Durabilities of Pyrochlore-Rich Titanate Ceramics Designed for Immobilization of Surplus Plutonium

Abstract

The chemical durabilities of two Pu-doped pyrochlore samples were studied by Single-Pass-Flow-Through (SPFT) tests at 70°C. The dissolution of pyrochlore is incongruent with preferential releases of Ca and Gd over Ti, close to stoichiometric releases of U and Ti, and lower releases of Hf and Pu than Ti. Altered pyrochlore and polymorphs of TiO2 (brookite and probably anatase) have been identified on the surface of the leached sample and the principal secondary phase is an unknown polymorph of TiO2 containing Hf and varying amounts of Gd and Pu. These surface alteration phases are consistent with reported studies of natural samples. The releases of U, Gd, Ca and Ti into solution follow linear kinetics, whereas the releases of Pu and Hf exhibit non-linear behavior. The presence of ∼5% PuO2 and trace amounts of glass does not appear to have an effect on the overall durability of the material. Further, the low Pu release rate and the similar kinetics for Pu and Hf releases limit the possibility of nuclear criticality under repository conditions. Overall, this study provides useful information on the lower bounds of durabilities of the materials.

Titanate ceramics for immobilisation of uranium-rich radioactive wastes arising from 99Mo production

“…The approximate composition was Ca 1.1 U 0.7 Zr 0.1 Al 0.05-Ti 1.95 O 7 and other workers have reported pyrochlores with Ca contents of >1 f.u. [20][21]. This has been attributed to some of the U being in a valence state of >4+.…”

Leach rates of uranium and thorium from murataite ceramics

“…Comparing our results with reference data [8][9][10][11] note that the average U leach rate obtained (1.42 × 10 −3 g/ (m 2 × d)) is close to the value of ∼ 0.89 × 10 −3 g/(m 2 × d) measured for U leaching from titanate pyrochlore-based ceramic at pH = 2 in work [9]. Like in our work, in the work [9] steady-state for U has not been reached.…”

“…Like in our work, in the work [9] steady-state for U has not been reached. The U leach rate at pH = 5.6 and T = 70 • C from pyrochlorebased ceramics was found to be ∼ 10 −4 g/(m 2 × d) [10] that is some higher than the value 2.4 × 10 −5 g/(m 2 × d) obtained in our previous work [6] at pH ≈ 6-6.5 and T = 90 • C. REEs (Ce, Gd, Lu) exhibit some higher leach rates from pyrochlore-based ceramics as compared with U under the same conditions [9][10][11] whereas Pu is lower leachable element -its average leach rate was found to be ∼ 8 × 10 −5 g/(m 2 × d) at pH = 2 and T = 90 • C [9] and ∼ 10 −6 -10 −5 g/(m 2 × d) at pH = 5.6 and T = 70 • C [10].…”

Pyrochlore-structured titanate ceramics for immobilisation of actinides: Hot isostatic pressing (HIPing) and stainless steel/waste form interactions

“…These latter waste forms are largely based on a major phase, e.g., brannerite [7,8], hollandite [9], zirconolite [10,11] and pyrochlore [12,13], etc. Pyrochlore-rich ceramics are among the specific compositions developed initially for immobilisation of surplus impure plutonium using sintering as the consolidation process [14] and have since been tailored to suit various actinide-rich waste streams. Nevertheless, all these waste forms contain sufficient amounts of targeted additional phases to allow compositional flexibility to deal with variations in waste chemistry and waste/ precursor ratio [12,13].…”