Chemical durability of hollandite ceramic for conditioning cesium Frédéric Angeli a, * , Peter McGlinn b , Pierre Frugier a a CEA Marcoule, DEN/DTCD/SECM/LCLT, F-30207 Bagnols-sur-Cèze, France b ANSTO, New Illawarra Road, Menai, PMB1, NSW 2234, Australia article info Article history: Received 7 March 2008 Accepted 15 July 2008 abstract The aqueous corrosion behavior of Cs-doped hollandite ceramic (BaCs 0.28 Fe 0.82 Al 1.46 Ti 5.72 O 16 ) was studied using several different static experimental protocols, with leachants of varying pH, and at different sur- face area to volume ratios, for periods ranging from six months to three years. All leach tests were carried out at 90 °C. X-ray diffraction (XRD) and scanning electron microscopy (SEM), coupled with energy dis- persive X-ray spectroscopy (EDS), were used to characterize the surfaces of the hollandite before and after leaching. The most pronounced elemental releases, and corresponding changes to surface composi- tion and microstructure, was evident at low pH, in particular pH 1. Cs and Ba releases were highest at low pH, with surface alteration exhibited by the formation of secondary rutile (prevalent at pH 1) and Al- and Ba-depleted hollandite (prevalent at pH 2). After rapid initial Cs release, the alteration rate was extremely low over the pH range from 2 to 10, as well as in pure water experiments with a sample-surface-area-to- solution-volume ratio ranging from 0.1 cm 1 to 1200 cm 1 . The rates were about 10 5 gm 2 d 1 , corre- sponding to alteration thicknesses of a few nanometers per year. Higher rates (5  10 3 gm 2 d 1 ) were observed only under very acidic conditions (pH 1). Congruency in Cs and Ba releases occurred only at pH 1, with incongruency between the two elements increasing with increasing pH. There were no apparent solubility constraints on Cs releases regardless of the SA/V ratio, whereas geochemical modeling sug- gested that Ba releases could have been affected by the formation of BaCO 3 , particularly at high SA/V ratios. Extended leaching (with the leachant renewed once after 261 days of leaching) confirmed the high durability of hollandite with altered thicknesses of less than one nanometer per year over the last two years. Whilst Cs depletion of the hollandite surface was evidenced when leachates were replenished with the fresh deionised water, the presence of a soluble Ba-bearing secondary phase was inferred. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Hollandite is one of the major phases of Synroc, a multiphase ceramic matrix also containing zirconolite and perovskite phases [1] that is being considered as a matrix for immobilizing Cs result- ing from enhanced separation of fission product solutions from spent fuel reprocessing [2]. The advantage of this material is that it also incorporates Ba, a decay element of Cs. It also exhibits good thermal conductivity [3], an advantage due to the radiogenic heat produced by the decay of 137 Cs and 134 Cs, as well as good electronic irradiation resistance [4]. The hollandite formula proposed for Cs immobilization is Ba 2þ x Cs þ y M 3þ 2xþy Ti 4þ 82xy O 16 (x 6 2) in which M is a trivalent cation, for example Ti 3+ , Al 3+ or Fe 3+ , ensuring the charge compensation necessary for loading Ba and Cs. With Ti 4+ , it forms octahedra that create a tunnel structure in which mono- or divalent cations (Cs and Ba, respectively) are inserted. The composition used in this study was a ferriferous hollandite containing Al: Ba 1 Cs 0.28 Fe 0.82 - Al 1.46 Ti 5.72 O 16 (which corresponds to the following oxide mass composition: 19.4% BaO, 5% Cs 2 O, 8.3% Fe 2 O 3 , 9.4% Al 2 O 3 , 57.9% TiO 2 ). Adding iron allows pressureless consolidation (in air) through better control of the redox conditions during the synthesis process, without subsequently degrading the chemical durability of the matrix [5]. This type of composition contains no soluble secondary phases [6]. This point is of particular importance for hollandite disposal in a geological formation as the conditioning matrix must be resis- tant to aqueous dissolution with radioelement retention properties minimizing their release into the environment. Hollandite formula- tions are known for their good aqueous corrosion resistance [5–9]. After leaching Fe-free hollandite in deionized water at 75 °C and 150 °C, Pham et al. [8] reported very rapid initial release of Ba and Cs followed by dissolution of the upper atomic layers. The leach rate then dropped by at least an order of magnitude over the ensuing week. This was attributed to the formation of a thin passivating layer rich in Al and Ti. The authors noted a drop in the pH due to hydrolysis of Al, which reportedly precipitated on the surface as AlOH and/or Al(OH) 3 . Carter et al. [6] measured very low Cs and Ba leach rates—below 0.001 g m 2 d 1 after 56 days at 90 °C in pure 0022-3115/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2008.07.003 * Corresponding author. E-mail address: frederic.angeli@cea.fr (F. Angeli). Journal of Nuclear Materials 380 (2008) 59–69 Contents lists available at ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat