Glass–ceramic nuclear waste forms obtained from SiO 2 –Al 2 O 3 –CaO–ZrO 2 –TiO 2 glasses containing lanthanides (Ce, Nd, Eu, Gd, Yb) and actinides (Th): study of internal crystallization P. Loiseau a , D. Caurant a, * , N. Baffier a , L. Mazerolles b , C. Fillet c a Laboratoire de Chimie Applique ´e de l’Etat Solide (UMR 7574), ENSCP, 11 rue Pierre et Marie Curie, 75231 Paris cedex 05, France b CECM (UPR 2801) 15 rue Georges Urbain, 94407 Vitry-sur-Seine, France c CEA, Direction de l’Energie Nucle ´aire, Centre de la Valle ´eduRho ˆne, DIEC/SCDV/LEBM, 30207 Bagnols-sur-Ce `ze, France Received 26 January 2004; accepted 18 May 2004 Abstract Glass–ceramic waste forms such as zirconolite (nominally CaZrTi 2 O 7 ) based ones can be envisaged as good candi- dates for minor actinides or Pu immobilization. Such materials, in which the actinides (or lanthanides used as actinide surrogates) would be preferentially incorporated into zirconolite crystals homogeneously dispersed in a durable glassy matrix, can be prepared by controlled crystallization (nucleation + crystal growth) of parent glasses belonging to the SiO 2 –Al 2 O 3 –CaO–ZrO 2 –TiO 2 system. In this work we present the effects of the nature of the minor actinide surrogate (Ce, Nd, Eu, Gd, Yb, Th) on the structure, the microstructure and the composition of the zirconolite crystals formed in the bulk of the glass–ceramics. The amount of lanthanides and thorium incorporated into zirconolite crystals is dis- cussed in relation with the capacity of the glass to accommodate these elements and of the crystals to incorporate them in the calcium and zirconium sites of their structure. Ó 2004 Elsevier B.V. All rights reserved. PACS: 81.05.Kf; 81.05.Pj; 81.40.Ef 1. Introduction Due to the ease of waste immobilization processes in glassy matrices (by melting and casting) and the high structural flexibility offered by such disordered materi- als, they are currently used (e.g. aluminoborosilicate glasses) to incorporate the broad range of high level radioactive wastes (HLW) – fission products + minor actinides–originatingfromthereprocessingofcommer- cial or defense spent fuels. Nevertheless, because of both the higher chemical durability and the higher capacity of several single-phase ceramic matrices such as zirconates, titanates and phosphates [1–3] to incorporate specific nuclear wastes (corresponding either to a specific ele- ment or to a family of elements with similar chemical properties), ceramic waste forms have been proposed for the immobilization of long-lived radionuclides such as excess weapons plutonium and minor actinides. 0022-3115/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2004.05.020 * Corresponding author. Tel.: +33 1 44 27 67 08/24; fax: +33 1 46 34 74 89. E-mail address: caurant@ext.jussieu.fr (D. Caurant). Journal of Nuclear Materials 335 (2004) 14–32 www.elsevier.com/locate/jnucmat