This is the first comprehensive study to compare the long term stability of U and Sr (bio)remediation end-points following oxidative remobilisation ( via oxygen/nitrate) yielding insights into retention capabilities and removal mechanisms.

Flow-through columns were used to assess potential long-term trends in ⁹⁰Sr biogeochemistry and transport in a Finnish near-surface very low-level waste (VLLW) repository concept. Experiments simulated the effects of water intrusion and flow through the repository barrier and backfill materials, examining impacts on ⁹⁰Sr migration. Artificial rainwater containing 2.0 mg/L stable Sr (as a proxy for ⁹⁰Sr) was pumped through column systems that had varying compositions from a matrix of rock flour (backfill material), bentonite (backfill/sealing material), and carbon steel (waste encapsulation material), for 295 days. Effluent geochemistry was monitored throughout. Sr retention behaviour in all column systems was broadly similar. Sr removal from influent rainwater was marked (~95% removed) at the beginning of the experiments, and this degree of removal was maintained for 20 days. Thereafter, Sr concentrations in the effluents began to rise, reaching ~2 mg/L by 295 days. Further, 56%–67% of added Sr was retained in the repository materials over the 295-day reaction period. Analysis of the effluents indicated that colloids did not form; as such, Sr output was likely to be aqueous Sr²⁺. Upon completion of the experiment, solid-associated Sr distribution and speciation in the columns were assessed through column sectioning and post-mortem analyses, which encompassed the following: total acid digests, sequential extractions, and XAS analysis. The total acid digests and sequential extractions showed that Sr was evenly distributed throughout the columns and that the majority (68%–87%) of solid-associated Sr was in the exchangeable fraction (MgCl₂). This suggested that a major part of the solid-phase Sr was weakly bound to the column materials via outer-sphere sorption. Interestingly, a smaller amount of Sr (7%–23%) could only be extracted by aqua regia, suggesting that a proportion of Sr may bind more strongly to the barrier materials. XAS analysis of select samples confirmed that the dominant Sr phase was sorbed to the rock flour and bentonite, but not corroded carbon steel. Columns were also subject to remobilisation experiments using artificial rain- and seawater without added Sr. While rainwater remobilised Sr slowly, high-ionic strength seawater remobilised Sr at much higher rates in the systems containing bentonite. Interestingly, Sr was well retained in the rock flour-only system following rain and seawater intrusion. Overall, the results indicate that the column materials provide reactive surfaces for Sr removal should it be released from waste packages; however, the backfill and barrier materials have limited retention capacity, and the dominant sorption interaction is relatively weak. The safety case for the shallow disposal of radioactive waste should consider the possibility of seawater intrusion and that the bentonite-bound Sr was significantly more susceptible to remobilisation following seawater, despite retaining slightly more Sr during sorption experiments.

Thermally-treated spent ion-exchange resins (OIXRs) were immobilized by mixing 1 wt% ash with a metakaolin geopolymer. Stable ¹³³ Cs simulated ¹³⁷ Cs. Year-long semi-dynamic leaching experiments at pH 12.5 monitored changes in pH, conductivity and dissolved ions. Na, Al, Si were rapidly released into solution and 32% of Cs leached within 28 days. Ca uptake dominated, driving Na-Ca exchange and progressive N–A–S–H to C–A–S–H transformation. XRD, FE-SEM–EDX and ²⁷ Al/ ²⁹ Si MAS-NMR showed depolymerization, Ca-rich precipitates and loss of tetrahedral Al. Despite 59% Cs loss after 1 year, results demonstrate the matrix’s long-term chemical evolution and its promise as a sustainable OIXR wasteform.

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 (50 %) external radiation dose from the particles. In contrast, UO₂ readily dissolved in ALF (∼75 % U 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.

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.