EPA-Provided Comment: Stakeholder comment received at the May 12, 2010 

Stakeholder meeting in Albuquerque 

Comment: Is DOE aware of these experiments (Plutonium nano-colloids) and have the 

results (preliminary?) been taken into account (if applicable) in the WIPP 

radionuclide release scenarios? 

Reference (provided at the Stakeholder meeting): 

Argonne National Laboratory News Release: “Scientists discover how the structure of 

plutonium nanocluster contaminants increases risk of spreading,” 2008. 

DOE Response: 

The DOE is aware of the plutonium nano-colloid experiments. These are not new results and 

different aspects of the Argonne work have been presented at various international meetings that 

were attended by researchers currently active in the WIPP project (Migration 2007, Plutonium 

Futures 2008, and Actinides 2009). In addition there is a more extensive description of the 

experimental work than the News Release provided at the Stakeholder meeting that was 

published (Soderholm et. al., 2008). 

The results described in the News Release and published paper are interesting results that 

specifically raise questions about the structure of plutonium colloids in aqueous media. It is 

proposed by the authors that plutonium colloids are best described as nanoclusters with a very 

specific size and structure that are formed by oxolation reactions (Pu-O-Pu bonding), rather than 

the more conventionally accepted olation (Pu-OH-Pu bonding) reactions. In this context, the 

colloids proposed have a much more rigid/defined structure that contrasts with the more 

generally accepted view that they are amorphous and less rigidly structured under 

environmentally-relevant conditions. Specific examples of highly specific structures were 

provided for colloids formed in alkali hydroxide media and reconstituted under highly acidic 

conditions in hydrochloric and nitric acid where high-energy x-ray scattering (HEXS) was used 

to establish their structure. The long-term stability of these colloids, their relevance to the much 

higher pH systems typically of concern in groundwater/brine systems, and transport properties 

were not addressed in this publication – so the overall relevance of these preliminary results to 

the WIPP case is not clearly established by what has been published. 

Although this study raised an important question about the possible structure of plutonium 

colloids, it does not, in fact, raise an issue that will significantly impact current WIPP PA 

calculations of plutonium release. The expected high-probability scenario in the WIPP is that 

there are no human intrusions after repository closure and the WIPP repository remains selfsealed with no net release of plutonium in any form – in this expected case the possible existence 

of colloids is not a significant issue. Under low-probability human intrusion scenarios, where 

brine enters the WIPP repository leading to a dissolved concentration release of plutonium (that 

would also include colloidal species), nano-colloids are essentially included as part of our 

dissolved concentration fraction and would not add to the total plutonium concentration available 

for release – so this pathway is insensitive to the nature/structure of the dissolved plutonium 
species. Lastly, the colloidal fraction of this dissolved plutonium concentration is only 

potentially important when subsurface migration is an important release pathway – a very low 

probability event. Even for this scenario, the WIPP already includes colloidal plutonium 

transport in its current PA and there is no significant release of colloidal plutonium predicted 

under the current release scenarios. Since conservative colloidal transport assumptions are used 

in these calculations, they are relatively insensitive to the detailed nature/structure of the colloid 

– which is the main issue addressed by the Soderholm et al., paper. 

DOE References: 

L. Soderholm, P. Almond, S. Skanthakumar, R. E. Wislons and P.C. Burns, “The Structure of the 

plutonium oxide nanocluster [Pu38O56Cl54(H2O)8]

14-, Angew. Chem Int. Ed., 47 (2008) 298-302. 
Argonne National Laboratory is a U.S. 

Department of Energy laboratory 

managed by UChicago Argonne, LLC. 

 Media Contact: Brock Cooper 

 (630) 252-5565 

 bcooper@anl.gov 

 

Scientists discover how the structure of plutonium nanocluster 

contaminants increases risk of spreading 

ARGONNE, Ill. (April 22, 2008) — 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 Advanced Photon Source (APS) at Argonne 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," said Argonne 

senior chemist Lynda Soderholm. "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 have 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. 

-- more-- 
Plutonium nanoclusters – add one 

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 their unique electric and magnetic properties – 

together with her collaborators S. Skanthakumar, Richard Wilson and Peter Burns of Argonne’s 

Chemical Sciences and Engineering Division. 

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. 

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