2, CEA Marcoule, Bagnols sur Cèze, , France
3, UCCS, Villeneuve d'Ascq, , France
The treatment and recycling of the spent nuclear fuel by the PUREX process in the AREVA-La Hague plant (France) allows to reduce the ultimate nuclear waste volume and radiotoxicity and to save natural resources by recovering valuable materials, such as plutonium and uranium. To do so, the spent fuel is dissolved in nitric acid then uranium and plutonium are co-extracted, separated in two flux and recovered from solution by precipitation into solid phases. Plutonium is precipitated by oxalic acid, then calcined in air in order to obtain PuO2, used as the main starting material for the Mixed OXide (MOX) fuel fabrication.
In the current industrial process, the plutonium oxalate is precipitated in the form of squared particles of PuIV(C2O4)2.6H2O. Interestingly, this morphology is maintained during the calcination in air which means that the morphology obtained at the precipitation step may influence the textural properties of the final oxide. Researches are currently carried out in collaboration between the CEA-Marcoule and the UCCS-Lille in order to vary the morphology of PuO2 by modifying the oxalic precipitation conditions. The aim is to be able to control the morphology as soon as the precursor precipitation step occurs.
The crystal shape inherently depends on internal parameters which are related to the symmetry of the crystal structure and to the reticular density. Besides these internal parameters, the size and morphology of crystals can be influenced by various external parameters such as the frequency of nucleation, the rate of crystal growth and the agglomeration of the particles, which are themselves related to the chemical conditions. We herein tempted to manage the morphology of PuO2 by acting both on internal and external parameters i.e. by modifying the crystal structure of the oxide precursors and by performing modifications of the synthesis conditions.
The experimental work was carried out by precipitating either plutonium (III) or plutonium (IV) oxalates in an acidic medium (HNO3 ~ 1M) and in presence of thermally labile additives. The chemical conditions were defined on the surrogates Nd2(C2O4)3(H2O)6.4H2O and Th(C2O4)2.6H2O prior to the implementation to the plutonium (III) and (IV) systems. The additives effects on the surrogate systems can be listed as follows: modification of the crystal structure and of the particle shape, decrease of the particle size without modification of the crystal structure, modulation of the particles morphology and/or agglomeration without modification of the crystal structure.
The presentation will be dedicated to the multidisciplinary approach combining solid-state chemistry, solution analysis and chemical engineering set up in order to explain the rule of additives on the mechanisms underlying the observed modulations. Another part will be devoted to the origins of the similarities and differences of the thorium (IV) oxalate and the plutonium (IV) oxalate systems towards the additives.