No abstract

SummaryThe Hanford Site is a former nuclear defense production facility. A groundwater plume containing uranium, originating from a combination of purposeful discharges of wastewater to cribs, trenches, and ponds, along with some accidental leaks and spills related to nuclear fuel fabrication activities, has persisted beneath the Hanford Site 300 Area for many years. Despite the cessation of uranium releases and the removal of shallow vadose zone source materials, the remedial action objective to lower the concentration of groundwater uranium to the U. S. Environmental Protection Agency maximum contaminant level concentration of 30 µg/L has not been achieved within the anticipated 10-year time period. Some unknown amount of contamination remains in the vadose zone beneath the lower extent of the excavation activities. Additional contamination also may remain beneath buildings and facilities in the southern portion of the 300 Area, which has not been decontaminated and decommissioned. The use of polyphosphate technology for source treatment in the vadose zone and capillary fringe is expected to accelerate the natural attenuation of uranium to thermodynamically stable uranium-phosphate minerals. This effort will complement the current 300 Area treatability test being conducted within the saturated zone (e.g., 300 Area aquifer) for in situ treatment of uranium-contaminated groundwater.Polyphosphate technology has been demonstrated for in situ precipitation of phosphate phases to control the long-term fate of uranium. A critical component of the development and testing is detailed evaluation to determine if polyphosphate technology could be modified for infiltration from ground surface or some depth of excavation to stabilize source uranium phases. This report presents results from bench-scale treatability studies conducted under site-specific conditions to optimize the polyphosphate amendment for implementation of a field-scale technology demonstration to stabilize uranium within the 300 Area vadose zone and capillary fringe of the Hanford Site. Documented in this report are data related to 1) the retardation of polyphosphate as a function of water content and pore water velocity, 2) the reaction between uranium-bearing solid phases and aqueous polyphosphate remediation technology as a function of polyphosphate composition and concentration, 3) the mechanism of autunite formation via the reaction of solid-phase calcite-bound uranium and aqueous polyphosphate remediation technology, 4) the transformation mechanism and reaction kinetics between uranyl-carbonate and -silicate minerals with the polyphosphate remedy under advective conditions, and 5) the extent and rate of uranium released and immobilized as a function of polyphosphate composition and the infiltration rate of the polyphosphate remedy. Kinetic rate law parameters were determined from single-pass flow-through experiments. Pressurized unsaturated flow tests were used to determine the effect of polyphosphate composition, concentration, and infiltration rate...

SummaryAssessing long-term performance of Category 3 waste cement grouts for radionuclide encasement requires knowledge of the radionuclide-cement interactions and mechanisms of retention (i.e., sorption or precipitation); the mechanism of contaminant release; the significance of contaminant release pathways; how waste form performance is affected by the full range of environmental conditions within the disposal facility; the process of waste form aging under conditions that are representative of processes occurring in response to changing environmental conditions within the disposal facility; the effect of waste form aging on chemical, physical, and radiological properties; and the associated impact on contaminant release. This knowledge will enable accurate prediction of radionuclide fate when the waste forms come in contact with groundwater. Numerous sets of tests were initiated in fiscal years (FY) 2006 through2009 to evaluate 1) diffusion of iodine (I) and technetium (Tc) from concrete into uncontaminated soil after 1 and 2 years, 2) I and rhenium (Re) diffusion from contaminated soil into fractured concrete, 3) I and Re (set 1) and Tc (set 2) diffusion from fractured concrete into uncontaminated soil, 4) the moisture distribution profile within the sediment half-cell, 5) the reactivity and speciation of uranium (VI) [U(VI)] compounds in concrete porewaters, 6) the rate of dissolution of concrete monoliths, and 7) the diffusion of simulated tank waste into concrete.In FY 2008, concrete-soil half-cells initiated during FY 2007 using fractured concrete prepared with and without metallic iron, half of which were carbonated, were sectioned to evaluate the diffusion of I and Re in the concrete part of the half-cell. Probit plots were constructed from this data set.A second set of diffusion experiments, which had been initiated during FY 2007 using concrete-soil half-cells containing Tc, was sectioned in FY 2008 to measure the diffusion profile in the soil half-cell. These half-cells were prepared with and without metallic iron (Fe) and set up under unsaturated conditions (4%, 7%, and 15% moisture content by weight). Probit plots were constructed from this data set. In FY 2008, a set of concrete-soil half-cells were initiated. The half-cells were sectioned in FY 2009 to measure the diffusion profile in the concrete half-cell. Concentration and probit analysis was performed on the half-cells.A study was initiated during FY 2004 to better understand the reactivity of limited solubility U(VI)-bearing compounds in Portland cement grout specimens. The U(VI) nitrate-spiked specimens were aged for various time spans ranging from 2 weeks to 1 year. The uranium phases in these specimens were identified to be soddyite, becquerelite, uranophane, and autunite. Reliable thermochemical data are not available for these phases under conditions present in concrete waste forms. Therefore, to gather such data, synthetic routes were developed for the precipitation of pure uranium phases. From FY 2007 to FY 2008, the solubility meas...

SummaryAssessing long-term performance of Category 3 waste cement grouts for radionuclide encasement requires knowledge of the radionuclide-cement interactions and mechanisms of retention (i.e., sorption or precipitation); the mechanism of contaminant release; the significance of contaminant release pathways; how waste form performance is affected by the full range of environmental conditions within the disposal facility; the process of waste form aging under conditions that are representative of processes occurring in response to changing environmental conditions within the disposal facility; the effect of waste form aging on chemical, physical, and radiological properties; and the associated impact on contaminant release. This knowledge will enable accurate prediction of radionuclide fate when the waste forms come in contact with groundwater. The information presented in the report provides data that 1) quantify radionuclide retention within concrete waste form materials similar to those used to encapsulate waste in the Low-Level Waste Burial Grounds (LLBG); 2) measure the effect of concrete waste form properties likely to influence radionuclide migration; and 3) quantify the stability of uranium-bearing solid phases of limited solubility in concrete.v

No abstract