(117a) Hydrogen Transport through Consolidated Particulate Suspensions of Nuclear Corrosion Products; Insights from X-Ray Tomography

The UK and USA have accumulated substantial inventories of highly heterogeneous radioactive particulate sludge wastes comprised of consolidated fuel cladding corrosion products and by-products from legacy reprocessing activities respectively. These consolidated sediments experience in situ hydrogen generation from metallic fuel corrosion and radiolysis of water due to water soluble fission products such as caesium and strontium, while hydrogen bubbles can be retained within the settled sludge matrix over long periods. Subsequently, significant interest has arisen in understanding gas retention in order to mitigate against periodic acute releases of flammable gas during decommissioning of legacy waste silos, compounding the existing research interest in biogenic methane release from natural aquatic sediments, which represents a significant contributor towards global greenhouse gas emissions.

High resolution x-ray computed tomography (CT) has been employed to visualise bubbles retained by two predominantly brucite, Mg(OH)₂, test materials, which represents the major corrosion product from Magnox fuel cladding used in first generation reactors in the UK. A bespoke corroded magnesium sludge test material differed from a commercial Mg(OH)₂ test material as it contained a fraction of coarser particles, in the 100-1000 µm range, and achieves around double the yield strength at comparable solids concentrations within the 30-40 % w/w range. Low field nuclear magnetic resonance (LFNMR) relaxation time distributions also revealed that the microstructure of CMgS is characterised by a more constricted pore size distribution, with a median pore throat radius of 152 nm within a 42 % w/w sample. These subtle distinctions in sediment microstructure and mechanical properties manifested in considerable differences in the retained bubble populations observed under x-ray CT. A 30 Pa yield stress sample of CMgS supports a coarser population of 0.9-8 mm bubbles as opposed to 0.1-2 mm bubbles within an equivalent strength commercial Mg(OH)₂ sample and the coarser bubble population exhibits substantially lower connectivity between adjacent bubbles. Over 98 % of bubbles within the commercial Mg(OH)₂ soft sediment were found to partially coalesce and form a continuous void-network, spanning the 71 mm test vessel, thereby enabling chronic gas release from the bed by diffusion through the void-space. Conversely, the coarser bubbles within CMgS demonstrated fewer coalescence events, with distinct chains of up to 40 bubbles comprising less than 10 % of the total voidage and spanning less than 30 mm in length. Consequently, the coarse bubbles within the CMgS test material lack the buoyant stress to rise through the sediment and are insufficiently networked to promote release by diffusion through the gas phase; in the absence of an adequent mechanism for chronic gas release, the sediment experiences substantial expansion to a voidage approaching 40 % of the bed. Void fractions of this magnitude would have a substantial impact on decommissioning the legacy waste silos at Sellafield, with bulk waste swell requiring the provision of excess interim storage capacity at £10s millions in expense, while high voidage, low density sediments also present an enhanced risk of acute flammable gas release from Rayleigh-Taylor instabilities.