Hanford Tanks 241-C-203 and 241-C-204: Residual Waste Contaminant Release Model and Supporting Data

Abstract

This report describes the development of release models for key contaminants that are present in residual sludge remaining after closure of Hanford Tanks 241-C-203 (C-203) and 241-C-204 (C-204). The release models were developed from data generated by laboratory characterization and testing of samples from these two tanks. Key results from this work are that future releases from the tanks of the primary contaminants of concern ( 99 Tc and 238 U) can be represented by relatively simple solubility relationships between infiltrating water and solid phases containing the contaminants. In addition, it was found that high percentages of 99 Tc in the sludges (20 wt% in C-203 and 75 wt% in C-204) are not readily water leachable, and, in fact, are very recalcitrant. This is similar to 99 Tc results found in related studies of sludges from Tank AY-102 (Lindberg and Deutsch 2003;Krupka et al. 2004). These release models are being developed to support the tank closure risk assessments performed by CH2M HILL Hanford Group, Inc., for the U.S. Department of Energy.The material used for testing consisted of sludge samples collected from the tanks in September 2003. Initial (Tier 1) testing of the sludges consisted of 1) fusion analysis and acid digestion to determine the total composition of the sludges and 2) water leaching to estimate the soluble portion of the solids. Based on the results of these tests, subsequent analyses were conducted (Tier 2). These included X-ray diffraction (XRD) measurements to identify crystalline solids and scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) analysis to provide a close-up view of the morphologies of the sludge material and identify the major constituents of individual minerals and amorphous solids. Periodic replenishment and selective extraction tests were also conducted during the Tier 2 tests to further evaluate water leachability and identify solid phases limiting the release of contaminants to solution. The test data were used to develop conceptual contaminant release models based on a series of solubility controls.The 99 Tc release model developed from the laboratory testing of C-203 and C-204 sludges is based on the concentrations and solubilities of technetium-bearing solids in contact with pore water migrating through the sludges. There are two stages of technetium release to solution: The 238 U release model for C-203 and C-204 sludges is based on the concentrations and solubilities of uranium-and sodium-bearing minerals in contact with pore water migrating through the sludges. There are three stages of uranium release to solution:1. Initially, soluble čejkaite [Na 4 (UO 2 )(CO 3 ) 3 ] and a sodium nitrate solid are present in the sludges at concentrations of 0.16 g U/g-sludge and 0.22 g NaNO 3 /g-sludge for C-203 and 0.068 g U/g-sludge and 0.04 g NaNO 3 /g-sludge for C-204. The common ion effect due to Na + limits the solubility of čejkaite to 0.19 g U/L until all the NaNO 3 is dissolved. The solubility of NaNO 3 is constant at 629 g/L.2. After all the ...

Residual Waste from Hanford Tanks 241-C-203 and 241-C-204. 2. Contaminant Release Model

“…On the other hand, the extraction results for both Fe and Al in the C-204 samples are generally consistent with their existence as relatively insoluble phases such as oxides; however, a surprisingly large amount of the Al (16%) dissolved during the deionized water extractions. This suggests that some Al exists in a relatively soluble phase, such as dawsonite [NaAlCO 3 (OH) 2 ] which has been observed in other residual tank sludges ( , ). However, the results of the C-204 characterization studies ( , ) do not provide any evidence for the presence of this phase.…”

“…This suggests that some Al exists in a relatively soluble phase, such as dawsonite [NaAlCO 3 (OH) 2 ] which has been observed in other residual tank sludges ( , ). However, the results of the C-204 characterization studies ( , ) do not provide any evidence for the presence of this phase. The solid-phase characteriza tion studies ( , ) did identify a Na−Al−P−O±H phase; however, most phosphate solids are not expected to be significantly soluble under these conditions.…”

“…Empirical Solubility Measurements. XRD and SEM/EDS characterization of C-203 and C-204 tank sludges indicated that the majority of the U in the sludges was in the form of the mineral čejkaite [Na 4 UO 2 (CO 3 ) 3 ] ( , ). The results of water-leaching experiments () indicated that nearly all the U in the C-203 and C-204 sludges was soluble at a solid-to-solution ratio of 1:100 and dissolved in less than 24 h at room temperature (final solution pH values near 9.5).…”

Summary of Group Development and Testing for Single Shell Tank Closure at Hanford

“…A study was carried out by Deutsch et al [27] from the Pacific Northwest National Laboratory on the leaching of Tc-99 and dissolution of other chemical and radioactive species from solid residual waste sampled from C-203 and C-204. These studies revealed that there is a highly soluble form of technetium (pertechnetate) and an insoluble and inaccessible form of technetium in the particles.…”

“…Since these samples were taken in 2003 prior to waste removal from the tanks, the highly soluble pertechnetate had yet to be removed in the washing/removal process. Deutsch et al [27] suggest that the intractable technetium is incorporated into a very insoluble iron mineral, assumed to be ferrihydrite [Fe(OH) 3 ].…”