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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 is.”

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 weapons components.

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 solution:
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 instrumentation.

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 Sciences.

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 of biosciences.

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.