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Remarkably, doping isovalent $d^{10}$ and $d^0$ cations onto the $B$'' site in $A_2B$'$B$''O$_6$ double perovskites has the power to direct the magnetic interactions between magnetic $B$' cations. This is due to changes in orbital hybridization, which favors different superexchange pathways, and leads to the formation of alternative magnetic structures depending on whether $B$'' is $d^{10}$ or $d^0$. Furthermore, the competition generated by introducing mixtures of $d^{10}$ and $d^0$ cations can drive the material into the realms of exotic quantum magnetism. Here, a W$^{6+}$ $d^0$ dopant was introduced to a $d^{10}$ hexagonal perovskite Ba$_2$CuTeO$_6$, which possesses a spin ladder geometry of Cu$^{2+}$ cations, creating a Ba$_2$CuTe$_{1-x}$W$_x$O$_6$ solid solution ($x$ = 0 - 0.3). Neutron and synchrotron X-ray diffraction show that W$^{6+}$ is almost exclusively substituted for Te$^{6+}$ on the corner-sharing site within the spin ladder, in preference to the face-sharing site between ladders. This means the intra-ladder interactions are selectively tuned by the $d^0$ cations. Bulk magnetic measurements suggest this suppresses magnetic ordering in a similar manner to that observed for the spin-liquid like material Sr$_2$CuTe$_{1-x}$W$_x$O$_6$. This further demonstrates the utility of $d^{10}$ and $d^0$ dopants as a tool for tuning magnetic ground states in a wide range of perovskites and perovskite-derived structures., Comment: 26+24 pages, 7+20 figures

The United Kingdom holds the largest stockpile of separated civil plutonium in the world, projected to reach 140t, at the end of this decade, when reprocessing operations are complete. UK Government policy is that this material should be reused as MOX fuel in Light Water Reactors. This policy is re-examined in the light of recent experience of the US plutonium disposition programme, in which the MOX Fuel Fabrication Facility is now considered to be potentially unaffordable. Problematic aspects of US programme, relevant to the UK scenario, are reviewed, to understand the possible impact on UK policy. Based on the US experience and inherent uncertainty regarding the capital and operational costs of MOX fuel fabrication and plutonium immobilisation facilities, and the associated technical risks, it is concluded that the UK policy should explicitly adopt a dual track strategy to plutonium management, with commitment that: any remaining plutonium which is not converted into MOX fuel, or otherwise reused, will be immobilised and treated as waste for disposal. This will also ensure that the UK is positioned and prepared to take forward an immobilisation and disposal programme for the plutonium stockpile, should reuse as MOX fuel not prove an economic or viable option.

X-ray absorption fine structure (XAFS) and x-ray emission spectroscopy (XES) are advanced x-ray spectroscopies that impact a wide range of disciplines. However, unlike the majority of other spectroscopic methods, XAFS and XES are accompanied by an unusual access model, wherein; the dominant use of the technique is for premier research studies at world-class facilities, i.e., synchrotron x-ray light sources. In this paper we report the design and performance of an improved spectrometer XAFS and XES based on the general conceptual design of Seidler, et al., Rev. Sci. Instrum. 2014. New developments include reduced mechanical degrees of freedom, much-increased flux, and a wider Bragg angle range to enable extended x-ray absorption fine structure (EXAFS) for the first time with this type of modern laboratory XAFS configuration. This instrument enables a new class of routine applications that are incompatible with the mission and access model of the synchrotron light sources. To illustrate this, we provide numerous examples of x-ray absorption near edge structure (XANES), EXAFS, and XES results for a variety of problems and energy ranges. Highlights include XAFS and XES measurements of battery electrode materials, EXAFS of Ni and V with full modeling of results to validate monochromator performance, valence-to-core XES for 3d transition metal compounds, and uranium XANES and XES for different oxidation states. Taken en masse, these results further support the growing perspective that modern laboratory-based XAFS and XES have the potential to develop a new branch of analytical chemistry.

In a conceptual UK geological disposal facility for nuclear waste within a high-strength, crystalline geology, a cement-based backfill material, known as Nirex Reference Vault Backfill (NRVB), will be used to provide a chemical barrier to radionuclide release. The NRVB is required to have specific properties to fulfil the operational requirements of the geological disposal facility (GDF); these are dependent on the chemical and physical properties of the cement constituent materials and also on the water content. With the passage of time, the raw materials eventually used to synthesise the backfill may not be the same as those used to formulate it. As such, there is a requirement to understand how NRVB performance may be affected by a change in raw material supply. In this paper, we present a review of the current knowledge of NRVB and results from a detailed characterisation of this material, comparing the differences in performance of the final product when different raw materials are used. Results showed that minor differences in the particle size, surface area and chemical composition of the raw material had an effect on the workability, compressive strength, the rate of hydration and the porosity, which may influence some of the design functions of NRVB. This study outlines the requirement to fully characterise cement backfill raw materials prior to use in a geological disposal facility and supports ongoing assessment of long-term post-closure safety.

A simple method to directly convert Cs-exchanged IONSIV® IE-911 into a ceramic wasteform by hot isostatic pressing (1100 °C/190 MPa/2 hr) is presented. Two major Cs-containing phases, Cs2TiNb6O18 and Cs2ZrSi6O15, and a series of mixed oxides form. The microstructure and phase assemblage of the samples as a function of Cs content were examined using XRD, XRF, SEM and TEM/EDX. The chemical aqueous durability of the materials was investigated using the MCC-1 and PCT-B standard test methods. For HIPed Cs-IONSIV® samples, the MCC-1 normalised release rates of Cs were

Cs2TiNb6O18 is a potential ceramic wasteform for the long-term immobilization\ud of radioactive cesium. Cs2TiNb6O18 was synthesized using the aqueous precursor\ud method and a solid-state reaction, and its crystal structure was determined from the\ud Rietveld refinement of synchrotron X-ray and neutron powder diffraction data. The\ud structure is a pyrochlore analogue, space group P3̅m1 with Cs in 9-fold coordination.\ud The calculated bond valence sum from analysis of neutron diffraction data of 0.84 and\ud high coordination number suggest that Cs has a strong bonding environment. The\ud chemical aqueous durability was investigated using the MCC-1 and PCT-B standard test\ud methods. The measured Cs leach rates of 3.8 × 10−3 and 2.1 × 10−3 g m−2 day−1 obtained\ud via the MCC-1 and PCT-B methods, respectively, demonstrate good promise of a safe long-term immobilization material\ud comparable to, if not better than, hollanditethe material in the multiphase titanate ceramics (Synroc) targeted for cesium\ud sequestration.

A surrogate approach of ion beam irradiation is employed to simulate alpha decay of actinides in iron\ud phosphate nuclear waste glasses. Bismuth and helium ions of different energies have been selected for\ud simulating glass matrix modification owing to radiolysis and ballistic damage due to recoil atoms.\ud Structural modification and change in coordination number of network former were probed by employing\ud Reflectance Fourier-Transform Infrared (FT-IR), and Raman spectroscopies as a consequence of ion\ud irradiation. Depolymerisation is observed in glass sample irradiated at intermediate energy of 2 MeV.\ud Helium blisters of micron size are seen in glass sample irradiated at low helium ion energy of 30 keV.

In the safety case for the geological disposal of nuclear waste, the release of radioactivity from the repository is controlled by the dissolution of the spent fuel in groundwater. There remain several uncertainties associated with understanding spent fuel dissolution, including the contribution of energetically reactive surface sites to the dissolution rate. In this study, we investigate how surface features influence the dissolution rate of synthesised CeO2 and ThO2, spent nuclear fuel analogues which approximate as closely as possible the mineral structure characteristics of fuel-grade UO2 but are not sensitive to changes in oxidation state of the cation. The morphology of grain boundaries (natural features) and surface facets (specimen preparation-induced features) were investigated during dissolution. The effects of surface polishing on dissolution rate was also investigated. We show that preferential dissolution occurs at grain boundaries, resulting in grain boundary decohesion and enhanced dissolution rates. A strong crystallographic control was exerted, with high misorientation angle grain boundaries retreating more rapidly than those with low misorientation angles, which may be due to the accommodation of defects in the grain boundary structure. The data from these simplified analogue systems support the hypothesis that grain boundaries play a role in the so-called "instant release fraction" of spent fuel, and should be carefully considered, in conjunction with other chemical effects, in safety performance assessements for the geological disposal of spent fuel. Surface facets formed during the sample annealing process also exhibited a strong crystallographic control and were found to dissolve rapidly on initial contact with dissolution medium. Defects and strain induced during sample polishing caused an overestimation of the dissolution rate, by up to 3 orders of magnitude.

Contamination of soils with depleted uranium (DU) from munitions firing occurs in conflict zones and at test firing sites. This study reports the development of a chemical extraction methodology for remediation of soils contaminated with particulate DU. Uranium phases in soils from two sites at a UK firing range, MOD Eskmeals, were characterised by electron microscopy and sequential extraction. Uranium rich particles with characteristic spherical morphologies were observed in soils, consistent with other instances of DU munitions contamination. Batch extraction efficiencies for aqueous ammonium bicarbonate (42–50% total DU extracted), citric acid (30–42% total DU) and sulphuric acid (13–19% total DU) were evaluated. Characterisation of residues from bicarbonate-treated soils by synchrotron microfocus X-ray diffraction and X-ray absorption spectroscopy revealed partially leached U(IV)-oxide particles and some secondary uranyl-carbonate phases. Based on these data, a multi-stage extraction scheme was developed utilising leaching in ammonium bicarbonate followed by citric acid to dissolve secondary carbonate species. Site specific U extraction was improved to 68–87% total U by the application of this methodology, potentially providing a route to efficient DU decontamination using low cost, environmentally compatible reagents. ISSN:0304-3894 ISSN:1873-3336

Contamination of soils with depleted uranium (DU) from munitions firing occurs in conflict zones and at test firing sites. This study reports the development of a chemical extraction methodology for remediation of soils contaminated with particulate DU. Uranium phases in soils from two sites at a UK firing range, MOD Eskmeals, were characterised by electron microscopy and sequential extraction. Uranium rich particles with characteristic spherical morphologies were observed in soils, consistent with other instances of DU munitions contamination. Batch extraction efficiencies for aqueous ammonium bicarbonate (42–50% total DU extracted), citric acid (30–42% total DU) and sulphuric acid (13–19% total DU) were evaluated. Characterisation of residues from bicarbonate-treated soils by synchrotron microfocus X-ray diffraction and X-ray absorption spectroscopy revealed partially leached U(IV)-oxide particles and some secondary uranyl-carbonate phases. Based on these data, a multi-stage extraction scheme was developed utilising leaching in ammonium bicarbonate followed by citric acid to dissolve secondary carbonate species. Site specific U extraction was improved to 68–87% total U by the application of this methodology, potentially providing a route to efficient DU decontamination using low cost, environmentally compatible reagents., Journal of Hazardous Materials, 263 (Part 2), ISSN:0304-3894, ISSN:1873-3336

Inorganic chemistry 61(9), 4033 - 4045 (2022). doi:10.1021/acs.inorgchem.1c03655, Isovalent nonmagnetic d$^{10}$ and d$^0$ B″ cations have proven to be a powerful tool for tuning the magnetic interactions between magnetic B′ cations in A$_2$B′B″O$_6$ double perovskites. Tuning is facilitated by the changes in orbital hybridization that favor different superexchange pathways. This can produce alternative magnetic structures when B″ is d$^{10}$ or d$^0$. Furthermore, the competition generated by introducing mixtures of d$^{10}$ and d$^0$ cations can drive the material into the realms of exotic quantum magnetism. Here, Te$^{6+}$ d$^{10}$ was substituted by Wu$^{6+}$ d$^0$ in the hexagonal perovskite Ba$_2$CuTeO$_6$, which possesses a spin ladder geometry of Cu$^{2+}$ cations, creating a Ba$_2$CuTe$_{1–x}$W$_x$O$_6$ solid solution (x = 0–0.3). We find W$^{6+}$ is almost exclusively substituted for Te$^{6+}$ on the corner-sharing site within the spin ladder, in preference to the face-sharing site between ladders. The site-selective doping directly tunes the intraladder, J$_{rung}$ and J$_{leg}$, interactions. Modeling the magnetic susceptibility data shows the d$^0$ orbitals modify the relative intraladder interaction strength (J$_{rung}$/J$_{leg}$) so the system changes from a spin ladder to isolated spin chains as W$^{6+}$ increases. This further demonstrates the utility of d$^{10}$ and d$^0$ dopants as a tool for tuning magnetic interactions in a wide range of perovskites and perovskite-derived structures., Published by American Chemical Society, Washington, DC