Deep groundwater physicochemical components affecting actinide migration

To better understand the migration behavior of actinides in deep groundwater (GW), the interactions between doped tracers and deep GW components were investigated. La, Sm, Ho, and U tracers (10 or 100 ppb) were doped into sedimentary rock GW samples collected from 250 to 350 m deep boreholes in the experimental gallery of the Horonobe Underground Research Laboratory (URL), Hokkaido, Japan. To evaluate the effect of GW composition on the chemical speciation of actinides, the same tracers were doped into crystalline rock GW samples collected from 300 to 500 m deep boreholes in the experimental gallery at the Mizunami URL, Gifu Prefecture, Japan. Each GW sample was sequentially filtered through a micro-pore filter (0.2 μm) and ultrafilters with a 10 kDa nominal molecular weight limit. Next, the filtrate solutions were analyzed using inductively coupled plasma-mass spectrometry to determine the concentration of tracers retained in solution during each filtration step, and the used filters were analyzed using time-of-flight secondary ion mass spectrometry element mapping and X-ray absorption fine structure spectroscopy to determine the chemical species of the tracers trapped on each filter. It was determined that lanthanide migration was controlled by the amount of phosphates in the Horonobe GW. Therefore, it was expected that the solubility of minor actinides (MAs), which exhibit a similar chemical behavior to that of lanthanides, would be controlled by the formation of phosphates in sedimentary rock GW. Moreover, the data on the Mizunami GW indicated that a fraction of lanthanides and MAs formed hydroxides and/or hydroxocarbonates.

Interaction of rare earth elements and components of the Horonobe deep groundwater

To better understand the migration behavior of minor actinides in deep groundwater, the interactions between doped rare earth elements (REEs) and components of Horonobe deep groundwater were investigated. Approximately 10 ppb of the REEs, i.e. Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er, Tm, and Yb were doped into a groundwater sample collected from a packed section in a borehole drilled at 140 m depth in the experiment drift of Horonobe Underground Research Laboratory in Hokkaido, Japan. The groundwater sample was sequentially filtered with a 0.2 μm pore filter, and 10 kDa, 3 kDa and 1 kDa nominal molecular weight limit (NMWL) ultrafilters with conditions kept inert. Next, the filtrate solutions were analyzed with inductively coupled plasma mass spectrometry (ICP-MS) to determine the concentrations of the REEs retained in solution at each filtration step, while the used filters were analyzed through neutron activation analysis (NAA) and TOF-SIMS element mapping to determine the amounts and chemical species of the trapped fractions of REEs on each filter. A strong relationship between the ratios of REEs retained in the filtrate solutions and the ionic radii of the associated REEs was observed; i.e. smaller REEs occur in larger proportions dissolved in the solution phase under the conditions of the Horonobe groundwater. The NAA and TOF-SIMS analyses revealed that portions of the REEs were trapped by the 0.2 μm pore filter as REE phosphates, which correspond to the species predicted to be predominant by chemical equilibrium calculations for the conditions of the Horonobe groundwater. Additionally, small portions of colloidal REEs were trapped by the 10 kDa and 3 kDa NMWL ultrafilters. These results suggest that phosphate anions play an important role in the chemical behavior of REEs in saline (seawater-based) groundwater, which may be useful for predicting the migration behavior of trivalent actinides released from radioactive waste repositories in the far future.