The State of the Science and Technology in Deep Borehole Disposal of Nuclear Waste

Incorporating Transition Metal Ions into Uranium Oxide Hydrates: The Role of Zn(II) and the Effect of the Addition of Cs(I) Ions

The synthesis of two zinc-bearing uranium oxide hydrate (UOH) materials has been achieved, and their crystal structures, obtained via single-crystal X-ray diffraction using synchrotron radiation, and additional structural and spectroscopic properties are reported herein. Although both structures incorporate Zn2+ cations, the two differ significantly. The compound Zn2(OH)2(H2O)5[(UO2)10UO14(H2O)3] (UOHF-Zn), forming a framework-type structure in the P1̅ space group, was composed of β-U3O8 layers pillared by uranyl polyhedra, with the Zn2+ cations incorporated within the framework channels. In contrast, the compound Cs2Zn(H2O)4[(UO2)4O3(OH)4]2·3H2O (UOH-Zn) crystallized in the Cmc21 space group with a schoepite-like uranyl oxide hydroxide layered topology and both Zn2+ and Cs+ cations making up the interlayer species. The apparent driving force for the differences in the structures was the change from KOH to CsOH during synthesis, with the smaller K+ ions excluded in lieu of a higher proportion of Zn2+ (U/Zn ratio of 5.5:1) in UOHF-Zn, whereas in UOH-Zn, the larger Cs+ ions were preferentially incorporated at the expense of fewer Zn2+ cations (U/Cs/Zn ratio of 8:2:1). Highlighted in this work is the effect of the chemical species and, in particular, their ionic radius on UOH formation, further improving the understanding of UO2 alteration in the setting of deep geological repositories.

Uranium Oxide Hydrate Frameworks with Dy(III) or Lu(III) Ions: Insights Into the Framework Structures With Lanthanide Ions

Two uranium oxide hydrate frameworks (UOHFs) with either Dy3+ or Lu3+ ions, Dy1.36(H2O)6[(UO2)10UO13(OH)4] (UOHF-Dy) or Lu2(H2O)8[(UO2)10UO14(OH)3] (UOHF-Lu), were synthesized hydrothermally and characterized with a range of structural and spectroscopic techniques. Although SEM-EDS analysis confirmed the same atomic ratio of ~5.5 for U : Dy and U : Lu, they displayed different crystal morphologies, needles for UOHF-Dy in the orthorhombic C2221 space group and plates for UOHF-Lu in the triclinic P-1 space group. Both frameworks are composed of β-U3O8 type layers linked by pentagonal bipyramidal uranium polyhedra, with the Dy3+/Lu3+ ions inside the channels. However, the arrangements of Dy3+/Lu3+ ions are different, with disordered Dy3+ ions well aligned at the centers of the channels and single Lu3+ ions well-separated in a zigzag pattern in the channels. While the characteristic vibrational modes were revealed by Raman spectroscopy, the presence of a pentavalent uranium center in UOHF-Lu was confirmed with diffuse reflectance spectroscopy. The formation of two types of UOHFs with lanthanide ions, high or low symmetry, and the structure trend were discussed regards to synthesis conditions and lanthanide ionic radius. This work highlights the complex chemistry driving the formation of UOHFs with lanthanide ions and has implications to the spent nuclear fuel under geological disposal.

Filling the gaps of uranium oxide hydrates with magnesium(II) ions: unique layered structures and the role of additional sodium(I) ions

Alkaline earth metal ions play an important role in the formation of secondary uranium minerals due to their abundance in the Earth's crust. Although uranium oxide hydrate (UOH) minerals and synthetic phases with calcium, strontium and barium ions have been investigated, their counterparts with magnesium ions are much less studied. In this work, synthetic UOH materials with magnesium ions have been investigated with three new compounds being synthesised and characterised. Compound Mg2(H3O)2(H2O)6[(UO2)3O4(OH)]2 (U-Mg1 with a U : Mg ratio of 3 : 1) crystallises in the monoclinic P21/c space group having a layered crystal structure, constructed by β-U3O8 layers with 6-fold coordinated Mg2+ ions as interlayer cations. Compound Na2Mg(H2O)4[(UO2)3O3(OH)2]2 (U-Mg2p with U : Mg : Na ratios of 6 : 1 : 2) crystallises in the triclinic P1̄ space group having a layered structure, constructed by a unique type of uranium oxide hydroxide layer containing both α-U3O8 and β-U3O8 features, with alternating layers of 6-fold coordinated Mg2+ and 6-/8-fold coordinated Na+ interlayer cations. Compound Na2Mg(H2O)4[(UO2)4O3(OH)4]2 (U-Mg2n with U : Mg : Na ratios of 8 : 1 : 2) crystallises in the triclinic P1̄ space group having a corrugated layer structure, constructed by a unique type of uranium oxide hydroxide layer with mixed 6-fold coordinated Mg2+ and 7-fold coordinated Na+ interlayer cations. The structural diversity in the UOH-Mg system was achieved by adjusting the solution pH using NaOH, highlighting the importance of solution pH control and the additional Na+ ions in the formation of UOH phases. The extra structural flexibility offered by the Na+ ions emphasizes the opportunity for synthesising UOHs with dual-cations to further improve our understanding of the alteration products of spent nuclear fuel under geological disposal.

Analytical and Numerical Estimation of Fracture Initiation and Propagation Regions around Large-Diameter, Deep Boreholes for Disposal of Long-Lived Intermediate-Level Waste

The safety of high-level radioactive waste disposal has been studied across the world considering mined geologic repositories. Here, we introduce large-diameter, deep borehole disposal as one of the potential solutions for small volumes of long-lived intermediate-level waste (ILW). The short- and long-term stability of deep disposal boreholes is critical for environmental safety and public health. In this paper, we first use a recently revisited extensional strain criterion for fracture initiation and apply analytical solutions of a two-dimensional stress model to predict the fracturing region around a 2 km deep and 0.7 m diameter disposal borehole. Analytical solutions of fracture initiation are compared with results from the numerical simulator FRACOD, while the latter model also predicts dynamic effects such as fracture propagation. Both analytical and numerical methods predicted similar fracture initiation characteristics around the minor horizontal compressive stress springline, consistent with literature data. Numerical results showed deeper fracturing zones than those predicted by analytical solutions, mainly because the analytical predictions provide static snapshots under specific given conditions, while the numerical model calculates additional dynamic effects of fracture propagation. Including stress dynamics is shown to further weaken the rock around the borehole. At the bottom plane of the borehole, three-dimensional numerical simulations showed the development of fracturing zones around the major horizontal compressive stress springline. Borehole stability analyses are essential to plan the safe operation of drilling operations while also giving insights as to what borehole depths are more prone to fracturing and hence potentially less suitable as a waste disposal zone.

Adsorption of Strontium onto Synthetic Iron(III) Oxide up to High Ionic Strength Systems

In this work, the adsorption behavior of Sr onto a synthetic iron(III) oxide (hematite with traces of goethite) has been studied. This solid, which might be considered a representative of Fe3+ solid phases (iron corrosion products), was characterized by X-Ray Diffraction (XRD) and X-Ray Photoelectron Spectroscopy (XPS), and its specific surface area was determined. Both XRD and XPS data are consistent with a mixed solid containing more than 90% hematite and 10% goethite. The solid was further characterized by fast acid-base titrations at different NaCl concentrations (from 0.1 to 5 M). Subsequently, for each background NaCl concentration used for the acid-base titrations, Sr-uptake experiments were carried out involving two different levels of Sr concentration (1 × 10−5 and 5 × 10−5 M, respectively) at constant solid concentration (7.3 g/L) as a function of −log([H+]/M). A Surface Complexation Model (SCM) was fitted to the experimental data, following a coupled Pitzer/surface complexation approach. The Pitzer model was applied to aqueous species. A Basic Stern Model was used for interfacial electrostatics of the system, which includes ion-specific effects via ion-specific pair-formation constants, whereas the Pitzer-approach involves ion-interaction parameters that enter the model through activity coefficients for aqueous species. A simple 1-pK model was applied (generic surface species, denoted as >XOH−1/2). Parameter fitting was carried out using the general parameter estimation software UCODE, coupled to a modified version of FITEQL2. The combined approach describes the full set of data reasonably well and involves two Sr-surface complexes, one of them including chloride. Monodentate and bidentate models were tested and were found to perform equally well. The SCM is particularly able to account for the incomplete uptake of Sr at higher salt levels, supporting the idea that adsorption models conventionally used in salt concentrations below 1 M are applicable to high salt concentrations if the correct activity corrections for the aqueous species are applied. This generates a self-consistent model framework involving a practical approach for semi-mechanistic SCMs. The model framework of coupling conventional electrostatic double layer models for the surface with a Pitzer approach for the bulk solution earlier tested with strongly adsorbing solutes is here shown to be successful for more weakly adsorbing solutes.