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Plutonium (Pu) is one of the primary actinides of concern for long-term disposal and storage of nuclear waste. Strong sorption of Pu onto colloids of iron oxide, clay, and silica could result in colloid-facilitated transport of this actinide in groundwater systems. However, fundamental data on Pu sorption to colloids is sparse, resulting in large uncertainties in long-term predictions of colloid-facilitated Pu transport. This sparseness of data and the potential to significantly reduce uncertainties in predictive models served as a motivation for this study. The authors investigated the sorption and desorption behaviors of 239Pu(V) on three types of inorganic colloids (hematite, montmorillonite and silica), with adsorption being measured as a function of temperature, ionic strength and colloid concentration for each colloid. Natural ground water collected from Well-13 near Yucca Mountain, Nevada, and synthetic ground water (SYN.J-13) were used in the experiments. The results indicated that the adsorption of 239Pu(V) onto hematite colloids was faster and had a higher partition coefficient (K p, ml g-1) than onto montmorillonite or silica colloids in both J-13 and SYN.J-13 waters. Temperature did not significantly influence the adsorption of 239Pu(V) onto hematite and silica colloids, but the adsorption of 239Pu(V) onto montmorillonite colloids increased significantly with increasing temperature. While ionic strength did not significantly influence the adsorption of 239Pu(V) onto hematite colloids, an increase in ionic strength decreased the adsorption of 239Pu onto montmorillonite and silica colloids. Adsorption of 239Pu(V) per unit mass of colloid was much higher at low colloid concentrations than at high colloid concentrations. Desorption of 239Pu from 239Pu-loaded colloids was considerably slower than the adsorption process.

This study presents and discusses groundwater colloid results from various geological formations ranging from crystalline to sedimentary, from organic rich to organic poor systems and from subsurface to very deep aquifers. Colloid presence and their potential mobility are justified on the basis of colloid stability properties in the investigated groundwaters. The colloid concentration is a function of pH, redox potential, concentrations of Na, K, Ca, Mg and organic carbon, as well as the status of the chemical and physical steady state of the hydrogeochemical system. The colloid properties are discussed with a non-site specific approach.

Moisture can accumulate under asphalt covers for waste-burial areas, potentially mobilizing components of the waste. This case study describes an asphalt cover emplaced in 1961 to contain plutonium, uranium, beryllium, and lead wastes in an area at the Los Alamos National Laboratory used for underground nonnuclear detonation tests. In this semiarid environment, moisture, apparently from meteoric sources, has accumulated in the soil and tuff below the cover. Saturated conditions are observed to a depth of ∼6 m, while adjacent uncovered areas remain dry. No migration of plutonium or other contaminants has been observed. Remediation of the saturated volume may consist of removal of the asphalt cover and its replacement with a multilayer engineered cover, possibly with venting of the subsurface. The potential for water accumulation should always be considered when asphalt has been used or is proposed as a cover for a waste site.

Characterization and performance assessment studies for the potential high-level nuclear waste repository at Yucca Mountain have identified 2 3 7 Np as a radionuclide of concern for the proposed repository. To predict the migration of neptunium after a repository breach, an understanding of the relevant hydrologic and geochemical processes is required. The hydrologic flow in the unsaturated zone at Yucca Mountain is dependent on the infiltration rate, the stratigraphy of the vadose zone, and the heat generated by the decaying radioactive waste. The geochemical processes that strongly affect 2 3 7 Np migration include: solubility-limited release of 2 3 7 Np from the near-field environment, aqueous speciation of neptunium into non-sorbing carbonate/hydroxy complexes and the sorbing NpO 2 + cation, sorption of neptunium onto the zeolitic tuffs via an ion exchange mechanism, and radioactive decay. The finite element heat and mass transfer (FEHM) code was used to investigate the coupled effects of chemical interactions and heat on neptunium transport from the potential repository to the water table. The selective coupling method is introduced to solve these reactive transport problems. The simulations indicate that in the absence of irreversible changes in the hydrologic and transport properties, the heat pulse does not significantly affect the migration of neptunium, as the time scale of heat pulse propagation is shorter than the time scales associated with neptunium release and migration. Water chemistry, particularly pH, calcium, and sodium concentration significantly affect the retardation of neptunium by the zeolitic rocks between the repository and the water table.

Groundwater colloids can act as a vector which enhances the migration of contaminants. While sorbed to mobile colloids, contaminants can be held in the aqueous phase which prevents them from interacting with immobile aquifer surfaces. In this study, an idealized laboratory set-up was used to examine the influence of amorphous silica colloids on the transport of cesium. Synthetic groundwater and saturated glass bead columns were used to minimize the presence of natural colloidal material. The columns were assembled in replicate, some packed with 150–210 μm glass bead and others packed with 355–420 μm glass beads. The colloids used in these experiments were 100 nm amorphous silica colloids from Nissan Chemical Company. In the absence of these colloids, the retardation factor for cesium was 8.0 in the 150–210 μm glass bead columns and 3.6 in the 355–420 μm glass bead columns. The influence of anthropogenic colloids was tested by injecting 0.09 pore volume slugs of an equilibrated suspension of cesium and colloids into the colloid-free columns. Although there was little noticeable facilitation in the smaller glass bead columns, there was a slight reduction in the retardation of cesium in the larger glass bead columns. This was attributed to cesium having less of a retention time in the larger glass bead columns. When cesium was injected into columns with a constant flux of colloids, the retardation of cesium was reduced by 14–32% in the 150–210 μm glass bead columns and by 38–51% in the 355–420 μm glass bead columns. A model based on Corapcioglu and Jiang (1993) [Corapcioglu, M.Y., Jiang, S., 1993. Colloid-facilitated groundwater contaminant transport, Water Resour. Res., 29 (7) 2215–2226] was compared with the experimental elution data. When equilibrium sorption expressions were used and the flux of colloids through the glass bead columns was constant, the colloid facilitated transport of cesium was able to be described using an effective retardation coefficient. Fully kinetic simulations, however, more accurately described the colloid facilitated transport of cesium.

Plutonium oxidation state distribution on Yucca Mountain tuff and synthetic pyrolusite (beta-MnO2) suspensions was measured using synchrotron X-ray micro-spectroscopy and microimaging techniques as well as ultrafiltration/solventextraction techniques. Plutonium sorbed to the tuff was preferentially associated with manganese oxides. For both Yucca Mountain tuff and synthetic pyrolusite, Pu(IV) or Pu(V) was initially oxidized to more mobile Pu(V/VI), but over time, the less mobile Pu(IV) became the predominant oxidation state of the sorbed Pu. The observed stability of Pu(IV) on oxidizing surfaces (e.g., pyrolusite), is proposed to be due to the formation of a stable hydrolyzed Pu(IV) surface species. These findings have important implications in estimating the risk associated with the geological burial of radiological waste in areas containing Mn-bearing minerals, such as at the Yucca Mountain or the Hanford Sites, because plutonium will be predominantly in a much less mobile oxidation state (i.e., Pu(IV)) than previously suggested (i.e., Pu(V/VI).

The possibility of toxic effects from several actinide elements to bacteria isolated from the Waste Isolation Pilot Plant (WIPP) site has been investigated. This study is part of an extensive ongoing research program that endeavors to validate the suitability and safety of the WIPP site as a transuranic (TRU) radioactive waste repository. The motivation for the toxicity studies was to determine the eventual fate of the actinides after their contact with microorganisms relevant to the WIPP site. The toxicity studies investigated possible adverse effects to the growth of the bacteria due to actinide toxicity. Actinide interactions with the bacteria will impact actinide transport or retardation by the bacterial species.

Neptunium has a high solubility in groundwaters from Yucca Mountain [1]. Uranium in nuclear reactors produces 237Np which has a half-life of 2.14 × 106 years. Consequently, the transport of 237Np through tuffs is of major importance in assessing the performance of a high-level nuclear waste repository at Yucca Mountain. The objective of this work is to determine the amount of Np retardation that is provided by the minerals in Yucca Mountain tuffs as a function of groundwater chemistry.

A method of deconvolution has been developed to determine the site-specific selectivity coefficients for divalent and trivalent exchange in relatively rigid exchangers. The technique involves the measurement of ion-exchange isotherms and the application of the numerical approach of regularization to effect deconvolution. The method has been designed and extensively tested with computer-generated isotherms. The results of these numerical studies indicate that this approach can successfully recover selectivity coefficient distributions from divalent and trivalent isotherms. If a careful accounting of the exchanging cations is maintained, this method can be applied to systems undergoing ion exchange by different mechanisms.

A numerical method of regularization has been used to effect the deconvolution of isotherms obtained from the literature describing the exchange of Li+ K+, and Rb+ into the synthetic faujasites NaX and NaY. The obtained selectivity coefficient distributions fit the univalent isotherms used for deconvolution. The relative number of K+ cations undergoing exchange with the obtained selectivity coefficients has been compared with published cation-site populations in K-X and K-Y, determined crystallographically.

The behavior of monovalent ion-exchange isotherms has been mathematically described by a Fredholm integral equation of the first kind (IFK). A numerical technique of regularization applied to deconvolute the IFK yielded the selectivity coefficient distributions from the ion-exchange isotherms. Regularization has successfully recovered normal and log-normal unimodal and biomodal distributions from computer-generated isotherms. This method has also been used to deconvolute ion-exchange isotherms obtained from the literature.

The size and density of a 242Pu colloid has been measured by autocorrelation photon spectrometry. The sedimentation rate of the colloid was determined by ultraspeed centrifugation. From the concentration profiles of 242Pu in the centrifuged test tubes, a standard sedimentation formula was used to calculate the density; the size of the colloid was known from the light scattering experiments. The determined density of the 242Pu colloid was only slightly lower than the estimated density of crystalline PuO2.