Martina Klinkenberg

N2-BET specific surface area of bentonites

The specific surface areas (SSA(N2BET)) of 36 different bentonites had larger values for Ca(2+)/Mg(2+) bentonites than for Na(+) bentonites. This trend could not be explained by the different d(001) values nor by the different microstructures. The investigation of Cu-triene-exchanged smectites, which on drying at 105 degrees C still had a d(001) value accounting for approximately 13A, proved that the SSA(N2BET) of low-charged smectites increased more than that of high-charged smectites. This could be explained by: (i) more space between the permanent charge sites in the case of low-charged smectites and (ii) the fact that the layers of Cu-triene smectites do not collapse at 105 degrees C. In contrast the SSA(N2BET) of Ca(2+)-exchanged bentonites could not be related to the layer charge density (LCD) as in the case of the Cu-triene-exchanged bentonites which is probably due to the varying number of collapsed layers. In conclusion, the SSA(N2BET) of bentonites which is known to be largely variable is probably determined by microporosity resulting from the quasi-crystalline overlap region and accessible areas of the interlayer. The number of layers per stack and the microstructure are supposed to play a subordinate role. The larger SSA(N2BET) of Ca/Mg bentonites compared to Na bentonites probably can be explained by the larger space between the charges in the case of the presence of divalent cations.

Water-uptake capacity of bentonites.

The present study compares the water-vapor adsorption capacity of bentonites (natural cation population) with the Enslin-Neff method. Water-vapor adsorption at 50% r.h. (relative humidity) or 70% r.h. is known to depend heavily on the amount of permanent charge and on the type of exchangeable cation. At ~80% r.h. Na+- and Ca2+/Mg2+-dominated bentonites take up equal amounts of water. Comparing the water-uptake capacity at 80% r.h. with the cation exchange capacity (CEC) revealed a close correlation between these two variables. Appreciable scatter apparent from this plot, however, suggests that additional factors influence the water-uptake capacity.Water adsorption at external surfaces was considered to be one of these factors and was, in fact, implicated by N2-adsorption data. The ratio of external/internal water ranged from 0 to 1, which suggests that water-adsorption values cannot be applied in the calculation of the internal surface area without correction for external water.The Enslin-Neff water-uptake capacity, on the other hand, is unaffected by microstructural features (e.g. specific surface area and porosity). The amount of exchangeable Na+ is themost important factor. However, the relationship between the Na+ content and the Enslin value is not linear but may be explained by percolation theory.

Uptake of Ra during the recrystallization of barite: a microscopic and time of flight-secondary ion mass spectrometry study.

A combined macroscopic and microanalytical approach was applied on two distinct barite samples from Ra uptake batch experiments using time of flight-secondary ion mass spectrometry (ToF-SIMS) and detailed scanning electron microscopy (SEM) investigations. The experiments were set up at near to equilibrium conditions to distinguish between two possible scenarios for the uptake of Ra by already existent barite: (1) formation of a Ba1-xRaxSO4 solid solution surface layer on the barite or (2) a complete recrystallization, leading to homogeneous Ba1-xRaxSO4 crystals. It could be clearly shown that Ra uptake in all barite particles analyzed within this study is not limited to the surface but extends to the entire solid. For most grains a homogeneous distribution of Ra could be determined, indicating a complete recrystallization of barite into a Ba1-xRaxSO4 solid solution. The maxima of the Ra/Ba intensity ratio distribution histograms calculated from ToF-SIMS are identical with the expected Ra/Ba ratios calculated from mass balance assuming a complete recrystallization. In addition, the role of Ra during the recrystallization of barite was examined via detailed SEM investigations. Depending on the type of barite used, an additional coarsening effect or a strong formation of oriented aggregates was observed compared to blank samples without Ra. In conclusion, the addition of Ra to a barite at close to equilibrium conditions has a major impact on the system leading to a fast re-equilibration of the solid to a Ba1-xRaxSO4 solid solution and visible effects on the particle size distribution, even at room temperature.

Effects of Te(IV) Oxo-Anion Incorporation into Thorium Molybdates and Tungstates.

The exploration of phase formation in the Th-Mo/W-Te systems has resulted in four mixed oxo-anion compounds from high-temperature solid-state reactions: ThWTe2O9, Th(WO4)(TeO3), ThMoTe2O9, and Th2(MoO4)(TeO3)3. All four compounds contain edge-sharing thorium polyhedra linked by MoO4/WO6 and different tellurium oxo-groups to form three-dimensional frameworks. In ThWTe2O9, each helical Th based chain is connected by four tungstotellurite clusters resulting in a building fragment which has a cross-section of four-leafed clovers. The structure of Th(WO4)(TeO3) exhibits a multilayer-sandwich framework composed of thorium tellurite layers with tungsten chains in between. In the case of the molybdate family, ThMoTe2O9 and Th2(MoO4)(TeO3)3 are built from puckered Th-Te sheets which are further interconnected by MoO4 tetrahedral linkers. The DSC-TG technique was performed to gain insight into the thermal behavior of the synthesized compounds. Raman spectra of as-prepared phases were obtained and analyzed for signature peaks.

Novel Fundamental Building Blocks and Site Dependent Isomorphism in the First Actinide Borophosphates

Three novel uranyl borophosphates, Ag2(NH4)3[(UO2)2{B3O(PO4)4(PO4H)2}]H2O (AgNBPU-1), Ag(2-x)(NH4)3[(UO2)2{B2P5O(20-x)(OH)x}] (x = 1.26) (AgNBPU-2), and Ag(2-x)(NH4)3[(UO2)2{B2P(5-y)AsyO(20-x)(OH)x}] (x = 1.43, y = 2.24) (AgNBPU-3), have been prepared by the H3BO3-NH4H2PO4/NH4H2AsO4 flux method. The structure of AgNBPU-1 has an unprecedented fundamental building block (FBB), composed of three BO4 and six PO4 tetrahedra which can be written as 9□:[Φ] □<3□>□|□<3□>□|□<3□>□|. Two Ag atoms are linearly coordinated; the coordination of a third one is T-shaped. AgNBPU-2 and AgNBPU-3 are isostructural and possess a FBB of two BO4 and five TO4 (T = P, As) tetrahedra (7□:□<4□>□|□). AgNBPU-3 is a solid solution with some PO4 tetrahedra of the AgNBPU-2 end-member being substituted by AsO4. Only two out of the three independent P positions are partially occupied by As, resulting in site dependent isomorphism. The three compounds represent the first actinide borophosphates.

Comparison of the dry densities of highly compacted bentonites

Abstract Bentonites are in worldwide use as candidate materials for the encapsulation of highlevel radioactive waste (HLRW). To effectively seal waste canisters, bentonite is compacted to bentonite blocks which can be used to build a wall around the canisters. Compaction significantly improves the swelling pressure, which is currently considered as one of the most important parameters for assessing barrier performance. Most of the studies on compressibility of bentonites consider a few different materials only, which does not lead to a general understanding of bentonite performance. In order to identify the actual compressibility differences of different bentonites, a sizeable set of well characterized materials was investigated with respect to the dry densities after compaction. Different results were obtained for bentonites that had been dried and bentonites that were equilibrated at 70% r.h. (relative humidity) prior to compaction. The dry density of dried bentonites depends on total porosity and particle density. However, the dry density of microporous bentonites depends on the microporosity rather than total porosity because microporosity is not reduced upon compaction. On the other hand, for the samples previously equilibrated at 70% r.h., the water content is most important. However, the water content, i.e. the water uptake capacity at 70% r.h., in turn largely depends on the CEC but also on microporosity. Therefore, under a given load, the 36 bentonites studied showed a significant range of resulting dry densities, depending on water content, CEC and porosity. In conclusion, for a given bentonite, the dry density after compaction explains some geotechnical parameters such as the swelling pressure. However, for reasons explained in the present study, the dry density cannot be generally used to predict these parameters.

Spent UO2 TRISO coated particles – instant release fraction and microstructure evolution

Abstract The impact of burn-up on the instant release fraction (IRF) from spent fuel was studied using very high burn-up UO2 fuel (∼ 100 GWd/t) from a prototype high temperature reactor (HTR). TRISO (TRi-structural-ISO-tropic) particles from the spherical fuel elements contain UO2 fuel kernels (500 μm diameter) which are coated by three tight layers ensuring the encapsulation of fission products during reactor operation. After cracking of the tight coatings 85Kr and 14C as 14CO2 were detected in the gas fraction. Xe was not detected in the gas fraction, although ESEM (Environmental Scanning Electron Micoscope) investigations revealed an accumulation in the buffer. UO2 fuel kernels were exposed to synthetic groundwater under oxic and anoxic/reducing conditions. U concentration in the leachate was below the detection limit, indicating an extremely low matrix dissolution. Within the leach period of 276 d 90Sr and 134/137Cs fractions located at grain boundaries were released and contribution to IRF up to max. 0.2% respectively 8%. Depending on the environmental conditions, different release functions were observed. Second relevant release steps occurred in air after ∼ 120 d, indicating the formation of new accessible leaching sites. ESEM investigations were performed to study the impact of leaching on the microstructure. In oxic environment, numerous intragranular open pores acting as new accessible leaching sites were formed and white spherical spots containing Mo and Zr were identified. Under anoxic/reducing conditions numerous metallic precipitates (Mo, Tc and Ru) filling the intragranular pores and white spherical spots containing Mo and Zr, were detected. In conclusion, leaching in different geochemical environments influenced the speciation of radionuclides and in consequence the stability of neoformed phases, which has an impact on IRF.

Corrosion of non-irradiated UAlx-Al fuel in the presence of clay pore solution: a quantitative XRD secondary phase analysis applying the DDM method

Abstract Corrosion experiments with non-irradiated metallic UAlx–Al research reactor fuel elements were carried out in autoclaves to identify and quantify the corrosion products. Such compounds, considering the long-term safety assessment of final repositories, can interact with the released inventory and this constitutes a sink for radionuclide migration in formation waters. Therefore, the metallic fuel sample was subjected to clay pore solution to investigate its process of disintegration by analyzing the resulting products and the remnants, i.e. the secondary phases. Due to the fast corrosion rate a full sample disintegration was observed within the experimental period of 1 year at 90°C. The obtained solids were subdivided into different grain size fractions and prepared for analysis. The elemental analysis of the suspension showed that, uranium and aluminum are concentrated in the solids, whereas iron was mainly dissolved. Non-ambient X-ray diffraction (XRD) combined with the derivative difference minimization (DDM) method was applied for the qualitative and quantitative phase analysis (QPA) of the secondary phases. Gypsum and hemihydrate (bassanite), residues of non-corroded nuclear fuel, hematite, and goethite were identified. The quantitative phase analysis showed that goethite is the major crystalline phase. The amorphous content exceeded 80 wt% and hosted the uranium. All other compounds were present to a minor content. The obtained results by XRD were well supported by complementary scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analysis.

Research reactor fuel element corrosion under repository relevant conditions: separation, identification, and quantification of secondary alteration phases of UAlx-Al in MgCl2-rich brine

Abstract The corrosion of the UAlx-Al research reactor fuel type in synthetic MgCl2-rich brine (static batch-type experiments) was investigated with respect to the long-term safety of directly disposed research reactor fuel elements in salt formations. During corrosion, crystalline secondary phases were formed, which may serve as a barrier against radionuclide migration. For an optimized identification and quantification of the secondary phases using X-ray diffraction, a sample treatment to separate and enrich the secondary phases is necessary. A grain size fractionation was carried out in iso-propanol. A chemical composition and phase characterization of the secondary phases was accomplished. The results of the chemical investigations reveal that only traces of Al and U were dissolved. The separation and enrichment of secondary phases were carried out reproducible and successfully. Due to the phase characterization by scanning electron microscopy/energy dispersive X-ray spectroscopy and X-ray diffraction the following secondary phases were unambiguously identified: Mg-Al-Cl layered double hydroxide, lesukite, Fe layered double hydroxide (green rust), lawrencite, Fe (elemental), and traces of uncorroded fuel (UAl4). The quantitative analysis showed that LDH compounds and lesukite are the major crystalline phases. All other observed compounds were only present in trace amounts, i.e. constituting accessories. The Rietveld analysis also revealed the high content of amorphous phases of approximately 30%, which are expected to include the uranium as U(OH)4.

Towards a unifying mechanistic model for silicate glass corrosion

Borosilicate glasses are currently used for the immobilization of highly radioactive waste and are materials of choice for many biomedical and research industries. They are metastable materials that corrode in aqueous solutions, reflected by the formation of silica-rich surface alteration layers (SAL). Until now, there is no consensus in the scientific community about the reaction and transport mechanism(s) and the rate-limiting steps involved in the formation of SALs. Here we report the results of multi-isotope tracer (2H,18O,10B, 30Si, 44Ca) corrosion experiments that were performed with precorroded and pristine glass monoliths prepared from the six-component international simple glass and a quaternary aluminum borosilicate glass. Results of transmission electron microscopy and nanoscale analyses by secondary ion mass spectrometry reveal a nanometer-sharp interface between the SAL and the glass, where decoupling of isotope tracer occurs, while proton diffusion and ion exchange can be observed within the glass. We propose a unifying mechanistic model that accounts for all critical observations so far made on naturally and experimentally corroded glasses. It is based on an interface-coupled glass dissolution-silica precipitation reaction as the main SAL forming process. However, a diffusion-controlled ion exchange front may evolve in the glass ahead of the dissolution front if SAL formation at the reaction interface significantly slows down due to transport limitations.Alteration layer formation: model me thisA unifying mechanistic model has been developed for silicate glass corrosion that can explain all critical observations made to-date. Borosilicate glasses are often used in biomedical devices and to store and dispose of radioactive waste. They decay in aqueous solution via the generation of a porous ‘surface alteration layer’ (SAL), the structure of which is different to the bulk. How the SAL forms is still not clear, however, an international team lead by Thorsten Geislern at the University of Bonn, Germany, has now used multi-isotope tracer experiments, to provide detailed insight into the distinct chemical and transport steps occurring during SAL formation. Their results suggest that an ‘interface-coupled dissolution-precipitation’ reaction is the main mechanism at play during SAL formation, but that, if slowed by transport limitations, it may be replaced by an ‘interdiffusion’ process.