Hanford Tank 241-C-106: Residual Waste Contaminant Release Model and Supporting Data

Abstract

The CH2M HILL Hanford Group, Inc. (CH2M HILL) is producing risk/performance assessments to support the closure of single-shell tanks at the U.S. Department of Energy's Hanford Site. As part of this effort, staff at Pacific Northwest National Laboratory were asked to develop release models for contaminants of concern that are present in residual sludge remaining in tank 241-C-106 (C-106) after final retrieval of waste from the tank. The primary contaminants of concern in the sludge are 99 Tc, 238 U, 129 I, and Cr because of their potential mobility in the environment and the long half-lives of the radionuclides. A key result from this work is that high percentages (>90%) of these primary contaminants are not readily leachable from the residual waste. This minimizes their future release rates to the environment, and is similar to 99 Tc results found in related studies of sludges from tanks AY-102, C-203, and C-204.Developing release models consists of laboratory testing to produce contaminant release data and a conceptual source release model. After development, the release model can be incorporated into a fate and transport model as part of a long-term risk assessment for the closed tank.Initial laboratory tests (Tier 1) were conducted to characterize the sludge and identify water-leachable constituents. Based on the results of Tier 1 tests, additional analyses were performed to augment the characterization of the material and determine the controlling mechanism(s) for release of contaminants. Tier 2 tests consisted of X-ray diffraction (XRD) and scanning electron microscopy/energy dispersive spectrometry (SEM/EDS) analyses of the solids to identify reactive phases, and selective extractions to quantify the release of contaminants from particular solid phases.The laboratory results of sludge and liquid testing were used to develop source term models that describe the release of contaminants as infiltrating water contacts the solids in the future. These models simulate the geochemical system in the tank sludge and take into account interactions between the solution phase and the contaminant-containing solids. The release models are simplifications of the complex geochemical interactions occurring; however, they adequately represent the release of the primary contaminants of concern from the sludge as measured in the laboratory tests.Because of the highly complex chemical nature of tank C-106 residual sludge, clear and quantitative phase associations of the contaminants of concern with the phases known to exist in the sludge are difficult to specify. Although the various characterization methods employed in this study have revealed a number of important observations and have provided valuable data for constructing a scientifically defensible release model, many questions remain. Because a thorough understanding of all the important phase associations for the contaminants of concern cannot be developed at this time, an empirically based release model has been developed. Although less satisfying from a mechanistic ...

Scientific Opportunities to Reduce Risk in Nuclear Process Science

“…The primary mineral salts in saltcake are nitratine (NaNO 3 ) and NaNO 2 ; however, other minerals and amorphous solids are likely trapped in the saltcake. , and dawsonite [NaAlCO 3 (OH) 2 ], have also been detected in one or more tanks (Krupka et al 2006;Deutsch et al 2005b). X-ray diffraction patterns of tank sludge material show that large percentages of some sludges are amorphous.…”

“…Compared to the minerals present in the waste, the 2.2 amorphous compounds have highly variable compositions. For example, analysis of an amorphous Mnoxyhydroxide in Hanford tank C-106 by scanning electron microscopy/energy dispersive spectroscopy showed the presence of Ca, Al, Si, Fe, Pb, rare earth elements (Ce), Cr, P, and C in addition to Mn (Deutsch et al 2005b). Compounding the composition complexity of the tank wastes is the presence of organic compounds in addition to the inorganic solids.…”

Contaminant Release Data Package for Residual Waste in Single-Shell Hanford Tanks

“…The major compositional features of the C-106 residual waste are discussed in this section. Detailed chemical composition data for residual waste from tank C-106 are presented in Deutsch et al (2007a). Figure 2.4 shows the concentrations of the major metals in the waste, based on fusion and acid digestion analyses of primary and duplicate samples.…”

“…The average result for NO 3 was determined from the sum of concentrations in sequential water extracts. A detailed description of the leaching tests for C-106 residual waste may be found in Deutsch et al (2006Deutsch et al ( , 2007a. It should be 2.8 noted that anion quantification by these leaching tests may be not be complete if the anion is present in insoluble minerals, such as phosphates.…”

Application of Scanning Electron Microscopy to Characterization of Radioactive Solid Wastes from Underground Storage Tanks

“…Their impact is greatly magnified when each technique is used in an iterative fashion to cross interpret the results from the other analysis method and then identify additional, more refined SEM/EDS and XRD analyses to be done. This strategy was necessary, for example, to refine analyses of the SEM/EDS and XRD results to identify the complex assemblage of solids for asreceived sludge from tank C-106 [2]. O), possibly 1one or two phases with AgHg±Fe±Pb±Cu, an REE-rich oxide, and unknown Mn-Al-Fe-Na-P-Si-Ca-O±C±H phase.…”