Contrasting coprecipitation and recrystallization mechanisms for Ra immobilization via (Ba,Ra)SO₄ solid solution formation in fractured crystalline rocks: Insights from 3D reactive transport modeling

Ra incorporation into (Ba,Ra)SO₄ solid solutions is a key control on Ra mobility in groundwater systems and typically occurs through coprecipitation and recrystallization. The effectiveness and persistence of these mechanisms under long-term reactive transport conditions remain poorly constrained, particularly in fractured crystalline rocks, where Ra migration is controlled by the coupled effects of heterogeneous flow, advective–diffusive transport, fracture-matrix mass exchange, reaction kinetics, and evolving hydrogeochemical conditions. Here, we employ 3D reactive transport modeling using PFLOTRAN to investigate Ra mobility under near-field conditions relevant to geological nuclear waste repositories. The models couple fluid flow, solute transport, and non-ideal solid solution-aqueous solution (SS-AS) interactions, incorporating a regular Guggenheim solid solution model and composition-dependent dissolution-precipitation kinetics for stoichiometric solid solutions. Simulations are conducted in a 10 m × 10 m × 10 m fracture-matrix domain upscaled from a discrete fracture network, with a constant-flux inflow boundary and fixed Ra concentration representing a sustained Ra source over 10,000 years. The results show that coprecipitation leads to strong but transient Ra immobilization, with substantial Ra uptake within the first ∼200 years, followed by progressive Ra remobilization as sulfate is depleted and previously formed solid solutions dissolve. Consequently, Ra retention decreases markedly and becomes minimal after ∼1000 years. In contrast, recrystallization supports persistent Ra immobilization throughout the entire 10,000-year simulation period, provided that sufficient barite remains available within fracture zones. This mechanism is sustained by kinetically controlled coupled dissolution-reprecipitation and produces a characteristic spatial zonation, with Ra-rich solid solutions near inflow regions and progressively Ba-rich compositions downstream. Sensitivity analyses further demonstrate that increasing Ba/Ra ratios in the inflowing fluid can reduce the long-term Ra retention under kinetically controlled reactive transport conditions, in contrast to predictions based solely on thermodynamic equilibrium. Fractures are identified as the dominant domains for long-term Ra immobilization, whereas the low-permeability matrix contributes only minimally due to limited diffusive accessibility. Once fractures lose their retention capacity, aqueous Ra is predominantly flushed from the system rather than retained within the matrix. Overall, these results suggest that equilibrium assumptions commonly adopted in radionuclide safety assessments are insufficient to predict Ra behavior in complex subsurface systems, and thus robust evaluation of long-term Ra mobility requires coupling reaction mechanisms and kinetics with flow and transport in evolving fracture-matrix systems. Although both coprecipitation and recrystallization form (Ba,Ra)SO₄ solid solutions, their distinct microscopic mechanisms lead to contrasting long-term behaviors when upscaled to the field scale. These findings have important implications for nuclear waste disposal and managing Ra contamination in geothermal and mining environments.