Alteration of a Zirconolite Glass-Ceramic Matrix under Hydrothermal Conditions
- PDF / 623,650 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 22 Downloads / 277 Views
Alteration of a Zirconolite Glass-Ceramic Matrix under Hydrothermal Conditions Christelle Martin, Isabelle Ribet and Thierry Advocat Commissariat à l’Énergie Atomique (CEA), Valrhô–Marcoule BP 17171, 30207 Bagnols-sur-Cèze Cedex, France ABSTRACT Glass-ceramic matrices based on zirconolite (CaZrTi2O7) are being considered for specific conditioning of plutonium or the minor actinides. The actinides are distributed throughout the zirconolite crystals and the residual glass phase. Since zirconolite alteration is extremely limited, however, actinide release from the glass-ceramic material is mainly attributable to alteration of the residual glass. Zirconolite glass-ceramic specimens and specimens corresponding to the residual glass phase alone were therefore altered under hydrothermal conditions (150°C) and under initial rate conditions (100°C) to compare their kinetic behavior and estimate the effect of the crystals on material alteration. Under hydrothermal conditions, alteration occurred during the first few days: SEM observations showed greater alteration of the glass-ceramic material due to a phenomenon of preferential glass alteration around the zirconolite crystals; after three days the alteration rate had considerably diminished and both specimens exhibited similar behavior. Under initial rate conditions the initial rates differed due to a variation in the reactive surface area of the glass-ceramic. INTRODUCTION Glass-ceramic matrices based on zirconolite (CaZrTi2O7) are being considered for specific conditioning of plutonium or the minor actinides. This application is contingent on a demonstration of the long-term aqueous corrosion resistance of the material. One investigative method consists in leaching monolithic specimens under hydrothermal conditions [1-4] to accelerate the dissolution kinetics and obtain data on advanced stages of reaction progress in a relatively short time. The test objectives are to substantiate the much greater contribution of the residual glass to the total actinide release, to compare the kinetic behavior of the two materials and assess the influence of the crystals on alteration of the glass-ceramic, and to predict the evolution of the zirconolite crystals in contact with the alteration film at the surface of the residual glass. EXPERIMENTAL PROTOCOLS Tests were conducted with a zirconolite glass-encapsulated ceramic doped with neodymium to simulate the actinides, and with a “reference” glass (Table I) corresponding to the residual glass matrix in the glass-ceramic. A small quantity of B2O3 was added to both materials as an alteration tracer. Monolithic specimens of each material were polished to grade 4000 with SiC paper. Table I. Reference glass composition (wt%) corresponding to the residual glass phase in the glass-ceramic Oxide
SiO2
Al2O3
CaO
TiO2
ZrO2
Nd2O3
B2O3
Residual glass
46.71
13.52
20.81
7.66
3.99
6.3
1
JJ6.4.1
Hydrothermal alteration tests were conducted in 0.125-liter stainless steel PARR vessels with PTFE seals, for periods of 1, 3, 10, 40, 60 and 85
Data Loading...
