Bubble population growth and gas release from packed beds of AW500 ion exchange pellets

This paper explores the phenomenon of in situ generated gas retention within densely packed cylindrical ion-exchange pellets. Initially, discrete element method (DEM) simulations were used to identify pertinent contact packing parameters, rigorously validated through packing experiments. Calibrated values for dynamic and static friction, and the coefficient of restitution were found in ranges of 0.8–1.0, 0–0.2, and 0–0.2, respectively, with the importance of dynamic friction highlighted. The DEM validation process achieved a packing fraction that exhibited < 4 % deviation from experimental data, with some cylinder alignment observed near the column base. Additionally, laboratory-scale gas generation tests were conducted to replicate the retention of hydrogen within densely packed beds, achieved through the decomposition of hydrogen peroxide. X-ray computed tomography was then used to quantify bubble population size and growth within the beds. Experiments suggested a dynamic bed retention gas fraction of ∼7 %, which was produced within the first 30 min of generation. In parallel, CT imaging of the bubble populations indicated that large bubbles become less spherical over time, likely from pore invasion, partial cavity expansion and secondary coalescence. These mechanisms led to bubbles located in the upper portion of the bed being more likely to be released buoyantly, compared to those situated in the bed's lower regions. Overall, results indicate that the pore network channels within the pellet beds are large enough to achieve relatively steady gas release. These dynamics reduce the risk of sudden hydrogen rollover occurring industrially, particularly within the specialised context of nuclear waste management.