Long-Term Technetium Interactions With Reducing Cementitious Materials

Abstract

Revision 0 can oxidize technetium in a portland cement or saltstone-like monolith. The shrinking core model used in the saltstone performance assessment describes the existence of an oxidized outer layer of concrete surrounding a shrinking core of reducing intact saltstone. A sharp boundary between the two zones moves slowly inward, resulting in oxidation of Tc(IV). This work largely reinforced our conceptual model of the shrinking core model, but more importantly provided clarity regarding process kinetics, mechanisms, and input values for future detailed modeling. The shrinking core model in itself is devoid of geochemical mechanisms. This study provided more geochemical detail to this conceptual numerical model. vii SRNL-STI-2010-00668 Revision 0

Distribution Coeficients (Kd) Generated From a Core Sample Collected From the Saltstone Disposal Facility

“…Previous desorption studies by Kaplan et al (2008) reported K d values similar to those measured for this report where they measured a K d in an oxidizing environment of 0.93 mL/g and a reduced value of 91.3 mL/g, although there is some question to the validity of the latter value because of interference in the analytical data used to calculate the K d (Kaplan et al 2008). In a kinetic study, Kaplan et al (2011) measured steadily increasing technetium K d values from 2 to 900 mL/g as the contact time of Tc(VII) increased from 1 to 56 days. In these latter studies, a glovebox was used and atmospheric O 2 was maintained much lower <0.5 ppm, (this work 30-60 ppm), and suspension Eh was typically -250 mV.…”

“…A concern is that the slightest amount of O2 oxidizes the Tc(IV) to Tc(VII). In a kinetic study, Kaplan et al (2011) measured steadily increasing Tc Kd values from 2 to 900 mL/g as the contact time of Tc(VII) increased from 1 to 56 days. In these latter studies, a glovebox was used and atmospheric O2 was maintained much lower, <0.5 ppm, and suspension Eh was typically -250 mV.…”

“…It is recommended that the best value technetium K d be lowered to 1000 mL/g for reducing conditions. The recommended values are based on several values (Kaplan et al 2011), including literature values and site specific values. One reason these values were discounted was because the partial pressure of oxygen was slightly higher (30 to 60 ppm) than is expected in a reducing cement, such as saltstone (<0.5 ppm O 2 ).…”

“…One reason these values were discounted was because the partial pressure of oxygen was slightly higher (30 to 60 ppm) than is expected in a reducing cement, such as saltstone (<0.5 ppm O 2 ). When O 2 <0.5 ppm, Tc K d = 1000 mL/g (Kaplan et al 2011). This K d value of 139 mL/g represents the K d near the oxygen front.…”

Technetium Sorption By Cementitious Materials Under Reducing Conditions

“…It is immobilized in saltstone by (1) minimizing diffusion within the pore spaces of the cementitious wasteform, and (2) promoting the reduction of the highly mobile Tc(VII) form to a highly immobile Tc(IV) form by blast furnace slag. Kaplan et al (2011; reporting the thesis work by Lilley (2010)) attempted to measure its equilibrium constant by conducting (ad)sorption TcO 4 -studies in a glovebag. The objective was to measure the distribution coefficient, K d value, between solid phase saltstone and the aqueous phase.…”