The subsurface transport of iodine is a critically important process for understanding and assessing the risk associated with nuclear accidents and the long-term disposition of nuclear waste, as well as nuclear non-proliferation monitoring. Unfortunately, iodine speciation and transport through intact geologic media is not well understood. This uncertainty compromises our ability to differentiate background radionuclide emissions from those produced by nuclear events. The goal of the current study is therefore to quantify the matrix diffusion of iodine species through saturated igneous rock cores. Diffusion experiments were run for iodide using a lithophysae-rich lava, lithophysae-poor lava, and welded tuff, while iodate diffusion was studied through welded tuff. Iodine transport was compared with conservative tracers, tritiated water (<sup>3</sup>HHO) and deuterated water (D<sub>2</sub>O), and the effective matrix diffusion coefficient was experimentally determined for each system. Results indicate that iodate diffusion is slower than iodide through all rock types tested and both are slower than the conservative tracer. Among the three iodide systems, transport is fastest through the lithophysae-rich lava. This trend differs from that of the tracer, which transported faster through lithophysae-poor lava, indicating processes which retard iodine diffusion are specific to rock geochemistry. Results provide much needed data for transport models that predict relative radioiodine to radioxenon migration after an underground detonation or accidental release.

Iodine diffusive transport through igneous rock

2019 Fall National Meeting

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