Experimental Speciation of Plutonium(IV) in Natural Seawater

Predominance of the alkaline earth(II) triscarbonatoactinyl(VI) complexes in different geochemical contexts: Review of existing data and estimation of potentially unidentified species

From the available thermodynamic data in the literature, a review of the impact of the formation of complexes between triscarbonatoactinyl(VI) and alkaline earth(II) (Ae) is estimated under varying conditions. First, after analyzing the literature data and using the ascertained thermodynamic data available from the commissioned reviews from the Nuclear Energy Agency (Organization for the Economic Cooperation and Development) Thermochemical DataBank Project on actinides (An) U, Np, and Pu, and from recently determined AenUO2(CO3)3(4-2n)- thermodynamic functions, the formation of AenAnO2(CO3)3(4-2n)- complexes for Pu(VI) and Np(VI) are estimated using linear free energy relationships (LFERs). The data are in good agreement with the sole determination of AePuO2(CO3)32- from Jo et al. (Dalton Trans. 49, 11605), which gives a relative confidence in the LFERs, and allows the application to actual situations. From existing uranium data, first, the impact of the origin of the data on the calculated predominance is addressed under 0.1 M NaCl and atmospheric CO2(g); second, the influence of ionic strength and salinity on predominance is estimated; and finally, the influence of temperature up to 50 °C on the solubility of uraninite in a deep geological radioactive waste storage or disposal site is calculated. For neptunium and plutonium, the impact of the potential log10β°(AenAnO2(CO3)3(4-2n)-) on Pourbaix diagrams of Pu and Np in Mg-Ca-CO3 media are estimated from Jo et al. (Dalton Trans. 49, 11605) and LFERs. Finally, the application to the speciation of Pu and Np in seawater is proposed.

Synthesis and multi-scale properties of PuO2 nanoparticles: recent advances and open questions

Due to the increased attention given to actinide nanomaterials, the question of their structure-property relationship is on the spotlight of recent publications. Plutonium oxide (PuO₂) particularly plays a central role in nuclear energetics and a comprehensive knowledge about its properties when nanosizing is of paramount interest to understand its behaviour in environmental migration schemes but also for the development of advanced nuclear energy systems underway. The element plutonium further stimulates the curiosity of scientists due to the unique physical and chemical properties it exhibits around the periodic table. PuO₂ crystallizes in the fluorite structure of the face-centered cubic system for which the properties can be significantly affected when shrinking. Identifying the formation mechanism of PuO₂ nanoparticles, their related atomic, electronic and crystalline structures, and their reactivity in addition to their nanoscale properties, appears to be a fascinating and challenging ongoing topic, whose recent advances are discussed in this review.

Size and structure of hexanuclear plutonium oxo-hydroxo clusters in aqueous solution from synchrotron analysis

The size and shape of a water-soluble hexanuclear plutonium cluster were probed by combining synchrotron small-angle X-ray scattering (SAXS) and extended X-ray absorption fine structure (EXAFS). A specific setup coupling both techniques and dedicated to radioactive samples on the MARS beamline endstation at Synchrotron SOLEIL is described. The plutonium hexanuclear cores are well stabilized by the 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid ligands and this allows a good evaluation of the setup to probe the very small plutonium core. The results show that, in spite of the constrained conditions required to avoid any risk of sample dispersion, the flux and the sample environment are optimized to obtain a very good signal-to-noise ratio, allowing the detection of small plutonium aggregates in an aqueous phase. The structure of the well defined hexanuclear cluster has been confirmed by EXAFS measurements in solution and correlated with SAXS data processing and modelling. An iterative comparison of classical fit models (Guinier or sphere form factor) with the experimental results allowed a better interpretation of the SAXS signal that will be relevant for future work under environmentally relevant conditions.

Novel DGT Configurations for the Assessment of Bioavailable Plutonium, Americium, and Uranium in Marine and Freshwater Environments

Plutonium, americium, and uranium contribute to the radioactive contamination of the environment and are risk factors for elevated radiation exposure via ingestion through food or water. Due to the significant environmental inventory of these radioelements, a sampling method to accurately monitor their bioavailable concentrations in natural waters is necessary, especially since physicochemical factors can cause significant temporal fluctuations in their waterborne concentrations. To this end, we engineered novel diffusive gradients in thin-film (DGT) configurations using resin gels, which are selective for UO₂²⁺, Pu(IV + V), and Am(III) among an excess of extraneous cations. In this work, we also report an improved synthesis of our in-house ion-imprinted polymer resin, which we used to manufacture a resin gel to capture Am(III). The effective diffusion coefficients of Pu, Am, and U in agarose cross-linked polyacrylamide were determined in freshwater and seawater simulants and in natural seawater, to calibrate these configurations for environmental deployments.

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

We review the state-of-the-art and recent advances in the determination of radionuclide speciation in seawater.