Abstract

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.

Highlights

  • The waste arising from >60 years of civil and military nuclear operations around the world contains long-lived radionuclides that must be contained and isolated from future populations

  • Deep geological disposal facilities (GDF) proposed by US and European waste management organisations[1−4] employ an engineered multibarrier approach (Figure 1) to retard the release of radioactive species from the waste in quantities that could be detrimental to life and the environment

  • It is critical to the safety case for the GDF to be able to demonstrate that the design performance is well understood but to show that conditions arising from design failures are accounted for and mitigated as far as possible

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Summary

Introduction

The waste arising from >60 years of civil and military nuclear operations around the world contains long-lived radionuclides that must be contained and isolated from future populations. The transport of the 99mTc through the uniform saturated flow field was modeled using a onedimensional (1-D) convection−dispersion equation for reactive solute transport (eq 4): R ∂C ∂t where, subject to specified initial and boundary conditions, C is the aqueous concentration of a tracer at a given distance along the center of mass from inlet x (m) and elapsed time t (s), μ (s−1) is a first-order decay coefficient describing irreversible removal from the mobile aqueous phase, R is a retardation factor describing equilibrium interaction with the solid phase, and D is a dispersion coefficient equal to the product of the longitudinal (m s−1).

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