Series‑connected fracture control on solubility‑Limited decay chain transport

Abstract

Colloid-facilitated Th-230–Ra-226 decay-pair transport in series-connected fractured rock is examined as a conceptual mechanistic study using a GoldSim triple-continuum framework for fracture, skin, and matrix domains. Three fracture-segment types with distinct wetted perimeters and colloid-coating properties are connected in series to form two contrasting pathway configurations. Solubility control is applied to Th-230 while Ra-226 is fully soluble, so that a solubility-limited source cell replaces the conventional step function and represents thermodynamically governed source depletion. Th-230 solubility is varied over nearly three orders of magnitude including repository-relevant values, producing only minor differences in breakthrough behaviour and indicating that the reported mechanisms are robust across the explored range of source conditions. In contrast, a two-order-of-magnitude variation in the Ra-226 distribution coefficient strongly modulates secondary Ra-226 peak fluxes and cumulative release, yielding empirical power-law scaling relationships. The two pathway configurations reveal that a segment with a deposited-colloid coating acts as an internal secondary Ra-226 source wherever it is placed, so that its position determines where along the pathway the Ra-226 mass rate increases: at 5 × 105 years the mass rate rises by approximately two orders of magnitude across this segment in either configuration, leaving the late-time midstream flux about two orders of magnitude higher when the segment lies midstream, even though outlet fluxes remain comparable. Spatial mass rate profiles resolve this sequence-dependent internal heterogeneity that breakthrough curves alone may not fully capture. The power-law relationships and spatial diagnostics offer practical guidance for performance assessment in crystalline-rock repository settings.