Sulfur incorporation in high level nuclear waste glass: a S K-edge XAFS investigation

We perform X-ray absorption fine structure (XAFS) spectroscopy measurements at the sulfur K-edge to elucidate the electronic and geometric bonding of sulfur atoms in borosilicate glass used for the vitrification of high level radioactive liquid waste. The sulfur is incorporated as sulfate, most probably as sodium sulfate, which can be deduced from the X-ray absorption near edge structure (XANES) by fingerprint comparison with reference compounds. This finding is backed up by Raman spectroscopy investigation. In the extended XAFS data, no second shell beyond the first oxygen layer is visible. We argue that this is due to the sulfate being present as small clusters located into voids of the borosilicate network. Hence, destructive interference of the variable surrounding prohibits the presence of higher shell signals. The knowledge of the sulfur bonding characteristics is essential for further optimization of the glass composition and to balance the requirements of the process and glass quality parameters, viscosity and electrical resistivity on one side, waste loading and sulfur uptake on the other side

Roentgenabsorptionsspektroskopie an Phosphorsalzperlen Bestimmung der geometrischen und elektronischen Struktur von metalloxid-dotierten Natriumphosphatglaesern

Sodium metaphosphate glasses doped with transition metal oxides show characteristic colors. X-ray absorption spectroscopy (XAS) investigations provide information whether the coloration stems from different electronic transitions or changes in the geometrical structure of the glasses. Even though the violet color of MnO_y-doped glasses is considered as an evidence for Mn"3"+-ions, Mn K-XAS reveals that the majority of the manganese ions are in the oxidation state +II and have a mixed coordination of four and six oxygen atoms, respectively. The oxygen coordination around the nickel ions in NiO-doped glasses with different metall oxide concentrations is always six. The change of color from citreous to auburn with increasing nickel oxide content is attributed to a systematic change in the bonding characteristic from mainly ionic-like to a small but significant contribution of covalent-like bonding. Analysis of higher coordination shells provides no indication of the formation of metal oxide clusters. (orig.)SIGLEAvailable from TIB Hannover: RN 4852(2004-03) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

High level nuclear waste glass corrosion in synthetic clay pore solution and retention of actinides in secondary phases

The corrosion of the simulated high level waste glass GP WAK1 in synthetic clay pore solution was studied in batch-type experiments at 323 and 363 K with special focus on the effect of high carbonate concentration in solution. The corrosion rate after 130 days was < 10(-4) g m(-2) d(-1) - no significant effect of the carbonate was identified. During glass corrosion, crystalline secondary phases (powellite, barite, calcite, anhydrite and clay-like Mg(Ca,Fe)-silicates) were formed. To obtain a molecular level picture of radionuclide speciation within the alteration layer, spectroscopic methods have been applied including grazing incidence X-ray absorption spectroscopy (XAS) to study the structural changes in the coordination of uranyl upon alteration layer formation. The number of equatorial oxygen atoms increases from 4 in the bulk glass to 5 in the alteration layer. Furthermore, reduced coordination symmetry was found. Hectorite, a frequently observed secondary clay mineral within the glass alteration layer, was synthesized in the presence of trivalent f-elements (e.g. Eu) and structurally characterized using time-resolved laser fluorescence spectroscopy. Structural incorporation into the octahedral layer is indicated. (c) 2008 Elsevier B.V. All rights reserved

Actinide Incorporation in a Zirconia Based Pyrochlore(Nd1.8An0.2)Zr2O7+x (An ¼ Th, U,Np,Pu,Am)

Actinides (thorium,uranium,neptunium,plutonium,andamericium)wereinfiltratedintoaporous Nd1.8Zr2O6.7 matrix, preparedbygel-supportedprecipitation.(Nd1.8An0.2)Zr2O7+x pyrochloreswere formedaftersinteringinAr/H2 and thepyrochlorestructureremainsduringoxidationat800 1C inair. X-raydiffractionrevealsalinearrelationshipbetweenthepyrochlorelatticeparameterandtheionic radii oftheactinides.EXAFSmeasurementsonactinide L3-edge showasplitshellofnearestneighbour oxygenatomssimilartothatsurroundingofNd.Theactinide¿oxygenbonddistancesdecreasewiththe actinideionicradii,whichverifiesthattheseactinidesadopttheNdsiteinthe(Nd1.8An0.2)Zr2O7+x pyrochlore.TheoxidationsusceptibilityofNpisrelatedtotheavailabilityofoxygenvacanciesandin contrasttostabilisedzirconiaNp(V)canbeobtainedinzirconiabasedpyrochlore.JRC.E.4-Nuclear fuel

Actinide incorporation in a zirconia based pyrochlore (Nd(1.8)An(0.2))Zr2O7+x (An = Th, U, Np, Pu, Am)

Actinides (thorium, uranium, neptunium, plutonium, and americium) were infiltrated into a porous Nd1.8Zr2O6.7 matrix, prepared by gel-supported precipitation. (Nd(1.8)An(0.2))Zr2O7+x pyrochlores were formed after sintering in Ar/H-2 and the pyrochlore structure remains during oxidation at 800 degrees C in air. X-ray diffraction reveals a linear relationship between the pyrochlore lattice parameter and the ionic radii of the actinides. EXAFS measurements on actinide L-3-edge show a split shell of nearest neighbour oxygen atoms similar to that Surrounding of Nd. The actinide-oxygen bond distances decrease with the actinide ionic radii, which verifies that these actinides adopt the Nd site in the (Nd(1.8)An(0.2))Zr2O7+x pyrochlore. The oxidation susceptibility of Np is related to the availability of oxygen vacancies and in contrast to stabilised zirconia Np(V) can be obtained in zirconia based pyrochlore. (C) 2008 Elsevier Inc. All rights reserved