Lessons Learned from Characterization, Performance Assessment, and EPA Regulatory Review of the 1996 Actinide Source Term for the Waste Isolation Pilot Plant

The Waste Isolation Pilot Plant (WIPP) is a US Department of Energy (DOE) facility for the permanent disposal of transuranic waste from defense activities. In 1996, the DOE submitted the Title 40 CFR Part 191 Compliance Certification Application for the Waste Isolation Pilot Plant (CCA) to the US Environmental Protection Agency (EPA). The CCA included a probabilistic performance assessment (PA) conducted by Sandia National Laboratories to establish compliance with the quantitative release limits defined in 40 CFR 191.13. An experimental program to collect data relevant to the actinide source term began around 1989, which eventually supported the 1996 CCA PA actinide source term model. The actinide source term provided an estimate of mobile dissolved and colloidal Pu, Am, U, Th, and Np concentrations in their stable oxidation states, and accounted for effects of uncertainty in the chemistry of brines in waste disposal areas. The experimental program and the actinide source term included in the CCA PA underwent EPA review lasting more than 1 year. Experiments were initially conducted to develop data relevant to the wide range of potential future conditions in waste disposal areas. Interim, preliminary performance assessments and actinide source term models provided insight allowing refinement of experiments and models. Expert peer review provided additional feedback and confidence in the evolving experimental program. By 1995, the chemical database and PA predictions of WIPP performance were considered reliable enough to support the decision to add an MgO backfill to waste rooms to control chemical conditions and reduce uncertainty in actinide concentrations, especially for Pu and Am. Important lessons learned through the characterization, PA modeling, and regulatory review of the actinide source term are (1) experimental characterization and PA should evolve together, with neither activity completely dominating the other, (2) the understanding of physical processes required to develop conceptual models is greater than can be represented in PA models, (3) experimentalists should be directly involved in model and parameter abstraction and simplification for PA, and (4) external expert review should be incorporated early in a project to increase confidence long before regulatory reviews begin

Actinide biocolloid formation in brine by halophilic bacteria

The authors examined the ability of a halophilic bacterium (WIPP 1A) isolated from the Waste Isolation Pilot Plant (WIPP) site to accumulate uranium in order to determine the potential for biocolloid facilitated actinide transport. The bacterial cell surface functional groups involved in the complexation of the actinide were determined by titration. Uranium, added as uranyl nitrate, was removed from solution at pH 5 by cells but at pH 7 and 9 very little uranium was removed due to its limited solubility. Although present as soluble species, uranyl citrate at pH 5, 7, and 9, and uranyl carbonate at pH 9 were not removed by the bacterium because they were not bioavailable due to their neutral or negative charge. Addition of uranyl EDTA to brine at pH 5, 7, and 9 resulted in the immediate precipitation of U. Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) analysis revealed that uranium was not only associated with the cell surface but also accumulated intracellularly as uranium-enriched granules. Extended X-ray absorption fine structure (EXAFS) analysis of the bacterial cells indicated the bulk sample contained more than one uranium phase. Nevertheless these results show the potential for the formation of actinide bearing bacterial biocolloids that are strictly regulated by the speciation and bioavailability of the actinide

Solid-state nuclear magnetic resonance spectroscopy of cements

Cement is the ubiquitous material upon which modern civilisation is built, providing long-term strength, impermeability and durability for housing and infrastructure. The fundamental chemical interactions which control the structure and performance of cements have been the subject of intense research for decades, but the complex, crystallographically disordered nature of the key phases which form in hardened cements has raised difficulty in obtaining detailed information about local structure, reaction mechanisms and kinetics. Solid-state nuclear magnetic resonance (SS NMR)spectroscopy can resolve key atomic structural details within these materials and has emerged as a crucial tool in characterising cement structure and properties. This review provides a comprehensive overview of the application of multinuclear SS NMR spectroscopy to understand composition–structure–property relationships in cements. This includes anhydrous and hydrated phases in Portland cement, calcium aluminate cements, calcium sulfoaluminate cements, magnesia-based cements, alkali-activated and geopolymer cements and synthetic model systems. Advanced and multidimensional experiments probe 1 H, 13 C, 17 O, 19 F, 23 Na, 25 Mg, 27 Al, 29 Si, 31 P, 33 S, 35 Cl, 39 K and 43 Ca nuclei, to study atomic structure, phase evolution, nanostructural development, reaction mechanisms and kinetics. Thus, the mechanisms controlling the physical properties of cements can now be resolved and understood at an unprecedented and essential level of detail

Experimental diagenesis of lime mud

Textures similar to those found in natural microcrystalline limestones (micrites) were produced by closed-system (70-200\sp\circC/1-15.6 bars/1-486 days) and open-system (90-95\sp\circC/1 bar/63-311 days; 10-200 mL/day) hydrothermal experiments on a variety of lime muds ($<$25 $\mu$m size fraction), including natural lime muds, laboratory precipitated carbonates, and disaggregated skeletal materials.Mean crystal size ranges of about 1-3 $\mu$m and 5-25 $\mu$m were determined for calcite-dominated precursor (CDP) micrites and for aragonite-dominated precursor (ADP) micrites, respectively. Mean crystal size ranges in natural samples interpreted as CDP and ADP micrites by Lasemi (1983) and Lasemi and Sandberg (1982, 1983, 1984) match those experimentally-determined results fairly well.Precipitation of secondary calcite in aragonitic lime muds was controlled by pre-existing calcite or Mg-calcite seed crystals. The mean size of the seed crystals acts as the lower limit for mean crystal size in the final micrite, whereas the mean distance between the centers of the seed crystals provides an upper size limit. The calcitization of Mg-calcite-rich mud during the formation of CDP micrites differs from the calcitization of aragonitic mud because some Mg\sp{2+} released during dissolution of Mg-calcite adsorbs onto secondary calcite grains and because all sediment is potentially available to serve as seeds for localized precipitation of secondary calcite. The result is smaller crystal sizes in CDP micrites.Growth of cements in fractures and intraskeletal voids was the first noticeable change during experimental diagenesis. Micrite crystal sizes were generally fairly uniform throughout the change from mud to micrite. Partially-transformed samples appeared to be mixtures of the starting material and end-product. After mineralogical transformation, crystal size did not change during extended reaction times (no evidence of multiple dissolution-reprecipitation). The similarity of textures in experimental and ancient ADP micrites support conclusions by earlier authors that substantial compaction is not as important in the formation of micrites as was once thought.Similarities in the sizes and distribution of aragonite relics and initial aragonite grains indicate that those aragonite relics were engulfed by secondary calcite during reactions. Absence of relics in reacted synthetic aragonite muds suggests that natural relic preservation is linked to the presence of resistive coatings (e.g., by organic compounds).U of I OnlyETDs are only available to UIUC Users without author permissio

Kinetic modeling of microbially-driven redox chemistry of radionuclides in subsurface environments: Coupling transport, microbial metabolism and geochemistry

Microbial degradation of organic matter is a driving force in many subsurface geochemical systems, and therefore may have significant impacts on the fate of radionuclides released into subsurface environments. In this paper, the authors present a general reaction-transport model for microbial metabolism, redox chemistry, and radionuclide migration in subsurface systems. The model explicitly accounts for biomass accumulation and the coupling of radionuclide redox reactions with major biogeochemical processes. Based on the consideration that the biomass accumulation in subsurface environments is likely to achieve a quasi-steady state, they have accordingly modified the traditional microbial growth kinetic equation. They justified the use of the biogeochemical models without the explicit representation of biomass accumulation, if the interest of modeling is in the net impact of microbial reactions on geochemical processes. They then applied their model to a scenario in which an oxic water flow containing both uranium and completing organic ligands is recharged into an oxic aquifer in a carbonate formation. The model simulation shows that uranium can be reduced and therefore immobilized in the anoxic zone created by microbial degradation

Metal Sorption on Dolomite Surfaces

Potential human intrusion into the Waste Isolation Pilot Plant (WIPP) might release actinides into the Culebra Dolomite where sorption reactions will affect of radiotoxicity from the repository. Using a limited residence time reactor the authors have measured Ca, Mg, Nd adsorption/exchange as a function of ionic strength, P{sub CO{sub 2}}, and pH at 25 C. By the same approach, but using as input radioactive tracers, adsorption/exchange of Am, Pu, U, and Np on dolomite were measured as a function of ionic strength, P{sub CO{sub 2}}, and pH at 25 C. Metal adsorption is typically favored at high pH. Calcium and Mg adsorb in near-stoichiometric proportions except at high pH. Adsorption of Ca and Mg is diminished at high ionic strengths (e.g., 0.5M NaCl) pointing to association of Na{sup +} with the dolomite surface, and the possibility that Ca and Mg sorb as hydrated, outer-sphere complexes. Sulfate amplifies sorption of Ca and Mg, and possibly Nd as well. Exchange of Nd for surface Ca is favored at high pH, and when Ca levels are low. Exchange for Ca appears to control attachment of actinides to dolomite as well, and high levels of Ca{sup 2+} in solution will decrease Kds. At the same time, to the extent that high P{sub CO{sub 2}} increase Ca{sup 2+} levels, JK{sub d}s will decrease with CO{sub 2} levels as well, but only if sorbing actinide-carbonate complexes are not observed to form (Am-carbonate complexes appear to sorb; Pu-complexes might sorb as well; U-carbonate complexation leads to desorption). This indirect CO{sub 2} effect is observed primarily at, and above, neutral pH. High NaCl levels do not appear to affect to actinide K{sub d}s

Identification and evaluation of appropriate backfills for the Waste Isolation Pilot Plant (WIPP)

A backfill system has been designed for the Waste Isolation Pilot Plant (WIPP) which will control the chemical environment of the post-closure repository to a domain where the actinide solubility is within its lowest region. The actinide solubility is highly dependent on the chemical species which constitute the fluid, the resulting pH of the fluid, and the oxidation state of the actinide which is stable under the specific conditions. The use of magnesium oxide (MgO) has the backfill material not only controls the pH of the expected fluids, but also effectively removes carbonate from the system, which has a significant impact on actinide solubility. The backfill selection process, emplacement system design, and confirmatory experimental results are presented