SALTSTONE AND RADIONUCLIDE INTERACTIONS: RADIONUCLIDE SORPTION AND DESORPTION, AND SALTSTONE REDUCTION CAPACITY

The overall objective of this study was to measure a number of key input parameters quantifying geochemical processes in the subsurface environment of the Savannah River Site's (SRS's) Saltstone Facility. For the first time, sorption (K{sub d}) values of numerous radionuclides were measured with Saltstone and Vault 2 concrete. Particular attention was directed at understanding how Tc adsorbs and desorbs from these cementitious materials with the intent to demonstrate that desorption occurs at a much slower rate than adsorption, thus permitting the use of kinetic terms instead of (or along with) the steady state K{sub d} term. Another very important parameter measured was the reduction capacity of these materials. This parameter is used to estimate the duration that the Saltstone facility remains in a reduced chemical state, a condition that maintains several otherwise mobile radionuclides in an immobile form. Key findings of this study follow. K{sub d} values for Am, Cd, Ce, Co, Cs, Hg, I, Np, Pa, Pu, Se, Sn, Tc, U, and Y for Saltstone and Vault 2 concrete were measured under oxidized and reduced conditions. Precipitation of several of the higher valence state radionuclides was observed. There was little evidence that the Vault 2 and Saltstone K{sub d} values differed from previous SRS K{sub d} values measured with reducing grout (Kaplan and Coates 2007). These values also supported a previous finding that K{sub d} values of slag-containing cementitious materials, tend to be greater for cations and about the same for anions, than regular cementitious materials without slag. Based on these new findings, it was suggested that all previous reducing concrete K{sub d} values be used in future PAs, except Np(V) and Pu(IV) K{sub d} values, which should be increased, and I values, which should be slightly decreased in all three stages of concrete aging. The reduction capacity of Saltstone, consisting of 23 wt-% blast furnace slag, was 821.8 microequivalents per gram ({micro}eq/g). This value was approximately the same value as the one measured for 100% blast furnace slag. The cause for this approximately four-fold greater reduction capacity than anticipated is not known, but may be the result of the higher pH of Saltstone (pH {approx}11) compared to blast furnace slag (pH {approx}8), the presence of reducing minerals in the fly ash used to make the Saltstone, or to the Saltstone possibly having semi-conductor properties. These reduction capacity values will result in a near four-fold increase in the estimated duration that the Saltstone facility will remain in a reduced chemical state. The implication of this result is that oxidation-state-sensitive contaminants, such as Pu, Np, and Tc, will remain for a longer duration in a much less mobile form than previously believed. The reduction capacity of vault concrete, which consisted of 10 wt-% blast furnace slag, was 240 {micro}eq/g. Essentially all Am, Cd, Ce, Co, Cs, Hg, Sr, and Y was (ad)sorbed within four hours, whereas <3% of the adsorbed metals desorbed from these solids after 90 hours of continuous leaching. In particular, desorption of Tc (under oxidizing conditions) was >10{sup 3} fold slower than (ad)sorption (under reducing conditions). An important implication of this finding is that if groundwater by-passes or short-circuits the reduction capacity of the Saltstone by flowing along a crack, the ability of the oxygenated water to promote Tc desorption is appreciably less than that predicted based on the K{sub d} value. Relatively low Tc K{sub d} values, 6 to 91 mL/g, were measured in these studies indicating that little if any of the Tc(VII) introduced into the Saltstone or Vault 2 concrete suspensions was reduced to Tc(IV). Such a reduction results in apparent K{sub d} values on the order of 10{sup 4} mL/g. As such, these Tc sorption/desorption experiments need additional investigation to fully represent Saltstone environmental conditions. It is important to understand the limits of these data. They do not provide insight into how radionuclides cured and immobilized in Saltstone will leach from the Saltstone. However they do provide insight into how radionuclides once released into porewater will interact with Saltstone or vault concrete. The use of these site-specific data would greatly improve the pedigree of the input data for the Saltstone performance assessment. Additionally, these studies provided important guidance and technical justification for the conceptual geochemical model to be used in the Saltstone performance assessment