Estimated duration of the subsurface reduction environment produced by the salt-stone disposal facility on the Savannah River Site

The formula for Savannah River Site (SRS) salt-stone includes ∼25 wt% slag to create a reducing environment for mitigating the subsurface transport of several radionuclides, including Tc-99. Based on laboratory measurements and two-dimensional reactive transport calculations, it was estimated that the SRS salt-stone waste form will maintain a reducing environment, and therefore its ability to sequester Tc-99, for well over 10,000 years. For example, it was calculated that ∼16% of the salt-stone reduction capacity would be consumed after 213,000 years. For purposes of comparison, a second calculation was presented that was based on entirely different assumptions (direct spectroscopic measurements and diffusion calculations). The results from this latter calculation were near identical to those from this study. Obtaining similar conclusions by two extremely different calculations and sets of assumptions provides additional credence to the conclusion that the salt-stone will likely maintain a reducing environment in excess of 10,000 years. (authors)