The Oxide Dispersion Strengthened (ODS) materials are potential candidates as cladding tubes for Sodium-cooled Fast Reactors. The nano-oxides are finely dispersed within the grains and confer excellent mechanical properties to these alloys. Hence, assessing nano-particle stability under irradiation remains crucial to guarantee safe use of these materials. Although neutron irradiation remains a binding and challenging experimental study to conduct, difficulties can be overcome by ion beam processing. Ion beam processing of the ODS material allows to identify the radiation-induced Ostwald ripening as the mechanism governing the nano-particle response under irradiation. The result is the increase in size and a decrease in density of the finely dispersed Y2Ti2O7 nano-particles. Under neutron irradiation, radiation-induced Ostwald ripening appears to be less effective since a slight growth of nano-particles is observed. Further, our approach shows that nanoparticle growth kinetics should scale as φ1/3, φ being the radiation flux. This suggests that the low irradiation flux is at the origin of the slower growth kinetics of the neutron irradiated particles. Both neutron and ion irradiation induce a modification of the nanoparticles/matrix interfaces which are generally flat and sharp prior to irradiation and present steps after irradiation. This could alter the nano-particle coarsening during irradiation.

This work is an overview of the physical approaches required for characterizing and understanding the long-term evolution of ceramics under irradiation. Because this subject is complex and has many ramifications, we have chosen to address the problem by looking at the behavior of a number of key ceramics. In the first part of this work, we present the physical mechanisms responsible for the production of primary defects, pointing out the main differences between metals, semiconductors, and insulators. In part two, we attempt to show how devoted experimental techniques can combine with transmission electron microscopy and x-ray techniques to provide a clearer picture of the long-term evolution of the microstructure of ceramics under irradiation. The last part of this work is devoted to discussing different approaches to explain and describe the long-term behavior of irradiated ceramics.

JANNUS (Joint Accelerators for Nanosciences and Nuclear Simulation), the unique triple beam facility in Europe, offers the possibility to produce three ion beams simultaneously for nuclear recoil damage and implantation of a large array of ions for well-controlled modeling-oriented experiments. The first triple beam irradiation was performed in March 2010. Along with irradiation developments, continuous efforts have been made to implement ex situ and in situ characterization tools. In this study, we set out the present status of the JANNUS facility of the Saclay site. We focus on the instrumentation used for conducting multi-ion beam irradiations and implantations as well as for characterizing bombarded samples. On-line control of irradiation parameters, in situ modification monitoring using Raman spectroscopy or ion beam induced luminescence, and ex situ characterization by ion beam surface analysis [Rutherford backscattering spectrometry (RBS), nuclear reaction analysis (NRA), and elastic recoil detection analysis (ERDA)] of implanted samples are detailed. Some examples of single, dual, and triple beam irradiation configurations are presented. Access to the facility is provided by the French network EMIR for national and international users (http://emir.in2p3.fr/).

The behaviour and diffusion mechanisms of helium in nuclear ceramics, such as uranium dioxide spent fuel matrix and zirconolite for the specific conditioning of minor actinides, significantly impact the possible evolution of those matrices in interim storage or disposal conditions. In the framework of spent fuel storage studies, the additional diffusion of gas and fission products in uranium dioxidematrix is also an essential aspect of the R&D. Specific experimental studies have been conducted, devoted to thermal diffusion under 1000 C. Data processing methods, lead to helium diffusion coefficient and associated activation energy of 1.05 eV in the zirconolite and 2 eV in UO2. Comparativelywith the uranium dioxide matrix, the helium diffusion coefficient in zirconolite is 1 to 100 million times higher; this parameter will have to be taken into account to dimension the waste form. Diffusion coefficients measurements between 800 C and 1000 C, investigated by SIMS, showed a very slow diffusion of volatile fission products Xe, I, Te and Cs, with coefficients two or three order of magnitude lower than for helium.

The leaching behaviour of sintered UO2 pellets has been studied in synthetic groundwater at 96 °C under strictly controlled conditions (CO2 partial pressure, O2 partial pressure, pH and redox potential). Results obtained in oxidizing conditions (O2 content varying from 5 to 25 ppm) show a strong initial uranium release in solution during the first five days and a solubility control essentially driven by the growth of U(VI) species such as oxides, hydrates and carbonates. Results obtained in reducing conditions (use of a mineralogical buffer of Ni(0)/Ni(ll) with E0 = -0.257 Volts) show a very weak uranium release in solution close to 10-11 mole/l and a solubility control essentially driven by the formation of U(IV) compounds such as coffinite or uraninite.

The Rutherford backscattering technique is useful for the determination of the concentration profiles of some heavy elements in the near surface region of glasses, but is not able to provide chemical information on the elements detected.