Patricia Paviet-Hartmann

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 the reason, treatment of the portland cement based waste-form with supercritical carbon dioxide (SCCO[sub 2]) is shown to satisfy regulations. The effect of SCCO[sub 2] treatment by applying different CO[sub 2] pressure and temperature conditions on the leachability, phase constitution, and microstructure of surrogate-groped 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)[sub 2], CaCO[sub 3]) as well as the microstructure were measured by X-ray diffraction and scanning electron microscopy. SCCO[sub 2] treatment at 8.4 MPa and 25 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 rate of the surrogates [sup 232]Th, and [sup 151/153]Eu.

Chemical speciation of heterogeneously reduced Pu in synthetic brines

Summary X-ray absorption fine structure (XAFS) spectroscopy has been used to determine the speciation of Pu precipitates prepared by the heterogeneous reduction of Pu(VI) with Al and Fe in 5 M NaCl and an ERDA-6 brine, a simulant from the Waste Isolation Pilot Plant in Carlsbad, New Mexico. NaOCl was added to some of these solutions to determine its effect on Pu speciation. Analysis of the Pu LIII spectra showed that all solids consisted of PuO2+x-y(OH)2y·z H2O, compounds with characteristics identical to those prepared by hydrolysis and with Pu-O and Pu-Pu distances identical to those treated at elevated temperature. Additionally, reduction with Al gave compounds with different site distributions than reduction with Fe, and reduction with Al or the addition of NaOCl appeared to suppress the formation of oxo groups and their associated Pu(V) sites.

Supercritical CO{sub 2} carbonation of cemented radioactive waste-forms: Influence on leachability and structure

The traditional practice of cementing transuranic (TRU) low-level waste poses several technical problems, the most serious of which is that many of the legacy TRU waste-forms do not meet Department of Transportation (DOT) prohibitions on decay heat and/or free liquid. To address these problems, a treatment of cemented waste-forms with supercritical CO{sub 2} (SCCO{sub 2}) has been proposed. This treatment method alters the bulk chemical and structural properties of cast cement by accelerating natural carbonation reactions, while at the same time reducing both free and bound water. Reducing the amount of the hydrogenous content of a cemented waste-form to below 30 wt% simultaneously fulfills the DOT free-liquid requirement and increases the maximum allowable decay heat of the TRU-PACT-II drums by a factor of 4. The effect of SCCO{sub 2} treatment applying different CO{sub 2} pressure and temperature conditions (8.4 MPa < p < 28 MPa, 35 C < T < 62 C) on the leachability, phase constitution, and microstructure of surrogate-doped Portland cement type I/II samples was investigated.

Ash Cements Stabilized by Supercritical Co2 Carbonation for Tailings Pond Overlayer

Estonia possesses large oil shale deposits. The kukersite shale, located in the northeastern part of the country, is arguably the most important mineral resource of Estonia. There are two principal deposits; the Estonian and the Tapa. The former has a productive seam thickness that varies from 2.7-3.0 m in the northern part of the deposit to 1.4-2.0 m in the southern and western parts. The Tapa deposit is situated southwest of the Estonian deposit with a seam depth of 60-170 m below the surface. This seam has a maximum thickness 2.0-2.3 m in the central part of the deposit. This deposit is not currently in use. These two deposits cover a combined area of about 5000 km2.