Abstract
A full-scale field experiment applying 4D (3D time-lapse) cross-borehole Electrical Resistivity Tomography (ERT) to the monitoring of simulated subsurface leakage was undertaken at a legacy nuclear waste silo at the Sellafield Site, UK. The experiment constituted the first application of geoelectrical monitoring in support of decommissioning work at a UK nuclear licensed site. Images of resistivity changes occurring since a baseline date prior to the simulated leaks revealed likely preferential pathways of silo liquor simulant flow in the vadose zone and upper groundwater system. Geophysical evidence was found to be compatible with historic contamination detected in permeable facies in sediment cores retrieved from the ERT boreholes. Results indicate that laterally discontinuous till units forming localized hydraulic barriers substantially affect flow patterns and contaminant transport in the shallow subsurface at Sellafield. We conclude that only geophysical imaging of the kind presented here has the potential to provide the detailed spatial and temporal information at the (sub-)meter scale needed to reduce the uncertainty in models of subsurface processes at nuclear sites.
Highlights
Whilst modern geophysical methodology is increasingly applied at nuclear legacy sites worldwide, the full potential of near-surface geophysics has yet to be fully recognized by the nuclear industry and its regulators
Quiescent baseline 3D fractional resistivity change during the quiescent baseline period and as a result of the Stage 1, 2 and 3 injections is shown in Fig. 5. 4D inverse modelling was performed at regular intervals, and the results were plotted as % changes relative to the inverted model at the start of the most recent injection stage
Automated Electrical Resistivity Tomography (ERT) monitoring was applied at a UK nuclear licensed site for the first time and a full-scale field experiment trialing the methodology at Magnox Swarf Storage Silos (MSSS) in Sellafield successfully detected, characterized, and tracked simulated leaks
Summary
Whilst modern geophysical methodology is increasingly applied at nuclear legacy sites worldwide, the full potential of near-surface geophysics has yet to be fully recognized by the nuclear industry and its regulators. At the Sellafield Site in Cumbria, United Kingdom, major long-term decommissioning and remediation projects of national priority and international significance are currently underway that can demonstrably benefit from non-invasive subsurface monitoring approaches. The reduction of risk and hazard at four Sellafield plants known as the Legacy Ponds and Silos is a strategic priority for the UK's Nuclear Decommissioning Authority (NDA), but presents unique technical challenges. The safe emptying and decommissioning of the Magnox Swarf Storage Silos (MSSS) is a flagship project that Sellafield Ltd. are currently undertaking on behalf of the NDA. The present study demonstrates the value of 4D subsurface geoelectrical monitoring in support of those efforts, and highlights its potential for underpinning risk management and environmental assurance for nuclear decommissioning programs
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