. About 20% of the
plutonium in the Trinity weapon, fissioned; releasing an energy
equivalent to about 20,000 tons of TNT.
An identical design was used in "Fat Man", dropped on Nagasaki, Japan,
on August 9, 1945, killing 35,000-40,000 people and destroying 68%-80%
of war production at Nagasaki. Only after the announcement of the
first atomic bombs was the existence and name of plutonium made known
to the public by the Manhattan Project's Smyth Report.
Cold War use and waste
========================
Large stockpiles of weapons-grade plutonium were built up by both the
Soviet Union and the United States during the Cold War. The U.S.
reactors at Hanford and the Savannah River Site in South Carolina
produced 103 tonnes, and an estimated 170 tonnes of military-grade
plutonium was produced in the USSR. Each year about 20 tonnes of the
element is still produced as a by-product of the nuclear power
industry. As much as 1000 tonnes of plutonium may be in storage with
more than 200 tonnes of that either inside or extracted from nuclear
weapons.
SIPRI estimated the world plutonium stockpile in 2007 as about 500
tonnes, divided equally between weapon and civilian stocks.
Radioactive contamination at the Rocky Flats Plant primarily resulted
from two major plutonium fires in 1957 and 1969. Much lower
concentrations of radioactive isotopes were released throughout the
operational life of the plant from 1952 to 1992. Prevailing winds from
the plant carried airborne contamination south and east, into
populated areas northwest of Denver. The contamination of the Denver
area by plutonium from the fires and other sources was not publicly
reported until the 1970s. According to a 1972 study coauthored by
Edward Martell, "In the more densely populated areas of Denver, the Pu
contamination level in surface soils is several times fallout", and
the plutonium contamination "just east of the Rocky Flats plant ranges
up to hundreds of times that from nuclear tests". As noted by Carl
Johnson in Ambio, "Exposures of a large population in the Denver area
to plutonium and other radionuclides in the exhaust plumes from the
plant date back to 1953." Weapons production at the Rocky Flats plant
was halted after a combined FBI and EPA raid in 1989 and years of
protests. The plant has since been shut down, with its buildings
demolished and completely removed from the site.
In the U.S., some plutonium extracted from dismantled nuclear weapons
is melted to form glass logs of plutonium oxide that weigh two tonnes.
The glass is made of borosilicates mixed with cadmium and gadolinium.
These logs are planned to be encased in stainless steel and stored as
much as 4 km underground in bore holes that will be back-filled with
concrete. The U.S. planned to store plutonium in this way at the Yucca
Mountain nuclear waste repository, which is about 100 mi north-east of
Las Vegas, Nevada.
On March 5, 2009, Energy Secretary Steven Chu told a Senate hearing
"the Yucca Mountain site no longer was viewed as an option for storing
reactor waste". Starting in 1999, military-generated nuclear waste is
being entombed at the Waste Isolation Pilot Plant in New Mexico.
In a Presidential Memorandum dated January 29, 2010, President Obama
established the Blue Ribbon Commission on America's Nuclear Future. In
their final report the Commission put forth recommendations for
developing a comprehensive strategy to pursue, including:
: "Recommendation #1: The United States should undertake an integrated
nuclear waste management program that leads to the timely development
of one or more permanent deep geological facilities for the safe
disposal of spent fuel and high-level nuclear waste".
Medical experimentation
=========================
During and after the end of World War II, scientists working on the
Manhattan Project and other nuclear weapons research projects
conducted studies of the effects of plutonium on laboratory animals
and human subjects. Animal studies found that a few milligrams of
plutonium per kg of tissue is a lethal dose.
For human subjects, this involved injecting solutions typically
containing 5 micrograms (μg) of plutonium into hospital patients
thought to be either terminally ill, or to have a life expectancy of
less than ten years either due to age or chronic disease. This was
reduced to 1 μg in July 1945 after animal studies found that the way
plutonium distributes itself in bones is more dangerous than radium.
Most of the subjects, Eileen Welsome says, were poor, powerless, and
sick.
In 1945-47, eighteen human test subjects were injected with plutonium
without informed consent. The tests were used to create diagnostic
tools to determine the uptake of plutonium in the body in order to
develop safety standards for working with plutonium. Ebb Cade was an
unwilling participant in medical experiments that involved injection
of 4.7 μg of plutonium on April 10, 1945, at Oak Ridge, Tennessee.
This experiment was under the supervision of Harold Hodge. Other
experiments directed by the United States Atomic Energy Commission and
the Manhattan Project continued into the 1970s. 'The Plutonium Files'
chronicles the lives of the subjects of the secret program by naming
each person involved and discussing the ethical and medical research
conducted in secret by the scientists and doctors. The episode is now
considered to be a serious breach of medical ethics and of the
Hippocratic Oath.
The government covered up most of these actions until 1993, when
President Bill Clinton ordered a change of policy and federal agencies
then made available relevant records. The resulting investigation was
undertaken by the president's Advisory Committee on Human Radiation
Experiments, and it uncovered much of the material about plutonium
research on humans. The committee issued a controversial 1995 report
which said that "wrongs were committed" but it did not condemn those
who perpetrated them.
Explosives
============
(239)Pu is a key fissile component in nuclear weapons, due to its ease
of fission and availability. Encasing the bomb's plutonium pit in a
tamper (a layer of dense material) decreases the critical mass by
reflecting escaping neutrons back into the plutonium core. This
reduces the critical mass from 16 kg to 10 kg, which is a sphere with
a diameter of about 10 cm. This critical mass is about a third of that
for uranium-235.
The Fat Man plutonium bombs used explosive compression of plutonium to
obtain significantly higher density than normal, combined with a
central neutron source to begin the reaction and increase efficiency.
Thus only 6 kg of plutonium was needed for an explosive yield
equivalent to 20 kilotons of TNT. Hypothetically, as little as 4 kg of
plutonium--and maybe even less--could be used to make a single atomic
bomb using very sophisticated assembly designs.
Mixed oxide fuel
==================
Spent nuclear fuel from normal light water reactors contains
plutonium, but it is a mixture of plutonium-242, 240, 239 and 238. The
mixture is not sufficiently enriched for efficient nuclear weapons,
but can be used once as MOX fuel. Accidental neutron capture causes
the amount of plutonium-242 and 240 to grow each time the plutonium is
irradiated in a reactor with low-speed "thermal" neutrons, so that
after the second cycle, the plutonium can only be consumed by fast
neutron reactors. If fast neutron reactors are not available (the
normal case), excess plutonium is usually discarded, and forms one of
the longest-lived components of nuclear waste. The desire to consume
this plutonium and other transuranic fuels and reduce the
radiotoxicity of the waste is the usual reason nuclear engineers give
to make fast neutron reactors.
The most common chemical process, PUREX ('P'lutonium-'UR'anium
'EX'traction), reprocesses spent nuclear fuel to extract plutonium and
uranium which can be used to form a mixed oxide (MOX) fuel for reuse
in nuclear reactors. Weapons-grade plutonium can be added to the fuel
mix. MOX fuel is used in light water reactors and consists of 60 kg of
plutonium per tonne of fuel; after four years, three-quarters of the
plutonium is burned (turned into other elements). MOX fuel has been in
use since the 1980s, and is widely used in Europe. Breeder reactors
are specifically designed to create more fissionable material than
they consume.
MOX fuel improves total burnup. A fuel rod is reprocessed after three
years of use to remove waste products, which by then account for 3% of
the total weight of the rods. Any uranium or plutonium isotopes
produced during those three years are left and the rod goes back into
production. The presence of up to 1% gallium per mass in weapons-grade
plutonium alloy has the potential to interfere with long-term
operation of a light water reactor.
Plutonium recovered from spent reactor fuel poses little proliferation
hazard, because of excessive contamination with non-fissile
plutonium-240 and plutonium-242. Separation of the isotopes is not
feasible. A dedicated reactor operating on very low burnup (hence
minimal exposure of newly formed plutonium-239 to additional neutrons
which causes it to be transformed to heavier isotopes of plutonium) is
generally required to produce material suitable for use in efficient
nuclear weapons. While "weapons-grade" plutonium is defined to contain
at least 92% plutonium-239 (of the total plutonium), the United States
have managed to detonate an under-20Kt device using plutonium believed
to contain only about 85% plutonium-239, so called '"fuel-grade"
plutonium. The "reactor-grade" plutonium produced by a regular LWR
burnup cycle typically contains less than 60% Pu-239, with up to 30%
parasitic Pu-240/Pu-242, and 10-15% fissile Pu-241. It is unknown if a
device using plutonium obtained from reprocessed civil nuclear waste
can be detonated, however such a device could hypothetically fizzle
and spread radioactive materials over a large urban area. The IAEA
conservatively classifies plutonium of all isotopic vectors as
"direct-use" material, that is, "nuclear material that can be used for
the manufacture of nuclear explosives components without transmutation
or further enrichment".
Power and heat source
=======================
Plutonium-238 has a half-life of 87.74 years. It emits a large amount
of thermal energy with low levels of both gamma rays/photons and
neutrons. Being an alpha emitter, it combines high energy radiation
with low penetration and thereby requires minimal shielding. A sheet
of paper can be used to shield against the alpha particles from
(238)Pu. One kilogram of the isotope generates about 570 watts of
heat.
These characteristics make it well-suited for electrical power
generation for devices that must function without direct maintenance
for timescales approximating a human lifetime. It is therefore used in
radioisotope thermoelectric generators and radioisotope heater units
such as those in the 'Cassini', 'Voyager', 'Galileo' and 'New
Horizons' space probes, and the 'Curiosity' and 'Perseverance' (Mars
2020) Mars rovers.
The twin 'Voyager' spacecraft were launched in 1977, each containing a
500 watt plutonium power source. Over 30 years later, each source
still produces about 300 watts which allows limited operation of each
spacecraft. An earlier version of the same technology powered five
Apollo Lunar Surface Experiment Packages, starting with Apollo 12 in
1969.
(238)Pu has also been used successfully to power artificial heart
pacemakers, to reduce the risk of repeated surgery. It has been
largely replaced by lithium-based primary cells, but there were
somewhere between 50 and 100 plutonium-powered pacemakers still
implanted and functioning in living patients in the United States. By
the end of 2007, the number of plutonium-powered pacemakers was
reported to be down to just nine. (238)Pu was studied as a way to
provide supplemental heat to scuba diving. (238)Pu mixed with
beryllium is used to generate neutrons for research purposes.
Toxicity
==========
There are two aspects to the harmful effects of plutonium:
radioactivity and heavy metal poisoning. Plutonium compounds are
radioactive and accumulate in bone marrow. Contamination by plutonium
oxide has resulted from nuclear disasters and radioactive incidents,
including military nuclear accidents where nuclear weapons have
burned. Studies of the effects of these smaller releases, as well as
of the widespread radiation poisoning sickness and death following the
atomic bombings of Hiroshima and Nagasaki, have provided considerable
information regarding the dangers, symptoms and prognosis of radiation
poisoning, which in the case of the Japanese survivors was largely
unrelated to direct plutonium exposure.
The decay of plutonium, releases three types of ionizing radiation:
alpha (α), beta (β), and gamma (γ). Either acute or longer-term
exposure carries a danger of serious health outcomes including
radiation sickness, genetic damage, cancer, and death. The danger
increases with the amount of exposure. α-radiation can travel only a
short distance and cannot travel through the outer, dead layer of
human skin. β-radiation can penetrate human skin, but cannot go all
the way through the body. γ-radiation can go all the way through the
body.
Even though α radiation cannot penetrate the skin, ingested or inhaled
plutonium does irradiate internal organs. α-particles generated by
inhaled plutonium have been found to cause lung cancer in a cohort of
European nuclear workers. The skeleton, where plutonium accumulates,
and the liver, where it collects and becomes concentrated, are at
risk. Plutonium is not absorbed into the body efficiently when
ingested; only 0.04% of plutonium oxide is absorbed after ingestion.
Plutonium absorbed by the body is excreted very slowly, with a
biological half-life of 200 years. Plutonium passes only slowly
through cell membranes and intestinal boundaries, so absorption by
ingestion and incorporation into bone structure proceeds very slowly.
Donald Mastick accidentally swallowed a small amount of plutonium(III)
chloride, which was detectable for the next thirty years of his life,
but appeared to suffer no ill effects.
Plutonium is more dangerous if inhaled than if ingested. The risk of
lung cancer increases once the total radiation dose equivalent of
inhaled plutonium exceeds 400 mSv. The U.S. Department of Energy
estimates that the lifetime cancer risk from inhaling 5,000 plutonium
particles, each about 3 μm wide, is 1% over the background U.S.
average. Ingestion or inhalation of large amounts may cause acute
radiation poisoning and possibly death. However, no human being is
known to have died because of inhaling or ingesting plutonium, and
many people have measurable amounts of plutonium in their bodies.
The "hot particle" theory in which a particle of plutonium dust
irradiates a localized spot of lung tissue is not supported by
mainstream research--such particles are more mobile than originally
thought and toxicity is not measurably increased due to particulate
form. When inhaled, plutonium can pass into the bloodstream. Once in
the bloodstream, plutonium moves throughout the body and into the
bones, liver, or other body organs. Plutonium that reaches body organs
generally stays in the body for decades and continues to expose the
surrounding tissue to radiation and thus may cause cancer.
A commonly cited quote by Ralph Nader states that a pound of plutonium
dust spread into the atmosphere would be enough to kill 8 billion
people. This was disputed by Bernard Cohen, an opponent of the
generally accepted linear no-threshold model of radiation toxicity.
Cohen estimated that one pound of plutonium could kill no more than 2
million people by inhalation, so that the toxicity of plutonium is
roughly equivalent with that of nerve gas.
Several populations of people who have been exposed to plutonium dust
(e.g. people living down-wind of Nevada test sites, Nagasaki
survivors, nuclear facility workers, and "terminally ill" patients
injected with Pu in 1945-46 to study Pu metabolism) have been
carefully followed and analyzed. Cohen found these studies
inconsistent with high estimates of plutonium toxicity, citing cases
such as Albert Stevens who survived into old age after being injected
with plutonium. "There were about 25 workers from Los Alamos National
Laboratory who inhaled a considerable amount of plutonium dust during
1940s; according to the hot-particle theory, each of them has a 99.5%
chance of being dead from lung cancer by now, but there has not been a
single lung cancer among them."
Marine toxicity
=================
Plutonium is known to enter the marine environment by dumping of waste
or accidental leakage from nuclear plants. Though the highest
concentrations of plutonium in marine environments are found in
sediments, the complex biogeochemical cycle of plutonium means it is
also found in all other compartments. For example, various zooplankton
species that aid in the nutrient cycle will consume the element on a
daily basis. The complete excretion of ingested plutonium by
zooplankton makes their defecation an extremely important mechanism in
the scavenging of plutonium from surface waters. However, those
zooplankton that succumb to predation by larger organisms may become a
transmission vehicle of plutonium to fish.
In addition to consumption, fish can also be exposed to plutonium by
their distribution around the globe. One study investigated the
effects of transuranium elements (plutonium-238, plutonium-239,
plutonium-240) on various fish living in the Chernobyl Exclusion Zone
(CEZ). Results showed that a proportion of female perch in the CEZ
displayed either a failure or delay in maturation of the gonads.
Similar studies found large accumulations of plutonium in the
respiratory and digestive organs of cod, flounder and herring.
Plutonium toxicity is just as detrimental to larvae of fish in nuclear
waste areas. Undeveloped eggs have a higher risk than developed adult
fish exposed to the element in these waste areas. Oak Ridge National
Laboratory displayed that carp and minnow embryos raised in solutions
containing plutonium did not hatch; eggs that hatched displayed
significant abnormalities when compared to control developed embryos.
It revealed that higher concentrations of plutonium have been found to
cause issues in marine fauna exposed to the element.
Criticality potential
=======================
Care must be taken to avoid the accumulation of amounts of plutonium
which approach critical mass, particularly because plutonium's
critical mass is only a third of that of uranium-235. A critical mass
of plutonium emits lethal amounts of neutrons and gamma rays.
Plutonium in solution is more likely to form a critical mass than the
solid form due to moderation by the hydrogen in water.
Criticality accidents have occurred, sometimes killing people.
Careless handling of tungsten carbide bricks around a 6.2 kg plutonium
sphere resulted in a fatal dose of radiation at Los Alamos on August
21, 1945, when scientist Harry Daghlian received a dose estimated at
5.1 sievert (510 rem) and died 25 days later. Nine months later,
another Los Alamos scientist, Louis Slotin, died from a similar
accident involving a beryllium reflector and the same plutonium core
(the "demon core") that had previously killed Daghlian.
In December 1958, during a process of purifying plutonium at Los
Alamos, a critical mass formed in a mixing vessel, which killed
chemical operator Cecil Kelley. Other nuclear accidents have occurred
in the Soviet Union, Japan, the United States, and many other
countries.
Flammability
==============
Metallic plutonium is a fire hazard, especially if finely divided. In
a moist environment, plutonium forms hydrides on its surface, which
are pyrophoric and may ignite in air at room temperature. Plutonium
expands up to 70% in volume as it oxidizes and thus may break its
container. The radioactivity of the burning material is another
hazard. Magnesium oxide sand is probably the most effective material
for extinguishing a plutonium fire. It cools the burning material,
acting as a heat sink, and also blocks off oxygen. Special precautions
are necessary to store or handle plutonium in any form; generally a
dry inert gas atmosphere is required.
Land and sea
==============
The usual transport of plutonium is through the more stable plutonium
oxide in a sealed package. A typical transport consists of one truck
carrying one protected shipping container, holding a number of
packages with a total weight varying from 80 to 200 kg of plutonium
oxide. A sea shipment may consist of several containers, each holding
a sealed package. The U.S. Nuclear Regulatory Commission dictates that
it must be solid instead of powder if the contents surpass 0.74 TBq
(20 curies) of radioactivity. In 2016, the ships 'Pacific Egret' and
'Pacific Heron' of Pacific Nuclear Transport Ltd. transported 331 kg
(730 lbs) of plutonium to a United States government facility in
Savannah River, South Carolina.
Air
=====
U.S. Government air transport regulations permit the transport of
plutonium by air, subject to restrictions on other dangerous materials
carried on the same flight, packaging requirements, and stowage in the
rearmost part of the aircraft.
In 2012, media revealed that plutonium has been flown out of Norway on
commercial passenger airlines--around every other year--including one
time in 2011. Regulations permit a plane to transport 15 grams of
fissionable material. Such plutonium transportation is without
problems, according to a senior advisor ('seniorrådgiver') at Statens
strålevern.
License
=========
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Original Article:
http://en.wikipedia.org/wiki/Plutonium
