Leachability of Nd, U, Th, and Sr from Cements in a CO2 Free Atmosphere

Abstract

AbstractTo determine the leachability and concentration-controlling solids for nuclides contained in solidified low-level nuclear wastes, cement doped with neodymium, uranium, thorium and strontium was prepared and cured in a CO2-free atmosphere. Leaching tests were conducted by equilibrating aliquots of the crushed doped cement with CO2-free deionized water at pcH+ values ranging from 7.5 to 12.5 in an effort to determine if solubility constraints could be used to predict leachable concentrations of Nd (an analog of trivalent actinides), U, Th, and Sr. Steady state concentrations of Nd, U, Th and Sr were attained within a few days. The observed Nd concentrations continually decreased with the increase in pcH+ from about 7.5 to 9.0, and were at detection limit at pcH+ values > 9.0. The comparison of these results with thermodynamic data suggests that Nd(OH)3(am) is the controlling solid phase. The observed U concentrations decreased by two orders of magnitude with a unit increase in pcH+ in the range of 7.5 to 9.0 and were at the detection limit at pcH+ > 9.0. From the multitude of U(VI) solids that can form under the test conditions, the data suggest that CaUO4 is the likely controlling solid. The concentration of Th was at the detection limit over the entire pcH+ range covered. Based on previous work, ThO2(am) is likely to be the controlling solid. In the absence of CO2, no solubility-controlling solid was found for Sr.