Coprecipitation of actinide peroxide salts in the U-Th and U-Pu systems and their thermal decomposition.

The uranium and plutonium co-conversion process constitutes a continuous subject of interest for MOx fuel fabrication. Among the various routes considered, chemical coprecipitation by the salt effect has been widely investigated regarding its simplicity of integration between the partitioning and purification steps of the PUREX process, and the straightforward recovery of precursors that are easily converted into oxide phases by thermal decomposition. The present study focuses on the coprecipitation behavior of U-Th and U-Pu actinide peroxide mixed systems by examining the precipitation yields and settling properties for nitric acidity in the range of 1 to 3 M and hydrogen peroxide concentration in the range of 4.5 to 7 M. The precipitated solids have been characterized by powder XRD, IR and Raman spectroscopy, laser granulometry and SEM-EDS analyses revealing the synthesis of studtite and actinide(IV) peroxo-nitrates as aggregated particles. The actinide solid phases are uniformly distributed within the filtered cakes. The precursor thermal decomposition results in the formation of oxide phases at low temperature according to a sequential release of water molecules, peroxide ligands and nitrate ions. The calcination step has a limited effect on the morphology of the powders which remain highly divided. The high precipitation rate of actinides makes this chemical route potentially interesting as a co-precipitation process.

Probing the local structure of nanoscale actinide oxides: a comparison between PuO2 and ThO2 nanoparticles rules out PuO2+x hypothesis.

Actinide research at the nanoscale is gaining fundamental interest due to environmental and industrial issues. The knowledge of the local structure and speciation of actinide nanoparticles, which possibly exhibit specific physico-chemical properties in comparison to bulk materials, would help in a better and reliable description of their behaviour and reactivity. Herein, the synthesis and relevant characterization of PuO2 and ThO2 nanoparticles displayed as dispersed colloids, nanopowders, or nanostructured oxide powders allow to establish a clear relationship between the size of the nanocrystals constituting these oxides and their corresponding An(iv) local structure investigated by EXAFS spectroscopy. Particularly, the first oxygen shell of the probed An(iv) evidences an analogous behaviour for both Pu and Th oxides. This observation suggests that the often observed and controversial splitting of the Pu-O shell on the Fourier transformed EXAFS signal of the PuO2 samples is attributed to a local structural disorder driven by a nanoparticle surface effect rather than to the presence of PuO2+x species.

Insights into the sonochemical synthesis and properties of salt-free intrinsic plutonium colloids.

Fundamental knowledge on intrinsic plutonium colloids is important for the prediction of plutonium behaviour in the geosphere and in engineered systems. The first synthetic route to obtain salt-free intrinsic plutonium colloids by ultrasonic treatment of PuO2 suspensions in pure water is reported. Kinetics showed that both chemical and mechanical effects of ultrasound contribute to the mechanism of Pu colloid formation. In the first stage, fragmentation of initial PuO2 particles provides larger surface contact between cavitation bubbles and solids. Furthermore, hydrogen formed during sonochemical water splitting enables reduction of Pu(IV) to more soluble Pu(III), which then re-oxidizes yielding Pu(IV) colloid. A comparative study of nanostructured PuO2 and Pu colloids produced by sonochemical and hydrolytic methods, has been conducted using HRTEM, Pu LIII-edge XAS, and O K-edge NEXAFS/STXM. Characterization of Pu colloids revealed a correlation between the number of Pu-O and Pu-Pu contacts and the atomic surface-to-volume ratio of the PuO2 nanoparticles. NEXAFS indicated that oxygen state in hydrolytic Pu colloid is influenced by hydrolysed Pu(IV) species to a greater extent than in sonochemical PuO2 nanoparticles. In general, hydrolytic and sonochemical Pu colloids can be described as core-shell nanoparticles composed of quasi-stoichiometric PuO2 cores and hydrolyzed Pu(IV) moieties at the surface shell.

Decay Mechanism of NO3(•) Radical in Highly Concentrated Nitrate and Nitric Acidic Solutions in the Absence and Presence of Hydrazine.

The decay mechanism of NO3(•) has been determined through a combination of experiment and calculation for 7 mol dm(-3) solutions of deaerated aqueous LiNO3 and HNO3, in the absence and presence of hydrazine (N2H4, N2H5(+), and N2H6(2+)). In the absence of hydrazine, the predominant NO3(•) decay pathways are strongly dependent upon the pH of the solution. For neat, neutral pH LiNO3 solutions (7 mol dm(-3)), NO3(•) produced by the pulse is fully consumed within 160 µs by OH(•) (37%), H2O (29%), NO2(-) (17%), and NO2 (17%). For acidic HNO3 solutions (7 mol dm(-3)), radiolytically produced NO3(•) is predominantly consumed within 1 ms by HNO2 (15%) and NO2 (80%). Intervening formulations exhibit the mechanistic transition from neat LiNO3 to neat HNO3. In highly acidic nitric acid solution, hydrazine exists mainly as N2H5(+) and N2H6(2+), both of which rapidly consume NO3(•) in addition to other decay mechanisms, with rate constants of 2.9 (±0.9) × 10(7) and 1.3 (±0.3) × 10(6) dm(3) mol(-1) s(-1), respectively.

Ultrasound-assisted reductive dissolution of CeO2 and PuO2 in the presence of Ti particles.

PuO2 is considered an important material for current and future nuclear fuel; however it is a very refractive compound towards dissolution. Among other techniques, its reprocessing can be performed via complexing dissolution in concentrated and boiling nitric acid containing hydrofluoric acid, or via oxidant dissolution in the presence of reagents with redox couples having high potentials such as Ce(iv)/Ce(iii), or Ag(ii)/Ag(i). Reductive dissolution can be performed under softer conditions and is considered an alternative to these methods which may suffer from several drawbacks (corrosion, effluent management, compatibility with nuclear waste disposal, etc.). In this study, a sonochemical and reductive approach is investigated for PuO2 dissolution under relatively mild conditions. At the first stage, the experiments are performed with CeO2 as an inactive surrogate for PuO2. The quantitative dissolution of both oxides can be achieved under ultrasound (20 kHz, 0.35-0.70 W mL(-1)) in 0.5 M HNO3/0.1 M [N2H5NO3]/2 M HCOOH sparged with Ar at 33-35 °C in the presence of Ti particles as a generating source of reductive species. Ultrasound enables the depassivation of the Ti surface (usually strongly passivated in nitric solutions) through acoustic cavitation which then allows further generation of the intermediate Ti(iii) reductive species. Dissolution rates and yields can be further increased with the injection of dilute fluoride aliquots (NH4F or HF) in the sonicated solution to favor Ti chemical depassivation. The rapid and complete dissolution of PuO2 under selected conditions is accompanied by Pu(iii) accumulation in solution.

Anion selectivity in ion exchange reactions with surface functionalized ionosilicas.

Mesoporous imidazolium functionalized surface functionalized ionosilicas have been investigated as anion exchange materials for the adsorption of oxo-anions in aqueous media. We studied particularly pertechnetate adsorption and could show that solids bearing long chain substituted imidazolium groups are highly efficient anion exchange materials often displaying high distribution coefficients between solid and liquid phases. We observed that the distribution coefficient of pertechnetate is a function of the presence of competing anionic species. As a consequence, our experiments allowed reproducing experimentally Hofmeister's series. However, pertechnetate adsorption on the material can completely be inhibited in the presence of highly fluorinated anions such as bis(trifluoromethylsulfonyl)amide (NTf2). This behaviour indicates a particularly imidazophilic behaviour of these anions, which have a particular importance due to their use in water immiscible ionic liquids. Finally, the adsorption process has been shown to be reversible. This feature is of importance in view of the regeneration of the anion exchange material.

Sonochemical redox reactions of Pu(III) and Pu(IV) in aqueous nitric solutions.

The behavior of Pu(iv) and Pu(iii) was investigated in aqueous nitric solutions under ultrasound irradiation (Ar, 20 kHz). In the absence of anti-nitrous reagents, ultrasound has no effect on Pu(iv), while Pu(iii) can be rapidly oxidized to Pu(iv) due to the autocatalytic formation of HNO(2) induced by HNO(3) sonolysis. In the presence of anti-nitrous reagents (sulfamic acid or hydrazinium nitrate), Pu(iv) can be sonochemically reduced to Pu(iii). The reduction follows a first order reaction law and leads to a steady state where Pu(iv) and Pu(iii) coexist in solution. The reduction process is attributed to the sonochemical generation of H(2)O(2) in solution. The kinetics attributed to the reduction of Pu(iv) are however higher than those related to the formation of H(2)O(2) which, after several hypotheses, is explained by the sonochemical erosion of the titanium-based sonotrode. Titanium particles thereby generated can be solubilized under ultrasound and generate Ti(iii) as an intermediate species, a strong reducing agent able to react with Pu(iv).

Molecular dynamics studies of concentrated binary aqueous solutions of lanthanide salts: structures and exchange dynamics.

Concentrated binary aqueous solutions of lanthanide (Nd(3+) and Dy(3+)) salts (ClO(4)(-), Cl(-), and NO(3)(-)) have been studied by means of classical molecular dynamics (MD) simulations with explicit polarization and UV-visible spectroscopy. Pair interaction potentials, used for the MD simulations, have been developed in order to reproduce experimental hydration properties. Nd(3+) and Dy(3+) have been chosen because of their position in the lanthanide series: Nd(3+) being a light lanthanide and Dy(3+) a heavy one. They are respectively coordinated to nine and eight water molecules, in pure water, involving changes in their salt hydration structures. Both MD simulations and UV-visible experiments highlight the stronger affinity of nitrate anions toward Ln(3+) compared to perchlorates and chlorides. Dissociation/association processes of Nd(3+)-Cl(-) and Nd(3+)-NO(3)(-) ion pairs in aqueous solution have been analyzed using potential of mean force profile calculations. Furthermore, from MD simulations, it appears that the affinity of anions (perchlorate, chloride, and nitrate) is stronger for Nd(3+) than Dy(3+).

Application of capillary electrophoresis for inorganic selenium speciation in the frame of high-level waste management.

Capillary electrophoresis (CE) with direct UV detection is proposed for speciation of inorganic Se in high-level liquid waste. In this aim, the optimal conditions of measurements (pH, electrolyte buffer concentration) and the influence of nitrate excess on the quantitative determination of Se(IV) and Se(VI) were studied. Different electrolyte buffers were considered: carbonate, phosphate and citrate. It was found, that citrate buffer is the most suitable for the application under consideration. Under the chosen optimal conditions (20 mmol L(-1) citrate buffer, pH 2.5), calibration curves for Se(IV) and Se(VI) are linear in the concentration range 10(-4)-10(-3) mol L(-1). The detection limits are 4x10(-6 )for Se(IV) and 2x10(-5) for Se(VI). The accuracy of the procedure was checked by calculating the recovery by spiking simulation solutions. Relative standard deviation (S(r)) is less than 10%.