Physicochemical Processes Controlling the Source-Term from Tank Residuals

McGrail, B. P. ; McNamara, B. K. ; Lumetta, G. J. ; Mattigod, S. V. ; Wallen, S. L.

Pacific Northwest National Laboratory (U.S.)

关键词: Sludges;    Diffusion;    Source Terms;    Dollars;    Sediments;   

DOI  :  10.2172/834781
RP-ID  :  EMSP-81893-2002
RP-ID  :  FG07-01ER63290
RP-ID  :  834781

美国|英语

来源: UNT Digital Library

【 摘 要 】

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.