Mechanisms involved in thermal diffusion of rare earth elements in apatite

In the context of radioactive waste disposal related to the back end of the nuclear fuel cycle, we study here the diffusion of lanthanide ions in hydroxyapatite. Two materials are considered as a possible migration barrier in the nuclear waste storage process. Lanthanum and europium have been chosen as representative elements of both fission products and actinide elements and were introduced in the apatite targets using ion implantation. The analysis of the evolution of profile distribution as a function of annealing conditions leads to low values of diffusion coefficients. A possible explanation is that the lanthanide ions substitute the calcium ions in the apatite structure and diffuse through these positions. Time-resolved laser induced luminescence measurements show that the europium ions substitute the calcium ions in both hydroxyapatite Ca(I) and Ca(II) sites. Probing the two sites has been performed by extended X-ray absorption fine structure (EXAFS) measurements in order to precise the europium environment. (C) 1999 Elsevier Science B.V. All rights reserved.