The objective of this study was to evaluate and quantify the potential transport of actinides such as americium in deep fractured crystalline rock formations, especially when sorbed to mobile mineral colloids. We selected a fractured/weathered granodiorite at the Grimsel Test Site (GTS) in Switzerland as a model system because the system has been well defined and field experiments involving actinides have already been conducted. Working on this system provides a unique opportunity to compare lab experimental results with field-scale observations.Weathered fracture fill material (FFM) from the GTS and bentonite used in the Full-scale Engineered Barriers Experiment (FEBEX) were shipped to Los Alamos National Laboratory, characterized by x-ray diffraction and microscopy, and used in batch sorption and column breakthrough experiments. Americium solutions were made by adding americium-241 to a synthetic Grimsel groundwater that matched the natural water chemistry found in the GTS groundwater. FFM samples were crushed, rinsed, equilibrated with synthetic groundwater and sorted to yield a 150-355 �m size fraction. Bentonite was crushed, sodium-saturated, equilibrated with synthetic Grimsel groundwater, and settled in columns to yield a stable suspension. Bentonite suspensions were equilibrated with americium before use.All experiments were conducted with Teflon� materials to limit sorption to system components. Column experiments were conducted using syringe pumps at low flow rate (<0.3 ml h-1) in small columns containing 5 g of material with pore volumes of 2-3 ml. Batch and column experiments showed that americium sorbed strongly to the FFM in the absence of bentonite. Pre-sorbing americium onto bentonite colloids prior to injection into the columns decreased retention times and sorption capacity. The measurement of americium concentrations as a function of time under these conditions and subsequent modeling with a two-site kinetic model allowed interrogation of desorption rates, which we believe control americium fate and transport over long time and distance scales.

Potential for colloid-facilitated transport of americium in a bentonite/granodiorite system

247th National Meeting (2014)

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