Pretreatment of Tc-Containing Waste and its Effect on Tc-99 Leaching from Grouts
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0985-NN10-02
Pretreatment of Tc-Containing Waste and its Effect on Tc-99 Leaching from Grouts Albert Aloy1, Elena N. Kovarskaya1, John R. Harbour2, Christine A. Langton2, and E. William Holtzscheiter2 1 RosAtom, "V.G. Khlopin Radium Institute" (KRI), 2-nd Murinsky str., 28, Saint Petersburg, 194021, Russian Federation 2 Savannah River National Laboratory, Washington Savannah River Company, Savannah River National Laboratory, Aiken, SC, 29808 ABSTRACT A salt solution (doped with Tc-99), that simulates the salt waste stream to be processed at the Saltstone Production Facility, was immobilized in grout waste forms with and without (1) ground granulated blast furnace slag and (2) pretreatment with iron salts. The degree of immobilization of Tc-99 was measured through monolithic and crushed grout leaching tests. Although Fe (+2) was shown to be effective in reducing Tc-99 to the +4 state, the strong reducing nature of the blast furnace slag present in the grout formulation dominated the reduction of Tc-99 in the cured grouts. An effective diffusion coefficient of 4.75 x 10-12 (Leach Index of 11.4) was measured using the ANSI/ANS-16.1 protocol. The leaching results show that, even in the presence of a concentrated salt solution, blast furnace slag can effectively reduce pertechnetate to the immobile +4 oxidation state. The measured diffusivity was introduced into a flow and transport model (PORFLOW) to calculate the release of Tc-99 from a Saltstone Vault as a function of hydraulic conductivity of the matrix. INTRODUCTION Previous work between Savannah River National Laboratory and Sandia National Laboratory demonstrated that ground granulated blast furnace slag (BFS), as part of the cementitious component of a grout formulation, can reduce and immobilize pertechnetate to Tc (+4) either in water or a dilute salt solution [1]. The grout formulation used in that work has been proposed as the reducing grout for Hanford Tank Closure. The Saltstone Production Facility at Savannah River will treat a waste stream with higher salt concentrations ([Na+] of ~2 to 6 M). It is therefore important to determine whether Tc-99 can also be immobilized through grouting in the presence of high concentrations of nitrate, nitrite, and hydroxide ions. Pretreatment of the salt solutions with iron salts was investigated to determine whether additional benefit could be obtained through pretreatment of pertechnetate. EXPERIMENTAL The composition of the simulant used in this study is provided in table I. This simulant had a density of 1.22 g/cm3, a pH of 12.4, and ~1.8 x 10-5 Ci/L of Tc-99. Technetium-99 was measured by detecting its β-activity using a Quantulus-1220 spectrometer manufactured by Wallac of Finland (with an Optiphase HiSafe2 liquid scintillator). The error of the radiometric measurements was ± 5 %, while the sensitivity of the measurements was 0.25 Bq per sample.
Table I. Composition of the salt waste stream that has been spiked with Tc-99. Content wt% M
Components H2O NaNO2 NaNO3 Na2SO4 NaOH NaAl(OH)4 Na2CO3 71 3.0 14.0 2.5 2.5
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