Development, characterization, and first application of a resonant laser secondary neutral mass spectrometry setup for the research of plutonium in the context of long-term nuclear waste storage.

Plutonium is a major contributor to the radiotoxicity in a long-term nuclear waste repository; therefore, many studies have focused on interactions of plutonium with the technical, geotechnical, and geological barriers of a possible nuclear waste storage site. In order to gain new insights into the sorption on surfaces and diffusion of actinides through these complex heterogeneous materials, a highly sensitive method with spatial resolution is required. Resonant laser secondary neutral mass spectrometry (Laser-SNMS) uses the spatial resolution available in time-of-flight secondary ion mass spectrometry (TOF-SIMS) in combination with the high selectivity, sensitivity, and low background noise of resonance ionization mass spectrometry (RIMS) and is, therefore, a promising method for the study and analysis of the geochemical behavior of plutonium in long-term nuclear waste storage. The authors present an approach with a combined setup consisting of a commercial TOF-SIMS instrument and a Ti:sapphire (Ti:Sa) laser system, as well as its optimization, characterization, and improvements compared to the original proof of concept by Erdmann et al. (2009). As a first application, the spatial distributions of plutonium and other elements on the surface of a pyrite particle and a cement thin section were measured by Laser-SNMS and TOF-SIMS, respectively. These results exemplify the potential of these techniques for the surface analysis of heterogeneous materials in the context of nuclear safety research.

Investigation of the Electrophoretic Mobility of the Actinides Th, U, Np, Pu, and Am in Different Oxidation States.

The electrophoretic mobilities (µe) of the actinides Th and U-Am in different oxidation states (prepared in 1 M HCl and 1 M HClO4) have been determined by capillary electrophoresis (CE)-inductively coupled plasma mass spectrometry (ICPMS) using 1 M acetic acid as the background electrolyte, which has proven to provide an excellent setup for trace analysis at environmentally relevant concentrations (1 × 10-9 M). The values are independent of the respective acid solution. The µe of the Pu oxidation states +III to +VI have been measured. They agree with both the available literature data and the redox-stable analogues (Eu(III), Th(IV), Np(V), U(VI)) that have also been investigated. The trend in the µe for the actinides U-Pu was found to be An(III) > An(VI) > An(V) > An(IV). The µe values of Am(III) (µe(Am(III)) = 3.86 × 10-4 cm2/(Vs)), U(IV) (µe(U(IV)) = 0.34 × 10-4 cm2/(Vs)), and U(VI) (µe(U(VI)) = 1.51 × 10-4 cm2/(Vs)) have been measured for the first time under these experimental conditions. Furthermore, the measured µe values show systematic trends that can be rationalized on the basis of the calculated species distribution of the actinides in 1 M acetic acid and the corresponding average effective charges (qeff).

Determination of the Stability Constants of the Acetate Complexes of the Actinides Am(III), Th(IV), Np(V), and U(VI) Using Capillary Electrophoresis-Inductively Coupled Plasma Mass Spectrometry.

Capillary electrophoresis-inductively coupled plasma mass spectrometry (CE-ICP-MS) was used to determine the stability constants of the actinides Am(III), Th(IV), Np(V), and U(VI) at an ionic strength of I = 0.3 M. The obtained stability constants were extrapolated to zero ionic strength by means of the Davies equation. For both U(VI) and Am(III), three consecutive acetate complexes with log(ß10) = 3.01 ± 0.12, log(ß20) = 5.27 ± 0.07, log(ß30) = 6.82 ± 0.09, and log(ß10) = 3.70 ± 0.09, log(ß20) = 5.35 ± 0.08, log(ß30) = 6.45 ± 0.09, respectively, could be identified. For Np(V), there was just one acetate complex, with log(ß10) = 1.56 ± 0.03. In the case of Th(IV), five different complex species could be determined: log(ß10) = 4.73 ± 0.16, log(ß20) = 8.92 ± 0.09, log(ß30) = 12.16 ± 0.11, log(ß40) = 12.96 ± 0.87, and log(ß50) = 14.39 ± 0.16. The actinides were selected with regard to their most stable oxidation state in aqueous solution so that four different oxidation states from +III to +VI could be investigated. A great benefit of CE-ICP-MS is the opportunity to measure at significantly lower concentrations compared to the available literature, allowing the study of actinide complexation in environmentally relevant concentration ranges. Furthermore, it is possible to analyze all four actinides simultaneously in one and the same sample.

Determination of kinetic parameters of redox reactions using CE-ICP-MS: A case study for the reduction of Np(V) by hydroxylamine hydrochloride.

The rate constants k of the reduction of 5 × 10-5  M Np(V) to Np(IV) by hydroxylamine hydrochloride (HAHCl) in 1 M HCl have been determined by CE-ICP-MS in the temperature range of ϑ = 30-70°C and with varying concentrations of HAHCl from 1 to 7.2 M. The reaction was found to have (pseudo)first order kinetics with respect to HAHCl. The experimental results for k ranged from 0.0029(1) min-1 (ϑ = 40°C, c(HAHCl) = 3 M) to 0.039(7) min-1 (ϑ = 60°C, c(HAHCl) = 7.2 M). The activation energy of the reaction was determined as EA  = (72 ± 10) kJ/mol. These results and a comparison with literature data show that the coupling of CE to ICP-MS provides a powerful analytical tool for the investigation of the kinetic aspects of redox reactions of actinides at low concentrations. On the basis of this proof-of-principle study, the method presented here can be extended to the investigation of the kinetic parameters of other redox systems containing different actinides (or transition metals) and oxidants/reductants.

Geochemical Interactions of Plutonium with Opalinus Clay Studied by Spatially Resolved Synchrotron Radiation Techniques.

Plutonium plays an important role within nuclear waste materials because of its long half-life and high radiotoxicity. The aim of this study was to investigate with high spatial resolution the reactivity of the more oxidized forms of Pu(V,VI) within Opalinus Clay (OPA) rock, a heterogeneous, natural argillaceous rock considered as a potential repository host. A combination of synchrotron based X-ray microprobe and bulk techniques was used to study the spatial distribution and molecular speciation of Pu within OPA after diffusion and sorption processes. Microscopic chemical images revealed a pronounced impact of geochemical heterogeneities concerning the reactivity of the natural barrier material. Spatially resolved X-ray absorption spectroscopy documented a reduction of the highly soluble Pu(V,VI) to the less mobile Pu(IV) within the argillaceous rock material, while bulk investigations showed second-shell scattering contributions, indicating an inner-sphere sorption of Pu on OPA components. Microdiffraction imaging identified the clay mineral kaolinite to play a key role in the immobilization of the reduced Pu. The findings provide strong evidence that reduction and immobilization do not occur as linked processes on a single reactive phase but as decoupled, subsequent, and spatially separated reactions involving different phases of the OPA.

Sensitive redox speciation of neptunium by CE-ICP-MS.

Capillary electrophoresis (CE) was used to separate the neptunium oxidation states Np(IV) and Np(V), which are the only oxidation states of Np that are stable under environmental conditions. The CE setup was coupled to an inductively coupled plasma mass spectrometer (Agilent 7500ce) using a Mira Mist CE nebulizer and a Scott-type spray chamber. The combination of the separation capacity of CE with the detection sensitivity of inductively coupled plasma mass spectrometry (ICP-MS) allows identification and quantification of Np(IV) and Np(V) at the trace levels expected in the far field of a nuclear waste repository. Limits of detection of 1 × 10(-9) and 5 × 10(-10) mol L(-1) for Np(IV) and Np(V), respectively, were achieved, with a linear range from 10(-9) to 10(-6) mol L(-1). The method was applied to study the redox speciation of the Np remaining in solution after interaction of 5 × 10(-7) mol L(-1) Np(V) with Opalinus Clay. Under mildly oxidizing conditions, a Np sorption of 31% was found, with all the Np remaining in solution being Np(V). A second sorption experiment performed in the presence of Fe(2+) led to complete sorption of the Np onto the clay. After desorption with HClO(4), a mixture of Np(IV) and Np(V) was found in solution by CE-ICP-MS, indicating that some of the sorbed Np had been reduced to Np(IV) by Fe(2+).

Speciation of Np(V) uptake by Opalinus Clay using synchrotron microbeam techniques.

Synchrotron-based X-ray absorption spectroscopy has been used to determine the chemical speciation of Np sorbed on Opalinus Clay (OPA, Mont Terri, Switzerland), a natural argillaceous rock revealing a micro-scale heterogeneity. Different sorption and diffusion samples with Np(V) were prepared for spatially resolved molecular-level investigations. Thin sections of OPA contacted with Np(V) solution under aerobic and anaerobic conditions as well as a diffusion sample were analysed spatially resolved. Micro-X-ray fluorescence (µ-XRF) mapping has been used to determine the elemental distributions of Np, Fe and Ca. Regions of high Np concentration were subsequently investigated by micro-X-ray absorption fine structure spectroscopy to determine the oxidation state of Np. Further, micro-X-ray diffraction (µ-XRD) was employed to gain knowledge about reactive crystalline mineral phases in the vicinity of Np enrichments. One thin section was also analysed by electron microprobe to determine the elemental distributions of the lighter elements (especially Si and Al), which represent the main elements of OPA. The results show that in most samples, Np spots with considerable amounts of Np(IV) could be found even when the experiments were carried out in air. In some cases, almost pure Np(IV) L(III)-edge X-ray absorption near-edge structure spectra were recorded. In the case of the anaerobic sample, the µ-XRF mapping showed a clear correlation between Np and Fe, indicating that the reduction of Np(V) is caused by an iron(II)-containing mineral which could be identified by µ-XRD as pyrite. These spatially resolved investigations were complemented by extended X-ray absorption fine structure measurements of powder samples from batch experiments under aerobic and anaerobic conditions to determine the structural parameters of the near-neighbour environment of sorbed Np.

Neptuniumnn(V) sorption and diffusion in opalinus clay.

The sorption and diffusion behavior of 8 x 10(-6) M Np(V) in Opalinus Clay (OPA) with synthetic pore water (pH 7.6) as mobile phase was studied under ambient conditions by batch and diffusion experiments, respectively. The Kd value determined by batch experiments with OPA suspensions is equal to 0.025 +/- 0.005 m3/kg. The diffusion-accessible porosity epsilon of intact OPA as determined by through- and out-diffusion experiments with tritiated water (HTO) is equal to 0.15 +/- 0.01. The diffusion coefficient De and the rock capacity factor alpha of 22Na+ in OPA were measured by through-, out-, and in-diffusion experiments and asserted the reliability of these diffusion techniques. For the diffusion of Np(V) in synthetic pore water, the capillary method gave Dw = (6.0 +/- 1.0) x 10(-10) m/s. Due to the strong sorption of Np(V) on CPA, the diffusion of Np(V) was investigated bythe in-diffusiontechnique.The diffusion parameters for Np(V) in OPA are De = (6.9 +/- 1.1) x 10(-12) m2/s and alpha = 243 +/- 4. This corresponds to Kd = 0.10 +/- 0.01 m3/kg for the sorption of Np(V) in intact OPA.

Spectroscopic characterization of the uranium carbonate andersonite Na2Ca[UO2(CO3)3] x 6H2O.

The uranium carbonate andersonite Na2Ca[UO2(CO3)3] x 6H2O was synthesized and identified with classical analytical and spectroscopic methods. The classical methods applied were powder X-ray diffraction (XRD), nitric acid digestion, and scanning electron microcopy combined with energy-dispersive spectroscopy (SEM/EDS). To characterize andersonite spectroscopically, time-resolved laser-induced fluorescence spectroscopy (TRLFS), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FT-IR) were used. Natural and synthetic andersonite samples were characterized with the nondestructive TRLFS by six fluorescence emission bands at 470.6, 486.1, 505.4, 526.7, 549.6, and 573.9 nm. In addition, andersonite was characterized by FT-IR measurements by the appearance of the asymmetric stretching vibration of the uranyl cation [v3(UO2(2+))] at 902 cm(-1) with a shoulder at 913 cm(-1). XPS measurements verified the composition of the synthetic andersonite sample. The measured intensity ratios of the XPS lines agree with the stoichiometry of Na2Ca[UO2(CO3)3] x 6H2O. The XPS features of the inner valence molecular orbitals are characteristic of the [UO2(CO3)3]4- structural moiety. These spectroscopic methods can be used to identify in a fingerprinting procedure secondary U(VI) phases in mixtures with other phases or as thin coatings on mineral and rock surfaces.