Experimental Study of Contaminant Release from Reducing Grout

¹ Center for Nuclear Waste Regulatory Analyses, San Antonio, Texas, U.S.A.
² U.S. Nuclear Regulatory Commission, Washington, D.C., U.S.A.

Abstract

A column experiment was conducted to study the release behavior of technetium, uranium, and selenium initially sequestered in reducing grout similar in composition to Savannah River Site (SRS) saltstone, a cementitious waste form made by mixing salt solution from SRS liquid waste storage tanks with a dry mix containing blast furnace slag, fly ash, and Portland cement. The data suggest that uranium was retained in the grout possibly as a CaUO₄ phase, whereas most of the selenium was released. Technetium release initially was relatively constant, and then increased significantly after 26 pore volumes. The increase in technetium release was slightly delayed relative to the observed Eh increase. The system Eh-pH started under conditions in which technetium solubility is low, constrained by Tc₃O₄ solubility, but eventually transitioned into the stability field of the pertechnetate ion. The delay in technetium release relative to the Eh increase was possibly due to slow oxidation of technetium at depth within the grout particles, which in turn was likely controlled by O₂ diffusion into the particles. In contrast to technetium and uranium, selenium release was not solubility limited and selenium likely was present in the pore solution initially as a HSe⁻ species.