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

摘要

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 (SCCO_2) is shown to satisfy regulations. The effect of SCCO_2 treatment by applying different CO_2 pressure and temperature conditions (S.4 MPa < p < 28 MPa, 35℃ < T< 62℃) 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, CaCO_3) as well as the microstructure were measured by X-ray diffraction and scanning electron microscopy. SCCO_2 treatment at 8.4 MPa and 35℃ can be shown as the most promising conditions to satisfy the requirements of the Depart- ment of Transportation (DOT) and to enhance the natural aging reaction of cement paste by carbonation, combined with the lowest release rates of the surrogates ~(232)Th, and ~(151/153)Eu.