Johan Vandenborre

Discrepancies in thorium oxide solubility values : a new experimental approach to improve understanding of oxide surface at solid/solution interface

Abstract The solubility of ThO2(cr) was studied since many years but a large discrepancy in solubility values is noticed in the literature. The present work suggests that this discrepancy is related to differences in the surface properties of thorium oxide. To understand the role of surface properties on solubility values, we conducted experiments with ThO2(cr) spheres with reproducable surface properties. Batch dissolution experiments were conducted in 0.01 M NaCl solution at pH=3.0 and 4.0 for periods of time up to 270 days. The solutions were spiked with 229Th to determine precipitation (sorption) rates of thorium, while dissolution rates were determined by measuring 232Th released from ThO2(cr) spheres. We assume that 229Th atoms are exchanged only with active sites involved in Th-dissolution. Using 229Th as local sensor of attachement and detachment processes at the ThO2(cr) surface under close-to-equilibrium conditions, allows to assess surface reactivity of the solid during solubility experiments.

Picosecond Pulse Radiolysis of Highly Concentrated Carbonate Solutions.

Highly concentrated potassium carbonate aqueous solutions are studied by picosecond pulse radiolysis with the purpose of exploring the formation processes of carbonate radical CO3(•-). The transient absorption band of solvated electron produced by ionizing is markedly shifted from 715 to 600 nm when the solute concentration of K2CO3 is 5 mol L(-1). This spectral shift is even more important than that observed for the solvated electron in 10 mol L(-1) KOH solutions. The broad absorption band of solvated electron in K2CO3 solutions overlaps with that of carbonate radical CO3(•-) formed at ultrashort time. Nitrate ion is used to scavenge the solvated electron and to observe the contribution of carbonate radical CO3(•-). The analysis of the amplitude and the kinetics of carbonate radical formation in highly concentrated solutions shows that CO3(•-) is formed within the electron pulse (7 ps) by two parallel mechanisms: a direct effect on the solute and the oxidation of the solute by water radical hole H2O(•+). These two mechanisms are followed by an additional one, by reaction between the solute and OH(•) radical especially in lower concentration. The radiolytic yield of each process is discussed.

Speciation of technetium in carbonate media under helium ions and γ radiation

Abstract Technetium carbonates complexes produced by chemical, electrochemical and radiolytic methods have been studied by UV-Visible, X-ray Absorption Fine Structure (XAFS) and Density Functional Theory methods. The (NH4)2TcCl6 salt was dissolved in 2 M KHCO3. The resulting purple solution was analyzed by XAFS and UV-Visible spectroscopy. The UV-Visible spectra exhibits a band centered at 515 nm. The XAFS results were consistent with the presence of polymeric species containing the [Tc2(μ−O)2]4+ core coordinated to carbonate ligand. Concerning the electrochemical methods, the pertechnetate anion was electrochemically reduced in concentrated carbonate solution [(CO32−)=5 M and (HCO3−)=0.5 M]. For the radiolytic reduction, the speciation of Tc under Helium ions particle beam and γ radiation was examined by UV-Visible and XAFS spectroscopy in high concentrated carbonate media. In concentrated carbonate solutions, pertechnetate as Tc(VII), was not reduced under irradiation due to the formation of carbonate radical which is a strong oxidant. Then, the solution proposed was the addition of formate to the solution which can scavenge hydroxyl radical 10 times faster than carbonate and prevent re-oxidation of reduced technetium. The XANES and EXAFS spectroscopies, approved by theoretical methods, revealed that the final product of the radiolytic reduction of pertechnetate is in the +IV oxidation state. The final structure of the reduced product by He2+ radiolysis was the same as electrochemical reduction. From this complex determination and evolution vs. the dose, this study is reporting the solubility of the Tc(IV) complex.

Assessment of surface reactivity of thorium oxide in conditions close to chemical equilibrium by isotope exchange 229Th/232Th method

Abstract This work aims to assess the solubility and the surface reactivity of crystallized thorium at pH 3.0 in presence of three types of solids: synthesized powder at 1300°C, crushed kernel, and intact kernel. In this study, the kernel is composed by the core solid from high temperature reactors (HTR) sphere particles. The originality of this work consisted in following in a sequential order the kinetic of dissolution, the surface reactivity in presence of isotope tracer 229Th, and its desorption process. Long time experiments (634 days) allowed to get deeper understanding on the behavior of the surface reactivity in contact with the solution. Solubility values are ranging from 0.3×10−7 mol·L−1 to 3×10−7 mol·L−1 with a dissolution rate of 10−6–10−4 g·m−2 day−1. PHREEQC modeling showed that crystallized ThO2(cr, 20 nm) phase controls the equilibrium in solution. Isotope exchange between 229Th and 232Th indicated that well-crystallized phase exist as an inert surface regarding to the absence of exchange between surface solid and solution.

Evolution of heavy ions (He2+, H+) radiolytic yield of molecular hydrogen vs. “Track-Segment” LET values

Abstract Ionizing radiation’s effects onto water molecules lead to the ionization and/or the excitation of them. Then, these phenomena are followed by the formation of radicals and molecular products. The linear energy transfer (LET), which defines the energy deposition density along the radiation length, is different according to the nature of ionizing particles. Thus, the values of radiolytic yields, defined as the number of radical and molecular products formed or consumed by unit of deposited energy, evolve according to this parameter. This work consists in following the evolution of radiolytic yield of molecular hydrogen and ferric ions according to the “Track-Segment” LET of ionizing particles (protons, helions). Concerning G(Fe3+) values, it seems that the energy deposited into the Bragg peak does not play the main role for the Fe3+ radiolytic formation, whereas for the G(H2) it is the case with a component around 40% of the Bragg peak in the dihydrogen production. Therefore, as main results of this work, for high energetic Helion and Proton beams, the G(Fe3+) values, which can be used for further dosimetry studies for example during the α radiolysis experiments, and the primary g(H2) values for the Track-Segment LET, which can be used to determine the dihydrogen production by α-emitters, are published.

Influence of Nitric Acid on the Helium Ion Radiolysis of Aqueous Butanal Oxime Solutions

Samples of butanal oxime in aqueous nitric acid solutions have been irradiated with the helium ion (4He2+) beam of the CEMHTI (Orléans, France) cyclotron. The consumption yield of butanal oxime has been measured by gas chromatography coupled with mass spectrometry. Gaseous products (mainly H2 and N2O) have also been monitored by micro-gas chromatography. Yields of liquid phase products (hydrogen peroxide and nitrous acid) have been determined by colorimetric methods. The influence of nitric acid on the radiation chemical behavior of butanal oxime depends on the nitric acid concentration. For a low concentration (≤0.5 mol L-1) butanal oxime is protected by the nitrate ions, which can efficiently scavenge the water radiolysis radicals. For higher concentrations, nitrous acid can accumulate in the medium, therefore leading to a strong increase of the butanal oxime degradation. The associated mechanism is an autocatalytic oxidation of butanal oxime by HNO2.