The electric field has a large effect on the stoichiometry and grain growth of UO2+x during Spark Plasma Sintering. UO2+x is gradually reduced to UO2.00 as a function of sintering temperature and time. A gradient in the oxidation state within the pellets is observed in intermediate conditions. The shape of the gradient depends unequivocally on the direction of the electrical field. The positive surface of the pellet shows a higher oxidation state compared to the negative one. An area with larger grain size is found close to the positive electrode, but not in contact with it. We interpret these findings with the redistribution of defects under an electric field, which affect the stoichiometry of UO2+x and thus the cation diffusivity. The results bear implications for understanding the electric field assisted sintering of UO2 and non-stoichiometric oxides in general.

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Spark plasma sintering (SPS) is a rapidly developing method for densification of powders into compacts. It belongs to the so-called “field assisted sintering techniques” that enable rapid sintering at much lower temperatures than the classical approaches of pressureless sintering of green pellets or hot isostatic pressing. In this paper, we report the successful integration of a SPS device into a hermetic glovebox for the handling of highly radioactive material containing radioisotopes of U, Th, Pu, Np, and Am. The glovebox implantation has been facilitated by the replacement of the hydraulic system to apply pressure with a compact electromechanical unit. The facility has been successfully tested using UO2 powder. Pellets with 97% of the theoretical density were obtained at 1000 °C for 5 min, significantly lower than the ∼1600 °C for 5-10 h used in conventional pellet sintering.

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Flash spark plasma sintering (FSPS) is a newly developed technique for consolidation of powder materials into dense compacts. This method belongs to the group of electrical field-assisted sintering (FAST) techniques. It combines the voltage/current characteristics of flash sintering (FS), 1,2 usually performed in pressure-less dog-bone configuration, with the sintering cell design typical of spark plasma sintering (SPS). 3 Recent investigations in FSPS 4-8 use mostly a modification of the classical SPS to achieve FS conditions. This is usually done through the insertion of a nonconductive sleeve (eg, BN 9) between the powder (or pre-compacted/partially pre-sintered pellet) and the graphite die, or alternatively in a die-free configuration. Commercial instruments enabling FSPS processing with higher voltage characteristics (>10 V), using independent external heating of the die or precise time application of the electric field, are under development or in early application stage. In the presented paper, we describe the pressure-assisted FS compaction of powder in an SPS configuration using such a newly developed instrument. We have chosen ceria as a sintering probe not only for its

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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.