Dependencies of flow velocity on migration behavior of radionuclides in an unsaturated zone around near-surface underground

The low-level radioactive wastes from the Fukushima Daiichi accident will be disposed in near-surface underground which is not always saturated by groundwater. In such an unsaturated porous media, migration behavior of radionuclides is predicted to be different from that in the saturated zone. Moreover, the flow rate of groundwater in near-surface underground is generally larger than that in deeper underground. For the performance assessment of near-surface disposal system, this study estimated the retardation coefficients and Peclet numbers under both saturated and unsaturated conditions by measuring breakthrough curve of tracer with a sand packed column. The flow velocities set in this study were based on the hydraulic conductivity that IAEA used to assess for a trench disposal. The results showed that the migration rate of tracer ions under the unsaturated condition exceeded that under the saturated condition, and increased under the condition of higher flow rate.