Hanford Tanks 241-C-202 and 241-C-203 Residual Waste Contaminant Release Models and Supporting Data

Abstract

SummaryAs directed by Congress, the U.S. Department of Energy (DOE) established the Office of River Protection in 1998 to manage DOE's largest, most complex environmental cleanup project -retrieval of radioactive waste from Hanford tanks for treatment and eventual disposal. Sixty percent by volume of the nation's high-level radioactive waste is stored at Hanford in aging deteriorating tanks. If not cleaned up, this waste is a threat to the Columbia River and the Pacific Northwest.CH2M HILL Hanford Group, Inc. is the Office of River Protection's prime contractor responsible for the storage, retrieval, and disposal of Hanford's tank waste. As part of this effort, CH2M HILL Hanford Group, Inc. contracted with Pacific Northwest National Laboratory (PNNL) to develop release models for key contaminants that are present in residual sludge remaining in Hanford Tanks 241-C-202 (C-202) and 241-C-203 (C-203). The release models were developed from data generated by laboratory characterization and testing of samples from these two tanks. These release models are being developed to support the tank risk assessments performed by CH2M HILL Hanford Group, Inc. for DOE.The contaminant release models developed for these tanks are based on empirical solubility release models. Sludge testing was not successful in identifying minerals in the solids that may be limiting contaminant release; thus, it was not possible to develop mechanistic release models for the residual sludge in these tanks. The empirical release models apply to two different tank scenarios. In the first scenario the tank is filled with a relatively inert material, such as sand, and the leaching solution that contacts sludge in the future is in equilibrium with calcite (CaCO 3 ). Equilibrium with calcite is the typical condition for Hanford vadose zone porewater and groundwater, and Ca 2+ and CO 3 2-/HCO 3 -are the common major cation and anions in solution. Alternatively, the tanks might be filled with a cementitious material, which would produce a Ca(OH) 2 dominated leaching solution while the cement is fresh. As the cement reacts with infiltrating water and ages, it would evolve to resemble the CaCO 3 solution of the first scenario. Empirical solubility release models for the primary contaminants of interest (U, Cr, and 99 Tc) have been developed from laboratory leaching tests of sludge samples using the Ca(OH) 2 and CaCO 3 leaching solutions. Results for 129 I results were below the detection limit.Key results from this work are that future release concentrations from these tanks of the primary contaminants of concern in most cases represent less than 10% of the total contaminant concentration in the sludge. That is, the contaminants are not appreciably soluble in the Ca(OH) 2 and CaCO 3 leaching solutions. For example, the cumulative amount of U leached by the Ca(OH) 2 leaching solution during six sequential stages of leaching represented only 0.13% and 0.11% of the total uranium in tanks C-202 and C-203, respectively. The corresponding percentages of U leached...

Thermodynamic Model for Uranium Release from Hanford Site Tank Residual Waste

“…Analytical and test methods used to characterize tank residual waste samples and leachates have been previously described. 3 A brief overview is provided in this section. The elemental and contaminant concentrations of the wastes were measured by complete dissolution of the bulk residual waste solids using fusion-dissolution procedures and acid digestions.…”

“…Analytical and test methods used to characterize tank residual waste samples and leachates have been previously described . A brief overview is provided in this section.…”

“…The same samples analyzed by XRD indicated above were also examined by SEM/EDS. A detailed analysis of the SEM/EDS data is provided in Deutsch et al, 3 along with all SEM micrographs and EDS analyses collected for the postretrieval samples from tanks C-202 and C-203. The major findings are summarized here.…”

“…Final waste retrieval for tanks C-202 and C-203 was accomplished using a vacuum system to remove as much sludge as possible. 3 A high pressure water spray was used with the vacuum to break up larger particles of waste that could not be removed solely with the vacuum. After the waste retrieval process was completed, samples of the residual material were collected for analysis and testing.…”

“…The solids left after this operation are referred to here as prefinal retrieval waste. Final waste retrieval for tanks C-202 and C-203 was accomplished using a vacuum system to remove as much sludge as possible . A high pressure water spray was used with the vacuum to break up larger particles of waste that could not be removed solely with the vacuum.…”