Topic Name: ANL Researchers Unveiled How Nanocluster Contaminants Increase Risk of Spreading Using Advanced Photon Source
Research persons: Lynda Soderholm
Location: Argonne National Laboratory, U.S. Department of Energy, United States
For almost half a century, scientists have struggled with
plutonium contamination spreading further in groundwater than expected,
increasing the risk of sickness in humans and animals.
It was known nanometer sized clusters of plutonium oxide were
the culprit, but no one had been able to study its structure or find a way to
separate it from the groundwater.
Scientists at the
U.S. Department of Energy’s
Argonne National Laboratory,
in collaboration with researchers from the University of Notre Dame, were able
to use high-energy X-rays from the Argonne Advanced Photon Source to finally
discover and study the structure of plutonium nanoclusters.
“When plutonium forms into the clusters, its chemistry is
completely different and no one has really been able to assess what it is, how
to model it or how to separate it Argonne senior chemist Lynda Soderholm said.
“People have known about and tried to understand the nanoclusters, but it was
the modern analytical techniques and the APS that allowed us understand what it
The nanoclusters are made up of exactly 38 plutonium atoms
and had almost no charge. Unlike stray plutonium ions, which carry a positive
charge, they are not attracted to the electrons in plant life, minerals, etc.
which stopped the ions’ progression in the ground water.
Models have been based on the free-plutonium model, creating
discrepancies between what is expected and reality. Soderholm said that with
knowledge of the structure, scientists can now create better models to account
for not only free-roaming plutonium ions, but also the nanoclusters.
The clusters also are a problem for plutonium remediation.
The free ions are relatively easy to separate out from groundwater, but the
clusters are difficult to remove.
“As we learn more, we will be able to model the nanoclusters
and figure out how to break them apart,” Soderholm said. “Once they are formed,
they are very hard to get rid of.”
Soderholm said other experiments have shown some clusters
with different numbers of plutonium atoms and she plans to examine -- together
with her collaborators S. Skanthakumar, Richard Wilson and Peter Burns of
Argonne’s Chemical Sciences and Engineering Division-- the unique electric and
magnetic properties of the clusters.
Note for Plutonium
Plutonium is a rare radioactive, metallic and toxic chemical element. It has the
symbol Pu and the atomic number 94. It is a fissile element used in most modern
nuclear weapons. The most significant isotope of plutonium is 239Pu, with a
half-life of 24,100 years. It can be made from natural uranium. The most stable
isotope is 244Pu, with a half-life of about 80 million years, long enough to be
found in extremely small quantities in nature, making 244Pu the nucleon-richest
atom that naturally occurs in the Earth's crust, albeit in small traces.
Plutonium has been called "the most complex metal" and "a physicist's dream but
an engineer's nightmare" for its peculiar physical and chemical properties. It
has six allotropes normally and a seventh under pressure. The allotropes have
very similar energy levels but significantly varying densities, making plutonium
very sensitive to changes in temperature, pressure, or chemistry, and allowing
for dramatic volume changes following phase transitions (in nuclear
applications, it is usually alloyed with a small amount of gallium, which
stabilizes it in the delta-phase). Plutonium is silvery in pure form, but has a
yellow tarnish when oxidized. It possesses a low-symmetry structure, causing it
to become progressively more brittle over time. Because it self-irradiates, it
ages both from the outside-in and the inside-out. However, self-irradiation can
also lead to annealing which counteracts some of the aging effects. In general,
the precise aging properties of plutonium are very complex and poorly
understood, greatly complicating efforts to predict future reliability of
The heat given off by alpha particle emission makes plutonium warm to the touch
in reasonable quantities. It displays five ionic oxidation states in aqueous
Pu(III), as Pu3+ (blue lavender)
Pu(IV), as Pu4+ (yellow brown)
Pu(V), as PuO2+ (thought to be pink; this ion is unstable in solution and will
disproportionate into Pu4+ and PuO22+; the Pu4+ will then oxidize the remaining
PuO2+ to PuO22+, being reduced in turn to Pu3+. Thus, aqueous solutions of
plutonium tend over time towards a mixture of Pu3+ and PuO22+.)
Pu(VI), as PuO22+ (pink orange)
Pu(VII), as PuO52- (dark red); the heptavalent ion is rare and prepared only
under extreme oxidizing conditions.
The actual color shown by Pu solutions depends on both the oxidation state and
the nature of the acid anion, which influences the degree of complexing of the
Pu species by the acid anion.
The isotope 239Pu is a key fissile component in nuclear weapons, due to its ease
of fissioning and availability. The critical mass for an unreflected sphere of
plutonium is 16 kg, but through the use of a neutron-reflecting tamper the pit
of plutonium in a fission bomb is reduced to 10 kg, which is a sphere with a
diameter of 10 cm. The Manhattan Project "Fat Man" type plutonium bombs, using
explosive compression of Pu to significantly higher densities than normal, were
able to function with plutonium cores of only 6.2 kg. Complete detonation may be
achieved through the use of an additional neutron source (often from a small
amount of fusion fuel). The Fat Man bomb had an explosive yield of 21 kilotons.
The isotope plutonium-238 (238Pu) has a half-life of 88 years and emits a large
amount of thermal energy as it decays. Being an alpha emitter, it combines high
energy radiation with low penetration (thereby requiring minimal shielding).
These characteristics make it well suited for electrical power generation for
devices which must function without direct maintenance for timescales
approximating a human lifetime. It is therefore used in radioisotope
thermoelectric generators such as those powering the Cassini and New Horizons
(Pluto) space probes; earlier versions of the same technology powered the ALSEP
and EASEP systems including seismic experiments on the Apollo Moon missions.
About Advanced Photon Source
The Advanced Photon Source (APS) at Argonne National Laboratory is a national
synchrotron-radiation light source research facility funded by the United States
Department of Energy, Office of Science, Office of Basic Energy Sciences.
Argonne National Laboratory is managed by UChicago Argonne, LLC, which is
composed of the University of Chicago, Jacobs Engineering Group Inc. and BWX
Technologies, Inc. (BWXT).
Using high-brilliance X-ray beams from the APS, members of the international
synchrotron-radiation research community conduct forefront basic and applied
research in the fields of materials science, biological science, physics,
chemistry, environmental, geophysical, planetary science, and innovative X-ray
Electrons are produced by a cathode that is heated to about 1,100°C (2,000°F).
The electrons are accelerated to 99.999% of the speed of light in a linear
accelerator. From the linear accelerator, the electrons are injected into the
booster synchrotron. Here, the electrons are sent around an oval racetrack of
electromagnets, providing further acceleration. Within one-half second, the
electrons reach 99.999999% of the speed of light. Upon reaching this speed, the
electrons are injected into the storage ring, a 1,104 meter (3 622 ft)
circumference ring of more than 1,000 electromagnets.
Once in the storage ring, the electrons produce x-ray beams that are available
for use in experimentation. Around the ring are 40 straight sections. One of
these sections is used to inject electrons into the ring, and four are dedicated
to replenishing the electron energy lost though x-ray emission by using 16
radio-frequency accelerating cavities. The remaining 35 straight sections can be
equipped with insertion devices. Insertion devices, arrays of north-south
permanent magnets usually called "undulators," cause the electrons to oscillate
and emit light in the invisible part of the electromagnetic spectrum. Due to the
relativistic velocities of the electrons, that light is Lorentz contracted into
the x-ray band of the electromagnetic spectrum.
Funding for the research was provided by the
U.S. Department of Energy,
Office of Science,
Office of Basic Energy
The mission of the Basic Energy Sciences (BES) program - a
multipurpose, scientific research effort - is to foster and support fundamental
research to expand the scientific foundations for new and improved energy
technologies and for understanding and mitigating the environmental impacts of
energy use. The portfolio supports work in the natural sciences, emphasizing
fundamental research in materials sciences, chemistry, geosciences, and aspects
Argonne National Laboratory brings the world’s brightest
scientists and engineers together to find exciting and creative new solutions to
pressing national problems in science and technology. The nation’s first
national laboratory, Argonne conducts leading-edge basic and applied scientific
research in virtually every scientific discipline. Argonne researchers work
closely with researchers from hundreds of companies, universities, and federal,
state and municipal agencies to help them solve their specific problems, advance
America’s scientific leadership and prepare the nation for a better future. With
employees from more than 60 nations, Argonne is managed by UChicago Argonne, LLC
for the U.S. Department of Energy's Office of Science.
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