The effect of supercritical carbon dioxide treatment on the leachability and structure of cemented radioactive waste-forms

The former process for the cementation of transuranic (TRU) low-level wastes poses several technical problems. Specifically in the US a TRU waste-form has not yet passed the Waste Isolation Pilot Plant prohibition for free liquid. For this reason, treatment of the portland cement based waste-form with supercritical carbon dioxide SCCO2 is shown to satisfy regulations. The effect of SCCO2 treatment by applying different CO2 pressure and temperature conditions (8.4 MPa < p < 28 MPa, 35 degrees C < T < 62 degrees C) on the leachability, phase constitution, and microstructure of surrogate-doped portland cement type I/II samples is presented. Leaching studies were performed using a synthetic groundwater leaching procedure. Changes in phase constitution of the major crystalline phases (Ca(OH)(2), CaCO3) as well as the microstructure were measured by X-ray diffraction and scanning electron microscopy. SCCO2 treatment at 8.4 MPa and 35 degrees C can be shown as the most promising conditions to satisfy the requirements of the Department of Transportation (DOT) and to enhance the natural aging reaction of cement paste by carbonation, combined with the lowest release rates of the surrogates Th-232 and (EU)-E-151/153. (C) 1999 Elsevier Science Ltd. All rights reserved.