B18-Core EXAFS
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Diamond Proposal Number(s):
[30629]
Abstract: Geopolymer cements are highly promising materials for long-term immobilisation of Strontium-90 radioactive waste, offering superior durability and cation binding sites compared to conventional Portland cement matrices. This study investigates the influence of prolonged leaching on the Sr immobilisation mechanism and structural integrity of metakaolin-based geopolymers using the ANSI/ANS 16.1 semi-dynamic leaching test. All geopolymers demonstrated high Sr retention, with Leachability Indices at least 14.7 for all samples, significantly exceeding the industry guideline of 6.0, confirming their effectiveness. Importantly, potassium silicate–activated geopolymers exhibited reduced Sr release and substantially lower leaching rates than sodium silicate–activated geopolymers. Multiscale spectroscopic and diffractometric analysis, including synchrotron X-ray absorption spectroscopy and multinuclear high-field solid-state MAS NMR probing 39K, 23Na, 27Al, and 29Si, revealed that the alkali aluminosilicate gel framework remained structurally stable after leaching for 28 days, with no significant alterations to Si and Al bonding environments. Sr release is primarily controlled by diffusion, and the dominant immobilisation mechanism is the formation of insoluble SrCO3. Atomic-level Sr structural analysis using XANES/EXAFS revealed an increase in the average Sr coordination number in both systems after leaching, with a more pronounced rise in potassium-based geopolymers, consistent with enhanced SrCO3 formation. Overall, these findings demonstrate that geopolymers maintain structural integrity during leaching and show for the first time that using potassium rather than sodium as an alkali activator is definitively more advantageous for maximising the long-term effectiveness of geopolymer wasteforms. This demonstrates their strong suitability as wasteforms for the safe long-term immobilisation of Sr-containing radioactive wastes.
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Jul 2026
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B18-Core EXAFS
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Diamond Proposal Number(s):
[37458]
Open Access
Abstract: This study investigates the influence of strontium (Sr) salt chemistry (Sr(OH)₂•8H₂O, SrCO₃, Sr(NO₃)₂, and SrSO₄) on the nanostructural evolution of potassium silicate-activated geopolymers. High-field multinuclear (²⁷Al, ²⁹Si, ³⁹K, and ⁸⁷Sr) MAS NMR, synchrotron XANES, EPMA, XRD, and FTIR showed that while the primary binding phase in all samples is a disordered, highly cross-linked K-A-S-H gel, the Sr immobilisation mechanism is governed by salt solubility. Soluble nitrate and hydroxide salts release Sr²⁺ ions that are chemically incorporated into the K-A-S-H gel framework in brewsterite-type pseudo-zeolitic environments. In contrast, insoluble carbonate and sulfate salts act primarily as physical fillers, and are encapsulated as discrete particles within the geopolymer matrix, though sulfate additionally reacts to form secondary crystalline kalistrontite (K₂Sr(SO₄)₂). Sr 2+ adsorption on metakaolin surfaces is found to inhibit early-stage Al dissolution, resulting in a Si-rich K-A-S-H gel that transitions to an Al-rich K-A-S-H gel over 28 days. These results provide new insight into the mechanisms of immobilisation of Sr in geopolymers, and highlight their potential as wasteforms for long-term management of ⁹⁰Sr radioactive waste.
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Jun 2026
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B18-Core EXAFS
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Diamond Proposal Number(s):
[24074]
Abstract: There is a global legacy of radioactively contaminated land at nuclear facilities, and 90Sr is an important radioactive contaminant. Typically, at these sites, significant volumes of light contamination concrete waste occur. On-site disposal of concrete waste, potentially with pretreatment (e.g., crushing), is one disposal option for this lightly contaminated waste. Developing a mechanistic understanding of the long-term speciation and fate of 90Sr in typical on-site disposal scenarios will underpin safety case development for this option. Here, uptake of stable Sr/radioactive 90Sr with crushed concrete waste was explored in batch experiments with representative groundwater using a multitechnique approach combining direct speciation and low-level radioanalytical techniques. Experiments were conducted under air-limited and air-equilibrated conditions and with and without phosphate amendment. In the absence of air, the Sr uptake was low. With air, the Sr uptake was increased due to Sr-incorporated calcite formation. Phosphate-pretreated and phosphate-equilibrated crushed concrete again showed enhanced uptake from the solution due to increased sorption/incorporation on newly precipitated calcium phosphates. Overall, crushed concrete acts as a long-term sink for 90Sr contamination when air recharge is sufficient to drive carbonate precipitation. Additionally, phosphate pretreatment of crushed concrete waste may enhance 90Sr uptake to the solid phase.
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Jun 2026
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B18-Core EXAFS
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Diamond Proposal Number(s):
[37458]
Open Access
Abstract: Geopolymers are a promising alternative to conventional Portland cement-based wasteforms for immobilising hazardous radioactive fission products such as caesium-137 and strontium-90, offering superior durability and lower leach rates. However, the specific mass transport mechanisms governing radionuclide release in geopolymers remain poorly understood, limiting implementation. This study reveals the incorporation and mass transport mechanisms of caesium and strontium in metakaolin-based geopolymers. Solid-state characterisation showed Sr incorporation via direct chemical binding in the chemical binding in the alkali aluminosilicate hydrate gel in chargebalancing extra-framework sites, replacing K + ions, and precipitation of SrCO 3 and Sr(OH) 2 , while Cs is predominantly bound within the charge-balancing sites in the alkali aluminosilicate gel. Leach testing confirmed low overall release rates, with all measured Leachability Indices significantly exceeding the industry minimum of 6 (Li > 13 for Cs; Li > 18 for Sr), outperforming conventional PC systems. Mass transport modelling revealed distinct mechanisms: Cs release is accurately described by a Diffusion/Surface Exchange Kinetics Model (DSEM), yielding high correlation (R 2 > 0.99). However, Sr exhibited a complex, staggered release profile. Standard mass transport models (diffusion, dissolution, surface exchange) could not satisfactorily capture this complex behaviour. We hypothesise this rate resumption is caused by the structural reordering or crystallisation of the amorphous K-A-S-H gel into a zeolitic phase, potentially excluding incorporated Sr. This finding highlights that simple diffusive models, commonly assumed for geopolymers, are inadequate for predicting the long-term per-1 formance of Sr-containing geopolymer wasteforms. The new insight presented here is critical development of geopolymers for radioactive waste disposal.
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Jan 2026
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B18-Core EXAFS
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Diamond Proposal Number(s):
[31395, 37736]
Abstract: Selenium-79, a radionuclide present in higher-activity radioactive wastes destined for deep geological disposal, is mobile under oxic conditions, where Se(IV) and Se(VI) dominate. Anoxic batch microcosm incubations were constructed containing Wyoming MX80 bentonite (a candidate buffer material in geological disposal) and artificial groundwater with or without steel coupons to represent canister materials. Se(VI)(aq) bioreduced and was removed by 7 days when lactate was added as an electron donor, after which sulfate reduction occurred. With H2 gas as the electron donor, Se(VI) bioreduction slowed, with complete removal at 14 days and minimal sulfate reduction thereafter. 16S rRNA gene sequencing highlighted the dominance of Anaerobacillus spp. (44% at 28 days) during Se(VI)-reduction, and in the lactate-amended systems, there was a subsequent enrichment in sulfate-reducing bacteria affiliated with Desulfosporosinus spp. (60% relative abundance at 84 days). Extended X-ray absorption fine structure (EXAFS) analyses identified monoclinic Se(0) as the bioreduction product after 28 days, but by 84 days this evolved to trigonal Se(0) in the absence of steel coupons or was further reduced to FeSe2 with steel present. The reduction of Se(VI)(aq) to poorly soluble Se(0)/FeSe2 mediated by indigenous bentonite microbial communities highlights their potential importance in promoting Se-79 retention during deep geological disposal.
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Jan 2026
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B18-Core EXAFS
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Diamond Proposal Number(s):
[37736]
Open Access
Abstract: 99Tc is a long-lived radioactive fission product whose subsurface mobility is governed by redox conditions. Under oxic conditions, soluble Tc(VII)O4– is mobile, whereas under reducing conditions, poorly soluble Tc(IV) phases limit transport. Microcosm studies have frequently reported TcO2-like solids and, less consistently, Tc(IV)-sulfides. The stability of Tc(IV)-sulfides under environmentally relevant conditions remains unclear. Here, we used flowing sediment columns representative of the Sellafield subsurface to examine Tc speciation and stability over ∼1 year. Under reducing conditions, >90% of added TcO4– (400 μg) was retained under both Fe(III)- and sulfate-reducing conditions. X-ray absorption spectroscopy showed TcO2-like phases dominated in Fe(III)-reducing columns, while Tc(IV)-sulfides dominated after sustained sulfate reduction. Sequential extractions indicated that Tc in sulfidic sediments was more recalcitrant (≤23% released by weak acids) than in Fe(III)-reducing systems (∼60% released). With oxic groundwater pumping, effluent Tc sourced from the sediments rose rapidly. Over 160 days, the sulfidic columns remobilized ∼25% of their Tc inventory compared to ∼50% in Fe(III)-reducing columns. The Tc(IV)-sulfides also gradually oxidized to form TcO2 phases. While Tc(IV)-sulfides may enhance Tc retention under reducing conditions, TcO2 phases more likely govern 99Tc mobility during long-term redox cycling. Our findings provide new constraints for modeling Tc fate at contaminated sites and in radioactive waste disposal.
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Dec 2025
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I19-Small Molecule Single Crystal Diffraction
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Thien D.
Duong
,
Jiangnan
Li
,
Ruohan
Li
,
Xin
Lian
,
Yinlin
Chen
,
Jiarui
Fan
,
Joseph
Hurd
,
Lixia
Guo
,
Daniel
Lee
,
Mark
Warren
,
Sihai
Yang
Diamond Proposal Number(s):
[41123]
Abstract: The capture of xenon (Xe) and krypton (Kr) from the off-gas of used nuclear fuel is of great importance to the treatment of radioactive wastes and production of high purity Xe. Solid sorbents, in particular metal–organic frameworks (MOFs), show promise in gas capture. However, the unknown radiation resistance of MOFs has limited their development. Herein, the efficient capture and separation of Xe/Kr by MFM-520, which strikes a remarkable stability toward 1750 kilogray (kGy) γ-irradiation, is reported. Under ambient conditions, dynamic breakthrough experiments confirm the efficient separation performance, yielding a Xe capacity of 66 and 0.2 mg g−1 from a by-product of air separation (Xe/Kr: 20/80; v/v) and off-gas (Xe/Kr: 400/40 ppm balance in air), respectively. In situ synchrotron X-ray single crystal diffraction and solid-state nuclear magnetic resonance (ssNMR) studies reveal that the optimal micropore of MFM-520 underpins specific host-guest interactions to Xe, resulting in selective Xe capture.
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Oct 2025
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B18-Core EXAFS
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Diamond Proposal Number(s):
[33573]
Open Access
Abstract: Borosilicate glass is a potential candidate for high-level radioactive waste conditioning, thus understanding the effects caused by the combined presence of uranium and actinides within these matrices is of great importance. The glass matrix was simultaneously loaded with UO3 and lanthanide oxides (CeO2, Nd2O3, and Eu2O3) as chemical surrogates for actinides. Neutron diffraction in combination with Reverse Monte Carlo simulation confirmed that the basic glass structure is comprised of tetrahedral SiO4, and BO3/BO4 units. X-ray absorption spectroscopy indicated the presence of Ce mainly as CeIII and the co-existence of UV and UVI. U acts as an intermediate oxide and reduces the number of four-coordinated B, lanthanide ions serve as modifiers, with their increasing concentration shifting the B-O coordination from 3 to 4. X-ray photoelectron spectroscopy revealed a depth-dependent variation in the UIV/UVI ratio. Leaching tests showed increased dissolution of Si, B, and Na, compared to the glass matrix.
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Aug 2025
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B18-Core EXAFS
I14-Hard X-ray Nanoprobe
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You Cheng
Khng
,
Gianni F.
Vettese
,
Satoshi
Utsunomiya
,
Joyce W. L.
Ang
,
Jessica M.
Walker
,
Julia
Parker
,
Thomas
Neil
,
Katherine
Morris
,
Liam
Abrahamsen-Mills
,
Mirkka
Sarparanta
,
Gareth T. W.
Law
Diamond Proposal Number(s):
[31916, 31395]
Open Access
Abstract: Uranium dioxide (UO₂) particles can be released from mines, nuclear fuel manufacturing, reactor accidents, and weapons use. They pose inhalation risks, yet their behavior in the human lung remains poorly understood. This study investigates the long-term chemical alteration and dissolution of µm-sized UO₂ particles in two model lung fluids: Simulated Lung Fluid (SLF) and Artificial Lysosomal Fluid (ALF), representing extracellular and intracellular lung environments, respectively. Particles were exposed to each fluid at 37°C for up to 180 days (SLF) and 900 days (ALF). In SLF, UO₂ showed low apparent solubility (<2% U released to solution), but solid-phase analyses revealed significant oxidation of U(IV) (~50%) and formation of autunite-like sheets on the UO2 surface. Secondary phase formation may lessen overall UO2 dissolution, promoting long-term particle retention, whilst modifying particle chemical toxicity and cell uptake. In contrast, Monte Carlo simulations indicate that the SLF-induced surface alteration would reduce (>50%) external radiation dose from the particles. In contrast, UO₂ readily dissolved in ALF (~75% uranium released to solution in 60 days, ~100% by 900 days). There was no evidence of secondary phase formation in ALF, but extensive particle matrix dissolution/disaggregation was observed by 30 days. Fragmentation of the UO2 polycrystalline matrix may lead to release of smaller UO₂ crystallites, which could translocate more readily. Overall, this work provides new mechanistic insight into the fate of inhaled UO₂ under physiologically relevant conditions, highlighting a possible need to consider particle reactivity and alteration processes in health risk assessments.
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Aug 2025
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B18-Core EXAFS
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Diamond Proposal Number(s):
[37736]
Abstract: There is a requirement to further understand the structural and thermal properties of candidate Pu wasteform materials, and moreover gain a better understanding of composition-driven variation in these properties as they can impact disposability. Zirconolite (CaZrTi2O7) phases are a candidate wasteform system to immobilise Pu at scale and therefore it is necessary to understand (as far as possible) the isolated influence of cation substitution on specific lattice sites. CaZr1-xHfxTi2O7 is a model system for understanding the microstructural effects of Hf4+ substitution and underpin its viability as a neutron absorbing additive that could feasibly be co-immobilised with Pu. Hf4+ was capable of wholly substituting for Zr4+ at low-to-moderate concentration (i.e. x ≤ 0.60) after which some minor Hf-phase segregation was observed. Powder X-ray diffraction, Rietveld analysis and Raman spectroscopy were consistent with Hf4+ substituting in the Zr4+ site and confirmed no additional zirconolite polytypes were formed in addition to 2M. Hf L3-edge EXAFS analysis was consistent with Hf4+ occupying the 7-fold Zr4+ site in the zirconolite-2M structure consistent with the targeted substitution scheme. The thermal diffusivity and thermal conductivity of the zirconolite ceramics was generally observed to increase with elevated Hf4+ content although no clear compositional trends were identified.
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Aug 2025
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