The Hanford Site in southeastern Washington State has been used extensively by the U.S. Department of Energy (DOE) to produce nuclear materials for the U.S. strategic defense arsenal. A large inventory of radioactive and mixed waste has accumulated in 177 buried single- and double-shell tanks. Liquid waste recovered from the tanks will be pre-treated to separate the low activity fraction from the high-level and transuranic wastes. The low-activity waste (LAW) will be immobilized in glass and placed in a near-surface disposal system on the Hanford Site. Vitrifying the LAW will generate over 160,000 m3 of glass. Before the immobilized low-activity waste (ILAW) can be disposed, DOE must approve a performance assessment (PA), which is a document that describes the long-term impacts of the disposal facility on public health and environmental resources. A sound scientific basis for determining the long-term release rates of radionuclides from LAW glasses must be developed if the PA is to be accepted by regulators and stakeholders. To conduct this calculation, Pacific Northwest National Laboratory (PNNL) used a methodology in which the waste form release rate was calculated by modeling the basic physical and chemical processes that are known to control dissolution behavior using a reactive transport code, STORM [1]. This methodology was used instead of empirical extrapolations from laboratory “leaching” experiments commonly used in other PA or in the phenomenological approach of SIA “Radon” [2]. This methodology is preferred because the dissolution rate, and hence radionuclide release rate, from silicate glasses is not a static variable—a constant that can be derived independently of other variables in the system. Glass dissolution rate is a function of three variables (neglecting glass composition itself): temperature, pH, and composition of the fluid contacting the glass. SIA Radon has been running a field experiment for over 12 years to evaluate the behavior of a high sodium glass buried in a loamy soil. The radioactive waste glass (K-26) made from actual intermediate-level waste from the Kursk (RBMK) reactor was manufactured and placed in a shallow trench. The waste stream was 86 mass% NaNO3, very similar to the salt content expected for Hanford LAW. The final glass composition had a Na2O content of roughly 16 mass%, making it very relevant to the glass formulations being considered at Hanford. A joint US DOE-SIA Radon project was devised to validate the modeling approach used for the ILAW PA by modeling glass corrosion in the subsurface experimental facility [3]. This paper gives an estimate of glass corrosion and ion exchange rates for K-26 waste glass based on field measurements.

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