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Accurate modeling and simulation of radionuclide migration in clay rocks such as the Opalinus Clay (OPA) play a key role in the safety assessment of deep geological repositories for nuclear wastes. At the continuum scale, the representative elementary volume (REV) is a fundamental constraint to quantify the effective diffusivity, which is a key parameter in reactive transport (RT) models. Therefore, an accurate estimation of the REV is essential for a meaningful continuum‐scale RT simulation in heterogeneous clay rocks. This study presents a comprehensive analysis of the heterogeneities of porosity and effective diffusivity in clay rocks by using the classical sampling theory and pore‐scale simulations. First, in this study, the two‐dimensional representative elementary area is correlated with the REV for porosity via a characteristic length. Next, it is shown that the REV for diffusivity is larger than the REV for porosity. Moreover, these two REVs can be correlated using Archie's law. In such a way, the REV for diffusivity can be determined by the developed correlations through analyzing two‐dimensional microstructures, thus significantly reducing the computational cost. Finally, the applicability of our approach for clay rocks is validated by experimental data on the diffusion of tritiated water in the heterogeneous sandy facies of OPA. From both the experimental data and the modeling prediction, the REV for diffusivity in the sandy facies of OPA is in the order of cubic centimeters. This study provides critical insights into the diffusion in heterogeneous clay rocks toward an enhanced predictability of radionuclide migration. Plain Language Summary: Contaminant migration in clay rocks is dominated by molecular diffusion due to their low permeability. To accurately simulate this process at the centimeter scale and above, the rocks can be assumed as homogeneous media only if their volume is larger than a critical volume, also known as the representative elementary volume (REV). Therefore, an accurate estimation of this critical volume is necessary for numerical modeling of contaminant migration in clay rocks. Calculating the REV in clay rocks is a major challenge. Therefore, in this study, the REV in clay rocks was systematically investigated, and easy‐to‐use correlations were established for rapid estimation of REV using two‐dimensional micrographs of the rocks. These critical findings contribute to a better understanding of rock microstructures and to improved calculations of contaminant migration in clay rocks. Key Points: A comprehensive study on the representative elementary volume (REV) for diffusive radionuclide transport in heterogeneous clay rocksDevelopment of a method to correlate the 3D REV for diffusivity to the 2D representative elementary area for porosity via Archie's law and a characteristic lengthValidation of the predicted REV for diffusivity using data from through‐diffusion experiments that confirm the general applicability of the proposed method [ABSTRACT FROM AUTHOR]

Single-phase monazite-type ceramics are considered as potential host matrices for the conditioning of separated plutonium and minor actinides. Sm-orthophosphates were synthesised and their behaviour under irradiation was investigated with respect to their long-term performance in the repository environment. Sintered SmPO4 pellets and thin lamellae were irradiated with 1, 3.5, and 7 MeV Au ions, up to fluences of 5.1 × 1014 ions cm−2 to simulate ballistic effects of recoiling nuclei resulting from α-decay of incorporated actinides. Threshold displacement energies for monazite-type SmPO4 subsequently used in SRIM/TRIM simulations were derived from atomistic simulations. Raman spectra obtained from irradiated lamellae revealed vast amorphisation at the highest fluence used, although local annealing effects were observed. The broadened, but still discernible, band of the symmetrical stretching vibration in SmPO4 and the negligible increase in P–O bond lengths suggest that amorphisation of monazite is mainly due to a breaking of Ln–O bonds. PO4 groups show structural disorder in the local environment but seem to behave as tight units. Annealing effects observed during the irradiation experiment and the distinctively lower dose rates incurred in actinide bearing waste forms and potential α-radiation-induced annealing effects indicate that SmPO4-based waste forms have a high potential for withstanding amorphisation. [ABSTRACT FROM AUTHOR]

The safe management of high‐level nuclear wastes, including their final disposal in a deep geological repository, requires a sound scientific understanding of the processes affecting the various materials present in the multibarrier system of the disposal facility, including the radioactive waste forms. Thus materials science aspects play an important role in the multidisciplinary and complex field of long‐term safety assessments. As many of these issues are related to mixed solid compounds, the aspect of structural radionuclide uptake by a host structure and subsequent solid solution formation is one of the key topics of the research related to nuclear waste management at Forschungszentrum Jülich. The adopted practice for deriving an in‐depth understanding of materials behavior by combining state‐of‐the‐art experimental and computational approaches is presented in the context of three examples: 1) corrosion of spent nuclear fuels, 2) radionuclide retention by secondary phases, and 3) innovative ceramic waste forms. New nano‐ and microanalytical tools, as well as advanced spectroscopic techniques and computational methods, further developed and tailored at Forschungszentrum Jülich, allowed for refined views on these materials. [ABSTRACT FROM AUTHOR]

This paper focuses on major phosphate-based ceramic materials relevant for the immobilisation of Pu, minor actinides, fission and activation products. Key points addressed include the recent progress regarding synthesis methods, the formation of solid solutions by structural incorporation of actinides or their non-radioactive surrogates and waste form fabrication by advanced sintering techniques. Particular attention is paid to the properties that govern the long-term stability of the waste forms under conditions relevant to geological disposal. The paper highlights the benefits gained from synergies of state-of-the-art experimental approaches and advanced atomistic modeling tools for addressing properties and stability of f-element-bearing phosphate materials. In conclusion, this article provides a perspective on the recent advancements in the understanding of phosphate based ceramics and their properties with respect to their application as nuclear waste forms. [ABSTRACT FROM AUTHOR]

Metastudtite, (UO2)O2(H2O)2, is one of two known natural peroxide minerals, but little is established about its thermodynamic stability. In this work, its standard enthalpy of formation, -1,779.6 ± 1.9 kJ/mol, was obtained by high temperature oxide melt drop solution calorimetry. Decomposition of synthetic metastudtite was characterized by thermogravimetry and differential scanning calorimetry (DSC) with ex situ X-ray diffraction analysis. Four decomposition steps were observed in oxygen atmosphere: water loss around 220 °C associated with an endothermic heat effect accompanied by amorphization; another water loss from 400 °C to 530 °C; oxygen loss from amorphous UO3 to crystallize orthorhombic α-UO2.9; and reduction to crystalline U3O8. This detailed characterization allowed calculation of formation enthalpy from heat effects on decomposition measured by DSC and by transposed temperature drop calorimetry, and both these values agree with that from drop solution calorimetry. The data explain the irreversible transformation from studtite to metastudtite, the conditions under which metastudtite may form, and its significant role in the oxidation, corrosion, and dissolution of nuclear fuel in contact with water. [ABSTRACT FROM AUTHOR]

Copyright of Safety of Nuclear Waste Disposal is the property of Copernicus Gesellschaft mbH and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Safety of Nuclear Waste Disposal is the property of Copernicus Gesellschaft mbH and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Safety of Nuclear Waste Disposal is the property of Copernicus Gesellschaft mbH and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Safety of Nuclear Waste Disposal is the property of Copernicus Gesellschaft mbH and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Safety of Nuclear Waste Disposal is the property of Copernicus Gesellschaft mbH and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Recrystallization and solid-solution formation with barite is considered as relevant retention mechanism for 226Ra in long-term scenarios of nuclear waste management. Here, we studied the effect of ionic strength and the presence of Sr in solution upon the Ra-uptake kinetics and final Ra concentrations in solution by recrystallizing barite in solution with varying Sr and NaCl concentration and temperature for up to 1000 days. Final Ra-concentrations were interpreted based on thermodynamic modelling. Our results indicate a slight decrease of the retention potential of barite for Ra but little effect on the uptake kinetics due to the increase of ionic strength from. 0.1 mol/kg to 1.0 mol/kg of NaCl. The final concentrations at solid/liquid ratio of 0.5 g/kg are well described based on available thermodynamic models whereas at 5 g/kg additional Ra uptake probably due to kinetic effects was observed. On the contrary, the presence of Sr in solution can have a significant inhibiting kinetic effect on the uptake kinetics and lower the final Ra-uptake. In some cases, with low solid/liquid ratio or at ambient conditions, Sr completely inhibits barite recrystallization. In all other cases, Ra, Ba and Sr were taken up as thermodynamically predicted at the end of the experiments. [ABSTRACT FROM AUTHOR]

Because of ability to incorporate actinides into their structure, the lanthanide phosphate ceramics ( L n PO 4 ) are considered as potential matrices for the disposal of nuclear waste. Here we present highly reliable ab initio prediction of the variation of heat capacities and the standard entropies of these compounds in zircon structure along lanthanide series ( Ln = Dy, …,Lu) and validate them against the existing experimental data. These data are helpful for assessment of thermodynamic parameters of these materials in the context of using them as matrices for immobilization of radionuclides for the purpose of nuclear waste management. [ABSTRACT FROM AUTHOR]

The safety assessment of deep geological disposal of nuclear waste requires a sound understanding of radionuclide retention in the repository host rocks. Here, we investigate the retention behaviour of the safety relevant radionuclide 226Ra in the heterogeneous sandy facies of the Opalinus Clay (OPA-SF) at the Mont Terri underground rock laboratory. Various rock samples were selected from a drill core (BAD-1) located in the lower sandy facies, which were representative for its textural and mineralogical heterogeneity. The samples were carefully characterized with respect to mineralogy and chemistry using X-Ray diffraction and electron microscopy. The amounts of quartz, carbonates and clay minerals in the samples ranged between 58 wt.% – 65 wt.%, 13 wt.% – 35 wt.% and 5 wt.% – 25 wt.%, respectively, underpinning the mineralogical heterogeneity. Rare (Ba,Sr)-sulphate precipitates were identified in the clay matrix by electron microscopy. The uptake behaviour of Ra was evaluated using batch sorption experiments. Corresponding to the mineralogical variability of the rock samples, the measured distribution ratios R d for the 226Ra retention varied between 20 L kg−1 and 260 L kg−1 after 120 days. The experimental results and thermodynamic modelling approaches suggest that various processes contribute to the uptake of 226Ra on different time scales: fast cation exchange and surface complexation reactions by clay minerals and the (slower) formation of solid solutions during recrystallisation of (Ba,Sr)-sulphates as well as retention by carbonate minerals; the latter mechanism still requiring further investigation. [ABSTRACT FROM AUTHOR]

The widely favored option for the safe storage of high-level radioactive waste is a multi-barrier disposal system with clay-based geotechnical barriers e.g. bentonite. Montmorillonite as a major constituent of bentonite (up to 90 wt.-%) is an important sink for potentially released radionuclides. The adsorption of 226Ra and Ba on Na-montmorillonite (SWy-2) was investigated as function of pH (edges) and of Ba concentration (isotherms) at different ionic strengths. Ba is often used as a chemical analogue for 226Ra due to their closely related physical and chemical properties e.g. the formation of complete Ba/Ra solid solutions. The experimental Ba/Ra adsorption data could be quantitatively described by considering cation exchange reactions on the planar sites of montmorillonite. To describe the pH dependent adsorption of Ba and Ra at high ionic strength and high pH, an additional reaction involving surface complexation was necessary. The fitted selectivity coefficients (K c) for the cation exchange of Ba and Ra with respect to Na are very similar at low ionic strength (0.02 M), whereas at higher ionic strength (0.3 M), Ra appears to be more selective. • Adsorption on montmorillonite is similar but not always identical for 226Ra and Ba. • A combined cation exchange and surface complexation model describes all data. • Surface complexation is only relevant for high pH. • Higher selectivity of 226Ra is observed at higher ionic strength. • For ionic strength <0.1 M NaCl and pH < 8, Ba is a good analogue for 226Ra. [ABSTRACT FROM AUTHOR]

The uptake of molybdenum, present in aqueous cementitious environments in the form of the molybdate ion (MoO 4 2-), by various cement hydration phases such as calcium-silicate-hydrates (C–S–H), monosulphate (AFm), ettringite (AFt), as well as hardened cement paste made from Ordinary Portland cement (OPC), was studied in batch-type sorption experiments under anoxic conditions. Uptake kinetics were generally fast, leading to sorption equilibrium in less than 30 days. In particular, a strong uptake of molybdate by AFm phases was observed as well as a distinct contribution of C–S–H phases to the molybdate retention in cementitious systems, the latter depending on the Ca/Si-ratio of the C–S–H and the alkali content in solution. In systems containing hydrogarnet, the neo-formation of a molybdate-bearing AFm phase was identified as an additional process contributing to molybdate retention. The findings have implications for selecting grouts for the immobilisation of radioactive waste streams containing Mo-93 and also enhance the data available on molybdate sorption and retention in cementitious systems. • Uptake of molybdenum by various cement hydration phases investigated. • Formation of molybdate-bearing AFm in systems with hydrogarnet. • High content of AFm and C–S–H with high Ca/Si ratios beneficial for solidification of Mo-93-bearing waste streams. • Data provided for component additive approach models and for evaluation of concrete clearance and disposal options. [ABSTRACT FROM AUTHOR]

Technetium-99 (99Tc) is an important radionuclide when considering the disposal of nuclear wastes owing to its long half-life and environmental mobility in the pertechnetate (Tc(VII)) redox state. Its behaviour in a range of potential cement encapsulants and backfill materials has been studied by analysing uptake onto pure cement phases and hardened cement pastes. Preferential, but limited, uptake of pertechnetate was observed on iron-free, calcium silicate hydrates (C–S–H) and aluminate ferrite monosulphate (AFm) phases with no significant adsorption onto ettringite or calcium aluminates. Diffusion of 99Tc through cured monolithic samples, representative of cements being considered for use in geological disposal facilities across Europe, revealed markedly diverse migration behaviour, primarily due to chemical interactions with the cement matrix rather than differential permeability or other physical factors. A backfill cement, developed specifically for the purpose of radionuclide retention, gave the poorest performance of all formulations studied in terms of both transport rates and overall technetium retention. Two of the matrices, pulverised fuel ash: ordinary Portland cement (PFA:OPC) and a low-pH blend incorporating fly ash, effectively retarded 99Tc migration via precipitation in narrow, reactive zones. These findings have important implications when choosing cementitious grouts and/or backfill for Tc-containing radioactive wastes. Image 1 • Limited uptake of 99Tc on C–S–H and AFm phases with no significant adsorption onto ettringite or calcium aluminates. • Marked variation in diffusion of 99Tc through cured cement monoliths due to chemical interactions with the cement matrix. • Blends containing fly ash effectively retard 99Tc migration via precipitation in narrow, reactive zones. [ABSTRACT FROM AUTHOR]