Gas retention and release from nuclear legacy waste

Many consolidated sediments experience in situ gas generation from methanogenesis, corrosion or radiolysis reactions, while the strength of the sediment matrix can immobilise bubbles for long periods. Particular research interest is motivated by the need to mitigate against periodic acute release of flammable hydrogen from nuclear legacy waste sludge at Sellafield, UK and Hanford, USA to ensure safe decommissioning of ageing facilities. Gas retention within nuclear legacy waste silos is mimicked at laboratory scale by decomposing hydrogen peroxide within Mg(OH)_2 sediments, using clinical x-ray computed tomography to capture the bubbles' size, shape and mobility. Tomography revealed that modest yield stresses of 7Pa were sufficient for sediments to inhibit buoyant migration of relatively large 6-9 mm diameter bubbles during 8 h of imaging, causing the bed to swell to significant voidages in the order of 30 %. Sediments up to 800Pa yield stress exhibited sufficient gas holdup to reduce the bulk sediment density to below that of an aqueous supernatant, raising the potential for acute gas release through Rayleigh-Taylor instabilities. In the absence of buoyant migration or deep drainage channels, chronic gas release from low-intermediate strength (7-1112Pa) sediments appears to be governed by diffusion along extensive networks of partially coalesced submillimetre microvoids, observed here for the first time. Lattice Boltzmann and Monte Carlo simulations reveal these networks to be highly gas-pervious, with permeabilities in the 10.1-151.6 × 10^−12 m^2 range and effective hydrogen diffusivities of 3.7-12.5 × 10^−5 m^2 s^−1. Bubbles within kiloPascal strength sediments were observed to grow by the induction of high aspect ratio lateral fractures, promoting low-tortuosity pathways for efficient gas migration and reduced holdup. Conversely, a 30Pa sediment with an additional coarse (100−800 μm) particle fraction and constricted pore throats supported large 1-8 mm bubbles of high sphericity, forming short range ganglia of < 42 bubbles which spanned < 25mm in range, thereby truncating the avenues for chronic hydrogen release.