Transmission electron microscopy (TEM) experiments were carried out on the JANNUS platform (Joint Accelerators for Nano-science and NUclear Simulation) at CSNSM (Center of Nuclear Spectrometry and Mass Spectrometry) laboratory in Orsay. The experiment was devoted to the study of the evolution of the xenon aggregate population with increasing implantation fluence. A thin UO2 foil was implanted at fluences ranging from 3 x 10(12) to 7 x 10(14) at cm(-2) with 390 key Xe3+ ions at an irradiation temperature of 873 K. The TEM results indicate the presence of nanometer size bubbles above a fluence of 6 x 10(12) Xe cm(-2) and an increase in the bubble number density was observed between 6 x 10(12) Xe cm(-2) and 2 x 10(14) Xe cm(-2). Above 2 x 10(14) Xe cm(-2), the number density levels off at 4 x 10(23) +/- 0.5 x 10(23) m(-3). (C) 2011 Elsevier B.V. All rights reserved.