Dissolution kinetics of pyrochlore ceramics for the disposition of plutonium

• Published in: The American mineralogist Vol. 91; no. 1; pp. 39 - 53
• Main Authors: Icenhower, Jonathan P, Strachan, Denis M, McGrail, B. Pete, Scheele, Randall D, Rodriguez, Elsa A, Steele, Jackie L, Legore, Virginia L
• Format: Journal Article
• Language: English
• Published: Washington Mineralogical Society of America 01.01.2006; De Gruyter; Walter de Gruyter GmbH
• Subjects: actinides; amorphization; amorphous materials; ceramic materials; Ceramics; CERIUM OXIDES; CHEMICAL REACTION KINETICS; Crystal structure; DISSOLUTION; dissolution rate; experimental; Flow rates; Kinetics; MANAGEMENT OF RADIOACTIVE WASTES, AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES; MATERIALS SCIENCE; metals; Mineralogy; nonsilicates; phase equilibria; PHASE STUDIES; phase transition; Phase transitions; Plutonium; PLUTONIUM OXIDES; PYROCHLORE; radiation damage; radioactive waste; solution; Titanium alloys; Titanium dioxide; TITANIUM OXIDES; URANIUM OXIDES; waste disposal; WASTE FORMS; X-ray diffraction data; XRD data; ZIRCONOLITE
• ISSN: 0003-004X; 1945-3027
• DOI: 10.2138/am.2006.1709

Single-pass flow-through (SPFT) experiments were conducted on a set of non-radioactive Ti-based ceramics at 90 °C and pH = 2 to 12. The specimens contained 27.9 to 35.8 wt% CeO2 as a surrogate for UO2 and PuO2. Compositions were formulated as TiO2-saturated pyrochlore (CeP1) and pyrochlore-rich baseline (CePB1) ceramic waste forms. Pyrochlore + Hf-rutile and pyrochlore + perovskite + Hf-rutile constituted the major phases in the CeP1 and CePB1 ceramics, respectively. Results from dissolution experiments between pH = 2 to 12 indicate a shallow pH-dependence with an ill-defined minimum. Element release rates determined from experiments over a range of sample surface areas (S) and flow rates (q) indicate that dissolution rates become independent of q/S values at 10-8 to 10-7 m/s. Dissolution rates dropped sharply at lower values of q/S, indicating rates that are subject to solution saturation effects as dissolved constituents become concentrated. Forward dissolution rates were 1.3(0.30) × 10-3 and 5.5(1.3) × 10-3 g/m2·d for CeP1 and CePB1 ceramics, respectively. Dissolution rates obtained in other laboratories compare well to the findings of this study, once the rate data are placed in the context of solution saturation state. These results make progress toward an evaluation of CeO2 as a surrogate for UO2 and PuO2 as well as establishing a baseline for comparison with radiation-damaged specimens.