Potential for criticality in Hanford tanks resulting from retrieval of tank waste

This report assesses the potential during retrieval operations for segregation and concentration of fissile material to result in a criticality. The sluicing retrieval of C-106 sludge to AY-102 and the operation of mixer pumps in SY-102 are examined in some detail. These two tanks (C-106, SY-102) were selected because of the near term plans for retrieval of these tanks and their high plutonium inventories relative to other tanks. Although all underground storage tanks are subcritical by a wide margin if assumed to be uniform in composition, the possibility retrieval operations could preferentially segregate the plutonium and locally concentrate it sufficiently to result in criticality was a concern. This report examines the potential for this segregation to occur

A Comparison of Computerized Chemical Models for Equilibrium Calculations in Aqueous Systems

A survey of computer programs which are currently being used to calculate the distribution of species in aqueous solutions, especially natural waters, has been made in order to 1) provide an inventory of available programs with a short description of their uses, 2) compare the consistency of their output for two given test solutions and 3) identify major weaknesses or problems encountered from their use. More than a dozen active programs which can be used for distribution of species and activity calculations for homogeneos equilibria among the major anions and cations of natural waters have been inventoried. Half of these programs can also accept several trace elements including Fe, Al, Mn, Cu, Ni, Zn, Cd, Pb, Ag, Hg, As, Ba, Sr, and B. Consistency between programs was evaluated by comparing the log of the molal concentrations of free ions and complexes for two test solutions: a hypothetical seawater analysis and a hypothetical river water analysis. Comparison of the free major ion concentrations in the river water test case shows excellent agreement for the major species. In the seawater test case there is less agreement and for both test cases the minor species commonly show orders of magnitude differences in concentrations. These differences primarily reflect differences in the thermodynamic data base of each chemical model although other factors such as activity coefficient calculations, redox assumptions, temperature corrections, alkalinity corrections and the number of complexes used all have an affect on the output

Reactions of Heavy Metals and Other Mineral Water Quality Constituents with Soil Materials in Rapid Infiltration Groundwater Recharge Systems Using Reclaimed Wastewaters

Solutions containing Ca, Mg, K, Na, Pb, Ni, Cd, Cr, Cu, Zn, NH4, SO4, Cl, HCO3, PO4 at concentrations that are typically encountered in wastewaters were allowed to percolate through soil columns 15 cm in diameter and 200 cm deep in two soils; San Emigdio and Superstition sands with known physical and chemical properties were used in these experiments. The analysis of the percolating waters indicated that the trace metal concentrations were attenuated to the extent of non-detectability by A.A. spectrometry. The San Emigdio soil showed greater attenuation of PO4 and NH4 in comparison with Superstition sand. Speciation calculations by using GEOCHEM indicated that the major fractions of the trace metals were in cationic form and therefore would be absorbed by the soil exchange complex. Simulations that included the presence of dissolved organics indicated that a major fraction of Cu, Ni and Zn were complexed by the organic ligands. The speciation of Cd and Pb on the other hand were mainly influenced by the inorganic ligands

Mercury separation from concentrated potassium iodide/iodine leachate using Self-Assembled Mesoporous Mercaptan Support (SAMMS) technology

A study was conducted to demonstrate the effectiveness of a novel adsorber, the Self-Assembled Mesoporous Mercaptan Support (SAMMS) material to remove mercury (Hg) from potassium iodide/iodine (KI/I{sub 2}) waste streams. This study included investigations of the SAMMS material`s binding kinetics, loading capacity, and selectivity for Hg adsorption from surrogate and actual KI/I{sub 2} waste solutions. The kinetics data showed that binding of Hg by the adsorber material occurs very rapidly, with 82% to 95% adsorption occurring within the first 5 min. No significant differences in the rate of adsorption were noted between pH values of 5 and 9 and at Hg concentrations of {approximately}100 mg/1. Within the same range of pH values, an approximate four-fold increase in initial Hg concentration resulted in a two-fold increase in the rate of adsorption. In all cases studied, equilibrium adsorption occured within 4 h. The loading capacity experiments in KI/I{sub 2} surrogate solutions indicated Hg adsorption densities between 26 to 270 mg/g. The loading density increased with increasing solid: solution ratio and decreasing iodide concentrations. Values of distribution coefficients (1.3x10{sup 5} to >2.6x10{sup 8} ml/g) indicated that material adsorbs Hg with very high specificity from KI/I{sub 2} surrogate solutions. Reduction studies showed that compared to metallic iron (Fe), sodium dithionite can very rapidly reduce iodine as the triiodide species into the iodide form. Adsorption studies conducted with actual KI/I{sub 2} leachates confirmed the highly specific Hg adsorption properties (K{sub d}>6x10{sup 7} to>1x10{sup 8} ml//g) of the adsorber material. Following treatment, the Hg concentrations in actual leachates were below instrumental detection limits (i.e., < 0.00005 mg/l), indicating that the KI solutions can be recycled

Physicochemical Processes Controlling the Source-Term from Tank Residuals

After remediation and closure of the high-level waste tanks at Hanford (and at other DOE sites), residual radionuclide-bearing solids will remain in the form of sludge and hard heel adhering to the inner surfaces of the tanks. Radionuclide release from these residuals represents a potentially significant source of contaminants migrating in the sediments underlying the tanks. A recent composite analysis for the Hanford Site has shown the radionuclide source term from the residual solids to be one of the most significant long-term dose contributors on site, essentially equivalent in magnitude to a number of well-known discharges from leaking single-shell tanks. However, this assessment was based on a highly conservative release model for the tank residuals. The conservative model is being used in lieu of a true scientific understanding of the processes controlling the release rate from the sludge that is applicable to the Hanford vadose zone environment. Currently, the U.S. Department of Energy (DOE) is planning to spend hundreds of millions of dollars to remove 99% or more of the waste from Hanford high-level waste tanks that may have no real long-term environmental benefit. Through this project, we will significantly improve the fundamental scientific basis for estimating the release rate of 99Tc, the principal long-term dose contributor from tank residual wastes. We envision an improved conceptual model that considers diffusion of water and oxygen in the sludge under conditions of partial hydraulic saturation, but 100% relative humidity, consistent with the subsurface environment at Hanford. Key chemical processes are also considered, including the oxidation of reduced Tc compounds in the sludge and the chemical changes in sludge phase assemblage that will occur over time. A combination of novel experimental methods is proposed to investigate these processes. This comprehensive study will provide a sound technical basis for DOE and local stake holders to make more informed cost/benefit/risk decisions regarding closure of Hanford high-level waste tanks

Fabrication and testing of engineered forms of self-assembled monolayers on mesoporous silica (SAMMS) material

A number of engineered forms such as flexible extrudates, beads, and rods were fabricated using thiol-SAMMS (Self-Assembled Monolayers on Mesoporous Silica) and tested for their mercury adsorption capacities. The flexible extrudate form had a mercury adsorption capacity of 340 mg/g but was found to be structurally unstable. A structurally sound bead form of thiol-SAMMS was fabricated with 5, 10, 25, and 40% by weight clay binder (attapulgite) and successfully functionalized. A structurally stable but non-optimized rod form of thiol-SAMMS was also fabricated. Bench-scale processes were developed to silanize and functionalize mesoporous silica beads made with attapulgite clay binder. Contact angle measurements were conducted to assess the degree of surface coverage by functional groups on mesoporous silica materials