Jacques Lacombe

Solubility study of Ti,Zr-based ceramics designed to immobilize long-lived radionuclides

Abstract Aqueous alteration of five Ti-Zr-oxide-based ceramics containing elements simulating long-lived radionuclides was studied experimentally by leaching at 90 °C in deionized water for more than one year under conditions of high solid/liquid ratios. Four of these ceramics were synthesized by coldcrucible melting (two Synroc-like materials, one zirconia, and one aluminotitanate) and the fifth ceramic was a hot-pressed Synroc. Melted Synroc-like ceramics have the same major constitutive phases as hot-pressed Synroc, but crystal sizes are very different, millimetric as opposed to micrometric, respectively. After the first seven days of leaching, the alteration appeared to cease as solution concentrations for all of the constituent elements attained constant values. The altered mass percentages determined from the release of Ca and Mo were less than 0.2% of the initial mass. Thermodynamic equilibrium calculations using data or estimations for pure phases, or using a model of ideal solid solutions, showed that the cessation of the alteration cannot be explained by the solubility limit of the primary phases of these ceramics. But, the data could be interpreted by the development of a passivating layer of secondary phases, e.g., hydroxides. Examination of the altered surfaces was carried out using SEM, XRD and XPS; however, the thickness of the alteration layer, estimated as 3-5 nm, was below the resolution limit of these techniques. Finally, despite the differences in the crystal size and therefore the amount of grain boundaries, and in the synthesis redox conditions, the leaching behaviors of melted and hot-pressed Synroc are the same for the present experiments.

Alteration of Cold Crucible Melter Titanate-Based Ceramics: Comparison with Hot-Pressed Titanate-Based Ceramic

Synroc ceramics were synthesized in an induction-heated cold crucible at laboratory scale (1 kg) from an oxide mixture, and at industrial prototype scale (45 kg) from Synroc previously produced by sintering under load at high temperature. After melting, both materials contained the major phases of Synroc-C. The chemical durability of both melted materials, as determined by static leaching of powder samples in initially pure water at 90°C with an SA/V ratio of 20000m −1 , was equivalent to that of conventional hot-pressed Synroc-C. Cerium, used in this investigation to simulate the presence of tri-and tetravalent actinides, was found in steady-state concentrations on the order of 1 ppb (i.e. NL(Ce) ≤ 10 −6 g·m −2 ). The concentration in the leachates was independent of the initial CeO 2 content of the Synroc (at least up to 10 wt%); moreover, it is similar to the results obtained with hot-pressed Synroc-C specifically formulated for conditioning long-lived actinides.

Strategy of coupling to model physical phenomena within molten glass bath heated by direct induction

Purpose – This paper aims to report on a vitrification process based on direct induction that has been developed by the French Atomic Energy Commission (CEA, France). This process is characterized by currents directly induced inside the molten glass and by the cooling of all the crucible walls. In addition, a mechanical stirring device is used to homogenize the molten glass. This paper presents a global modelling of coupled phenomena that take place within the glass bath.Design/methodology/approach – Electromagnetic, thermal and hydrodynamic phenomena are modelled. The aim of this study is to develop strategy of coupled modelling between these aspects. The thermohydrodynamic calculations are achieved with the Fluent software (distributed by Fluent France) and the electromagnetic aspects are solved by the OPHELIE program based on integral methods (developed in EPM laboratory).Findings – Two configurations are considered: the first deals with thermal convection in an unstirred bath and the second takes into...

Computation of eddy currents in highly conductive particles dispersed in a moderately conductive matrix

In this article, we report 3D numerical simulations of highly conductive non-magnetic particles dispersed in a moderately conductive matrix, subject to an AC magnetic field in a range of several hundred kHz. We address the issue of the scaling of current loops and heating power with respect to the volume fraction of the dispersed phase. Simulations are performed in two steps. First, a static electric potential gradient is imposed between two opposite faces of the simulation domain and an effective conductivity is computed in good agreement with percolation models. Second, the particles are constrained in a spherical sub-region and an AC magnetic field is imposed at the boundary of the domain. For small volume fractions, the induced Joule power is in good agreement with an analytical model of dilute dispersions. As the volume fraction increases, wider current loops form, until the percolation threshold is reached. Then the induced power in the spherical aggregate is well described by the power induced in an equivalent sphere with a volume-fraction-dependent conductivity.

Altération en présence d'argile humide à 70°C de céramiques à base de Ti et Zr, de type Synroc

Abstract The chemical durability of three zirconotitanate ceramics was quantified on contact with humid clay at 70°C. These materials are innovative candidates for the containment of fission products or long-lived actinides such as Np, Am and Pu. Synroc C containing molybdates is the only sample showing significant mass losses after nine months of interaction with humid clay. This may be due to a migration of Mo to the reaction interface. Mo was recovered in clay as calcium molybdate.