Real-time gamma imaging of technetium transport through natural and engineered porous materials for radioactive waste disposal

We present a novel methodology for determining the transport of technetium-99m, a γ-emitting metastable isomer of ⁹⁹Tc, through quartz sand and porous media relevant to the disposal of nuclear waste in a geological disposal facility (GDF). Quartz sand is utilized as a model medium, and the applicability of the methodology to determine radionuclide transport in engineered backfill cement is explored using the UK GDF candidate backfill cement, Nirex Reference Vault Backfill (NRVB), in a model system. Two-dimensional distributions in ^99mTc activity were collected at millimeter-resolution using decay-corrected gamma camera images. Pulse-inputs of ∼20 MBq ^99mTc were introduced into short (<10 cm) water-saturated columns at a constant flow of 0.33 mL min^-1. Changes in calibrated mass distribution of ^99mTc at 30 s intervals, over a period of several hours, were quantified by spatial moments analysis. Transport parameters were fitted to the experimental data using a one-dimensional convection-dispersion equation, yielding transport properties for this radionuclide in a model GDF environment. These data demonstrate that ⁹⁹Tc in the pertechnetate form (Tc(VII)O₄^-) does not sorb to cement backfill during transport under model conditions, resulting in closely conservative transport behavior. This methodology represents a quantitative development of radiotracer imaging and offers the opportunity to conveniently and rapidly characterize transport of gamma-emitting isotopes in opaque media, relevant to the geological disposal of nuclear waste and potentially to a wide variety of other subsurface environments.