Synthesis of Nanostructured Fuels: Mimicking the High Burn-Up Structure

During its lifetime in the core of a nuclear reactor, the fuel undergoes significant changes in its physical, chemical and morphological characteristics. In outer regions of the fuel pellets the so called “high burn-up” or “rim” structure may form. In this region UO₂ grains, with an original size of about 10 μm in fresh dense fuel, are reorganized into a porous structure with grain size 0.1–0.3 μm and porosity fraction up to 20 %. The mechanical, thermal and fission product retention properties of the high burn-up structure have encouraged further interest and attempts to mimic this morphology in fresh fuel. The JRC-ITU has studied various techniques for the synthesis of uranium and thorium dioxide in aqueous or nonaqueous media. Such nanoparticles can serve as starting material for production of material having similar characteristics as the high burn-up structure, as has been proved using Zr(Y)O₂ nanoparticles. Recently, efforts have been focused on the compaction of the nanoparticle powders. A spark plasma sintering device (SPS, FCT Systeme GmbH) has been commissioned in the JRC-ITU and tested using various nonradioactive materials. The present study is oriented on pressing and sintering of Hf(Y)O₂ and ZrO₂ into nanostructured pellets. Final products have been characterized by optical and electron microscopy, X-ray powder diffraction and density measurements.