Christophe Den Auwer

Coffinite, USiO4, Is Abundant in Nature: So Why Is It So Difficult To Synthesize?

Coffinite, USiO4, is the second most abundant U(4+) mineral on Earth, and its formation by the alteration of the UO2 in spent nuclear fuel in a geologic repository may control the release of radionuclides to the environment. Despite its abundance in nature, the synthesis and characterization of coffinite have eluded researchers for decades. On the basis of the recent synthesis of USiO4, we can now define the experimental conditions under which coffinite is most efficiently formed. Optimal formation conditions are defined for four parameters: pH, T, heating time, and U/Si molar ratio. The adjustment of pH between 10 and 12 leads probably to the formation of a uranium(IV) hydroxo-silicate complex that acts as a precursor of uranium(IV) silicate colloids and then of coffinite. Moreover, in this pH range, the largest yield of coffinite formation (as compared with those of the two competing byproduct phases, nanometer-scale UO2 and amorphous SiO2) is obtained for 250 °C, 7 days, and 100% excess silica.

X-Ray Absorption Spectroscopy of Plutonium Particles at the Rocky Flats US Nuclear Weapons Production Site

The Rocky Flats Environmental Technology Site (RFETS) was a U.S. Department of Energy (DOE) environmental cleanup site located about 15 miles northwest of downtown Denver. Nearly 40 years of nuclear weapons production had left behind a legacy of contaminated facilities, soils, surface and groundwater. Ultrafiltration studies had shown that plutonium was associated with particles. We employed X-ray absorption spectroscopy to demonstrate that plutonium in soils at the Site was in the insoluble chemical form of PuO2. This information coupled with ultrafiltration studies was used to make the case for particle transport mechanisms as the basis of plutonium and americium mobility, rather than aqueous solubility processes, and established a successful scientific basis for the dominance of physical transport processes by wind and water. This understanding allowed Site contractors to rapidly move to application of soil erosion and sediment transport models as the means of predicting plutonium and americium transport, which led to design and application of site-wide soil erosion control technology to help control downstream concentrations of plutonium and americium in streamflow. Good scientific understanding in the public interest helped the most extensive cleanup in the history of Superfund legislation to finish 1 year ahead of schedule, ultimately resulting in billions of dollars in taxpayer savings.

On the structure of thorium and americium adenosine triphosphate complexes

Abstract Purpose: The actinides are chemical poisons and radiological hazards. One challenge to better appraise their toxicity and develop countermeasures in case of exposure of living organisms is to better assess pathways of contamination. Because of the high chemical affinity of those actinide elements for phosphate groups and the ubiquity of such chemical functions in biochemistry, nucleotides and in particular adenosine triphosphate nucleotide (ATP) may be considered critical target building blocks for actinides. Materials and methods: Combinations of spectroscopic techniques (Fourier transformed Infra Red [FTIR], Electrospray Ionization Mass Spectrometry [ESI-MS], and Extended X-ray Absorption Fine Structure [EXAFS]) with quantum chemical calculations have been implemented in order to assess the actinides coordination arrangement with ATP. Results: We describe and compare herein the interaction of ATP with thorium and americium; thorium(IV) as a representative of actinide(IV) like plutonium(IV) and americium(III) as a representative of all heavier actinides. In the case of thorium, an insoluble complex is readily formed. In the case of americium, a behavior identical to that described previously for lutetium has been observed with insoluble and soluble complexes. Conclusions: The comparative study of ATP complexation with Th(IV) and Am(III) shows their ability to form insoluble complexes for which a structural model has been proposed by analogy with previously described Lu(III) complexes.

Focus on speciation assessment in marine radiochemistry using X-ray absorption spectroscopy

The solubility, migration behavior and bioavailability of radionuclides in the marine environment strongly depend on their speciation. This focus article reviews the state-of-the-art and recent advances in the determination of radionuclide speciation in seawater, which is still challenging because of the very high ionic strength of the medium associated with ultra-trace concentrations of these elements in the oceans. In particular, we have highlighted the contribution and usefulness of synchrotron-based techniques such as X-ray absorption spectroscopy. Within this scope, we overview some major radionuclides in seawater, their natural or anthropogenic origin, and their reactivity and natural concentrations. We outline the theoretical speciation models currently used, based on thermodynamic stability constants, and compare them to published experimental data recently obtained from spectroscopic investigation of radionuclides in natural seawater samples. Finally, we discuss some leading perspectives on radionuclide speciation using X-ray absorption spectroscopy in environmental samples at concentrations that must deal with spectroscopy detection limits.