Abstract
Nowadays, the French Pressurized Water Reactors (PWR) provided about 70% of the electricity production in 2019. Around 15% of the nuclear fuel used is Mixed Oxides fuel (MOX), originating from spent nuclear fuel recycling in order to use plutonium. It is a crucial issue within Generation IV International Forum framework to preserve natural uranium resources, decrease plutonium inventory and increase the nuclear fuel burn-up. In this context, it is interesting to find new routes to produce homogenous oxide mixtures with adequate powder morphology and a low impurities content in order to facilitate the fuel production. New routes are studied for the direct conversion of actinide nitrate to U1-yPuyO2 and avoid the necessity to ground together UO2 and PuO2. Among them, Syntheses by Combustion in Solution are known since 1970s to allow the production of metals oxides [1], including actinides oxides [2], [3], [4], [5], [6], [7]. These reactions are based on precursor dissolution (metal nitrate in this case) in an aqueous solution with an organic compound named fuel. This solution is then dehydrated and leads to the gelation of the reactive media. A low temperature thermal treatment allows the ignition of that gel, thanks to a vigorous exothermic self-sustaining phenomenon. Combustion can occur in two ways: propagating combustion when ignition is local and spreads in the reactive media and volume combustion when gel is subjected to homogeneous heat and ignition takes place instantaneously. During ignition, a flame is visible and very high temperatures are reached; for example, 1200K in the case of glycine assisted SCS of uranium oxide [8].This flame temperature is strongly dependent on the fuel/actinide ratio and allows actinide oxide crystallites formation and growth, leading to well crystallized oxide powder.Previous studies on SCS showed the possibility to obtain U1 xThxO2 [2], [3], [4], [5] and U1 xCexO2 [6], [7] mixed oxides using citric acid or glycine as fuels for the reaction. A combustion synthesis of PuO2 was also reported in the literature [5] but the experiment was conducted on a hot plate in propagating conditions. In this study, we examined the volume combustion instead of propagating combustion in order to reach higher flame temperature. In the scope of plutonium high content MOX fuels, this work aim is to study the feasibility of U1 xPuxO2 formation thanks to combustion synthesis. Preliminary studies on surrogates were carried out in our laboratory. Gadolinium oxides synthesis was studied with the assistance of three fuels within more common ones in literature: glycine, citric acid and urea. Glycine and citric acid were selected because of products high crystallinity and low impurities content. Different thermal treatment and temperature rates were tested in order to improve ignition conditions. The next step to synthetize U1-xPuxO2 was to determine fuel/actinide optimal ratio to obtain UO2 with glycine and citric acid. In order to do that, uranium nitrate UO2(NO3)2·6H2O and glycine/citric acid gels were prepared at different molar ratios in the range from 1.1 to 2.5 for glycine and 0.2 to 0.6 for citric acid were prepared by aqueous solution dehydration. Uranium oxides were obtained after thermal treatment and characterized in order to select the optimal ratio for each fuel. In this study, particular consideration was paid to the alumina crucibles geometry. Indeed, ignition and so products crystallinity is dependent on the shape and size of the crucible. Several tests on uranium allowed us to choose the most relevant alumina crucible, which was later used for plutonium. Attempts to prepare PuO2 took place in hot-labs on ATALANTE facility. A study was conducted on plutonium nitrate with glycine and citric acid as fuels with some minor modifications in the protocol due to the plutonium chemistry, including plutonium precursor, which is manipulated as a nitric solution. Fuel/actinide ratio varied in the range from 0.7 to 3.7 for glycine and from 0.2 to 1.4 for citric acid. Optimal molar ratios were determined for each fuel. The current challenge is the determination of optimal conditions to produce U1-xPuxO2. A study on (U,Pu)O2 synthesis is being carried out for each fuel on ATALANTE facility, using varying Pu/(U+Pu) ratios in order to single out/find the optimal fuel/actinide ratio associated. The first results indicate the possibility to obtain U1-xPuxO2. On the other hand, the understanding of reaction mechanism constitutes a key point to frame the optimal reactive conditions.Finally, some important issues remain about the scale up in order to consider production of pellets of (U,Pu)O2 with different Pu contents. [1] A. E. Danks, Material Horizons 2016, 3, 91-112[2] J. Monnier, Ph.D. Thesis, 2019, Montpellier University[3] G. Peter Soldani, Ph.D. Thesis, 2014, Lille University [4] S. Anthonysamy, Journal of Nuclear Materials 2000, 278, 346-357[5] V. Chandramouli, Journal of Nuclear Materials 1999, 265, 255-261 [6] A. Jain, Journal of Nuclear Materials 2005, 345, 245-253[7] D. Maji, Journal of Nuclear Materials 2018, 502, 370-379[8] J. M. Roach, Inorganic Chemistry 2021, 60, 18938-18949