U1-xPuxO2 more or less 948; fuel precursor synthesis through advanced thermal denitration in presence of organic additive

International audienceWithin the U and Pu recycling process from nuclear spent fuels, the conversion of purified U and Pu solutiononto oxide powder is a key step at the interface between the separation / purification processes and thefabrication of uranium-plutonium oxide fuels called MOx ("Mixed Oxides"). In order to simplify theindustrial process, conversion must be able to integrate fluxes with the minimum of adjustments whileleading to oxides or mixtures of oxides adapted to a direct shaping of fuel pellets. In the framework of Pumulti-recycling, new conversion routes have to be developed to suit new needs: increase of the quantity ofsynthetized oxides, reduction of dust generation, no redox adjustments of feed actinide solutions, simplifiedeffluent management, no proliferation risks. To meet these future requirements, CEA is developing a"direct" conversion route based on advanced thermal denitration to obtain mixed actinide oxide U1-xPuxO2±δ.This synthesis method is based on the NPG (nitrate polyacrylamide gel) route developed for non-radioactivematerials [1]. Several adjustments of this synthesis were performed prior its application to the productionof U1-xPuxO2±δ (x = 0, 0.20, 0.25, 0.50 and 0.75) batches. For each sample uranium nitrate (UO2(NO3)2) wasdissolved in a solution of nitric acid containing Pu at the stoichiometry of the targeted oxide. Total U+Puconcentration was equal to 40 g.L-1, total nitrate concentration was equal to 3.0 mol.L-1. In parallel, a mixtureof mainly acrylic acid AA and N, N'-methylene bis (acrylamide) MBAM in a molar ratio of 20:1 (AA:MBAM) was prepared and then added to the actinide nitrate solution and stirred. Once homogeneous, theresulting solution was heated up to 100°C and supplemented with 25 μL of radical initiator to quickly allowcomplete polymerization into a gel. A xerogel was then formed by dehydration of the polymeric gel at150°C. Xerogel conversion at 800°C in reconstituted air produced intimate mixtures of U3O8 and PuO2.After the second calcination, performed under Ar/H2-5% at 800°C, single-phase oxides were obtained asconfirmed by XRD analyses showing the presence of a single fluorite structure (Fm-3m) for the samplesU1-xPuxO2±δ (x = 0 to 0.75) (Figure 1a), exhibiting a 3D morphology (Figure1b). For U0.80Pu0.20O2±δ, theamount synthesized allowed the fabrication of a pellet. In this objective, the powder was pressed under 500MPa in a three part die, no grinding step was used prior to the pelletization. The green density reached was70% of the theoretical density (TD). The green pellet was then sintered in a dilatometer under Ar/H2-4%with 1200 ppm of H2O and the axial shrinkage was recorded as function of temperature (Figure 2a). Themaximum shrinkage rate is observed at 1455°C. The pellet was then maintained at 1700°C for 2h. At theend of this process, the density of the pellet reached 94±2% of TD and exhibited oxygen to metal ratio of2.00. Visually the pellet did not show any cracks (Figure 2b).The main objective of this work was to explore the possibility to synthetize and sinter mixed U/Pu oxidesby “advanced thermal denitration in the presence of organic additives”. Feasibility is demonstrated here bythe successful synthesis of U1-xPuxO2±δ, whatever Pu content. The direct sintering of the obtained powderwithout milling step before shaping is possible. Therefore a significant breakthrough in the methodscurrently used for MOx fabrication is demonstrated. Indeed, this new process allows the synthesis of mixedoxides from solutions resulting from reprocessing without any redox adjustment (i.e. using U(VI)/Pu(IV)nitrate solution) and does not require any filtration and grinding steps, limiting highly contaminant dustgeneration