January 17, 2008
Book Chapter

Appendix D: Contaminant Release from Residual Waste in Closed Single Shell Tanks and Other Waste Forms Associated with the Tanks

Abstract

This appendix of the RFI report describes the potential release of contaminants for the onsite waste forms associated with closure of the Hanford single-shell tanks (SSTs). These waste forms include residual sludge (and perhaps saltcake) in the SSTs, low-activity waste glass produced by the Waste Treatment Plant, and glass produced by the bulk vitrification process. Contaminant release from waste encapsulated in cement is also discussed, as well as contaminants present in the vadose zone that was affected by tank leaks or other discharges near the SST waste management areas. Each waste form is described, and conceptual contaminant release mechanisms from these sources are discussed. The results of testing contaminant release from the waste forms are provided, as well as modeling methodologies to use the release data in performance assessment calculations. At this time, the most sophisticated release model available is for glass waste forms. A kinetic release model has been developed for glass, and a testing methodology has been implemented to provide the necessary rate parameters. Progress is being made on understanding the influence on the dissolution rate of secondary mineral formation on the glass surface. A comprehensive testing methodology also has been developed for residual tank waste; however, the waste material appears to be primarily amorphous and not amenable to common characterization techniques. For the few tanks that have been studied, only empirical release models have been developed because of the difficulty in characterizing the waste and developing more sophisticated mechanistic models. Studies of contaminant release from cementitious waste forms have focused for the past 35 years on a diffusional model in which the contaminants move through the pores of the cement to the surface of the waste form. Advances have been made in understanding the evolution of cement pore water and the chemical alteration of cement solids with time; however, this knowledge has not been incorporated into a more advanced model of contaminant interaction with and release from cementitious waste forms. The release model for contaminants on affected soil near the tanks is also a fairly simple model based on the linear adsorption (Kd) approach. The lack of a more mechanistic model for contaminant release from soil requires a large Kd database to account for differences in soil properties and solution compositions throughout the vadose zone.

Revised: March 11, 2008 | Published: January 17, 2008

Citation

Deutsch W.J., K.J. Cantrell, R.J. Serne, and E.M. Pierce. 2008. Appendix D: Contaminant Release from Residual Waste in Closed Single Shell Tanks and Other Waste Forms Associated with the Tanks. In The Resource Conservation and Recovery Act Facility Investigation (RFI) Report for Hanford Single-Shell Tank Waste Management Areas, DOE/ORP-2008-01 Revision 0, edited by FM Mann. Richland, Washington:U.S. Department of Energy, Office of River Protection. PNNL-SA-31924.