EPA-Provided Comment: Stakeholde r comment received at the May 12, 2010 Stakeholder meeting in Albuquerque
Comment: Is DOE aware of these experiment s (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 pr esented at various international meetings that were attended by researchers cu rrently active in the WIPP pr oject (Migration 2007, Plutonium Futures 2008, and Actinides 2009). In addition th ere is a more extensive description of the experimental work than the News Release pr ovided at the Stakeholder meeting that was published (Soderholm et. al., 2008). The results described in the News Release and published paper are inte resting results that specifically raise questions about the structure of plutonium colloid s in aqueous media. It is proposed by the authors that plutonium colloids ar e 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/defi ned structure that cont rasts with the more generally accepted view that they are amorphous and less rigidly structured under environmentally-relevant conditions. Specific ex amples of highly specific structures were provided for colloids formed in alkali hydroxide media and reconstitu ted under highly acidic conditions in hydrochloric and nitric acid wher e high-energy x-ray scattering (HEXS) was used to establish their structure. The long-term stabil ity of these colloids, thei r relevance to the much higher pH systems typically of concern in groun dwater/brine systems, and transport properties were not addressed in this publi cation – so the overall relevance of these pr eliminary results to the WIPP case is not clearly establishe d by what has been published. Although this study raised an important questi on about the possible st ructure 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 plut onium 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 diss olved concentration rel ease of plutonium (that would also include colloidal species), nano-coll oids are essentially in cluded as part of our dissolved concentration fraction a nd would not add to the total pl utonium concentration available for release – so this pathway is insensitive to the nature/structure of the dissolved plutoniumspecies. Lastly, the colloidal fraction of this dissolved plut onium 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 sign ificant release of colloid al 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 [Pu 38 O 56 Cl 54 (H 2 O) 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 struc ture of plutonium nanocluster contaminants increases risk of spreading ARGONNE, Ill. (April 22, 2008) — For almost half a century, scien tists have struggled with plutonium contamination spreading furthe r in groundwater than ex pected, increasing the
risk of sickness in humans and animals. It was known nanometer-sized clusters of pl utonium oxide were th e culprit, but no one had been able to study its stru cture or find a way to separa te it from the groundwater. Scientists at the U.S. Department of Energy’s Argonne National Laboratory, in
collaboration with researchers from the Universi ty 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 plutoni um 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 ha ve known about and tried to understand the nanoclusters, but it was the modern analyti cal techniques and the APS that allowed us
understand what it is." The nanoclusters are made up of exactly 38 pl utonium 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-plutoni um model, creating di screpancies between what is expected and reality. Soderholm said that with knowledge of the st ructure, scientists can
now create better models to account for not onl y free-roaming plutonium ions, but also the
nanoclusters.

-- more--Plutonium nanoclusters – add one

The clusters also are a problem for plutoni um remediation. The free ions are relatively easy to separate out from groundwater, bu t the clusters are difficult to remove. "As we learn more, we will be able to mode l the nanoclusters and figure out how to break them apart," Soderholm said. "Once they are fo rmed, 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. Skanthakum ar, Richard Wilson and Peter Burns of Argonne’s
Chemical Sciences a nd Engineering Division. Funding for the research was provided by th e U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. The mission of the Basic Energy Sciences (B ES) program – a multi purpose, scientific research effort – is to foster and support fundamental research to expand the scientific foundations for new and improved energy technologi es and for understanding and mitigating the environmental impacts of energy use. The portf olio supports work in the natural sciences, emphasizing fundamental research in materials sc iences, chemistry, geosciences, and aspects of
biosciences. Argonne National Laboratory brings the worl d’s brightest scientists and engineers together to find exciting and crea tive new solutions to pressing national problems in science and technology. The nation’s first national laborato ry, Argonne conducts leading-edge basic and applied scientific research in virtually every scien tific discipline. Argonn e researchers work closely with researchers from hundreds of companies, universities, and federal, state and municipal agencies to help them solve their sp ecific problems, advan ce America’s scientific leadership and prepare the nation for a better fu ture. 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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