Retention of Hydrogen Bubbles Generated from Water Radiolysis in Carbonate Slurry

The retention of hydrogen (H₂) bubbles generated by water radiolysis was quantitatively studied in a high-viscous suspension of carbonate slurry consisting of a mixture of suspended solid (SS) of magnesium (Mg) and calcium (Ca) precipitates under strongly alkaline conditions, like the radioactive wastes discharged from the coagulation sedimentation (co-precipitation) process at the multinuclide removal equipment in the Fukushima Daiichi Nuclear Power Station.

The H₂ retention properties were evaluated in two types of carbonate slurry with different hydrophilicity: the hydrophilic “current type” and the hydrophobic “return type.” Then, their properties were compared with those in another suspension of clay suspension of bentonite. The hydrophilicity of the surface of the SS particles was evaluated from two different adsorption isotherms of water (H₂O) vapor on the surface, as well as those of Mg(OH)₂ and CaCO₃ particles, simulating the precipitates in the slurry. The H₂O molecules were strongly chemically adsorbed on the Mg(OH)₂, while they were weakly physically adsorbed on the CaCO₃. Since the amount of chemical adsorption on the SS surface of the hydrophilic slurry was almost the same as that on the Mg(OH)₂ surface, Mg(OH)₂ was found to be dominant on the SS surface.

From the comparison between the amounts of chemical adsorption and H₂O in the slurry, it was confirmed that H₂O molecules must be shared among the SS particles, and this sharing of H₂O molecules formed the structural viscosity in the slurry, different from that in the clay suspension where electrostatic bonding between the fine clay minerals forms the viscosity.

The retention of H₂ bubbles in (by) the slurry was evaluated from the difference in the amount of H₂ observed with and without stirring the slurry after ⁶⁰Co γ irradiation. The observed amount increased linearly with increasing absorbed dose by the slurry to obtain H₂ generation yield (G-value): G_total (with stirring) and G_stat (without stirring). The G_total was independent of the sample height, whereas the G_stat decreased with increasing the height. This indicated that the higher the height, the more difficult it was for H₂ bubbles to be released. From the direct observation of H₂ bubbles after irradiation, the growth of H₂ bubbles with increasing the dose was evaluated to obtain the resisting force of H₂ bubbles to the slurry. The H₂ bubbles were confirmed to be retained in the slurry because the resistance was lower than the shear strength of the slurry (yield value at rest).

The ability of the slurry to retain the H₂ bubbles was evaluated from the sample height dependence of the relative G-value, G_stat/G_total. The log-log plot of the G-value and height gave a straight line, and the slope of the hydrophilic slurry became larger than that of the hydrophobic one. This difference indicated the effect of structural viscosity, which was observed in the H₂ bubble retention in the clay suspension. In addition, the slopes for the hydrophobic slurry and treated water were significantly different, and this indicated the effect of steric hindrance by the SS particles. Therefore, H₂ bubbles were retained by not only the structural viscosity but also the steric hindrance in the hydrophilic slurry.