Experimental investigations have made it possible to develop a technology for obtaining uranium dioxide fuel pellets with anomalously high thermal conductivity at 600-800°C and higher. The thermal conductivity of pellets is increased, without and with the addition of very small amounts of tin or titaniun dioxide, by improving the deposition process, making it possible to obtain simultaneously "large" particles and nanoparticles followed by annealing of the deposit at optimal temperature. It is pointed out that there is an analogy with the temperature dependence of the thermal conductivity for single-crystal uranium dioxide. The radial heat flux method showed that the temperature gradient from the center to the periphery for modified pellets is approximately three times smaller than for pellets fabricated by the standard factory technology. The modified pellets contain uranium of different valence and have an unusual microstructure, characterized by the presence of close-packed grains. Small spherical pores 0.1-0.2 µm in diameter are observed inside grains; a minimal number of polyhedral pores 1-2 µm in size lie along the grain boundaries.Uranium dioxide is the most widely used and reliable form of fuel for VVÉR. However, low thermal conductivity and brittleness limit the use of uranium diocxide fuel pellets in reactors operating in maneuvering regimes and limit the possibilities for reaching deep burnup.Work on increasing the thermal conductivity, plasticity, and heat-resistnce of uranium dioxide pellets by means of a wet technology is being done at the Physics and Power-Engineering Institute. The crux of the method lies in adding a very small amount of nanoparticles when a precipitate of ammonium polyuranate is obtained. This makes it possible to prepare sintered pellets with an unusual microstructure and anomalously high thermal conductivity. This method can be used to obtain other oxide ceramic materials with elevated thermal conductivity, plasticity, and heat resistance. Specifically, the technology has been drveloped for producing MgAl 2 O 4 , MgO, Gd 2 O 3 , (U, Th)O 2 , PuO 2 + MgO, PuO 2 + Fe + MgO, BaPuO 3 , and (Pu, Th)O 2 [1-4].The basic technlogical operations involved in making fuel pellets are as follows [5, 6]: preparation of a solution of uranyl-nitrate (with or without additives), precipitation of ammonium polyuranate from solution, filtration, heating of the precipitate at optimal temperature, reduction of the powder in a hydrogen atmosphere, mixing with a plasticizing agent, and molding and sintering the pellets.Properties of the Powder. The characteristics of the density, porosity, mechanical strength, microstructure, external appearance, and so on of the ceramic oxide fuel depend most strongly on the properties of the intermediate technological products (precipitates, oxides) [5, 6]. Annealing of the powders has the decisive effect.It has been asserted [7] that to increase dynamical strength the powder must be obtained under conditions where crystals form at a high temperature in order to ach...
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Fast reactors for purposes of efficient utilization of plutonium from power plants and weapons are currently under study and development in Russia and other countries (France and Japan). It is important in this connection to develop and substantiate new plutonium-containing fuel compositions with 238U replaced by an inert diluent.The Main Science Center of the Russian Federation -Power-Engineering Institute has been conducting since 1994 work on choosing a technology and studying the properties of a fuel material in a magnesium oxide matrix. Magnesium oxide was not chosen randomly as an inert diluent [1-3]: it does not form intermediate phases with plutonium dioxde; it does not interact with sodium; it is highly radiation resistant; it is easily reprocessed by dissolution; and, it possesses a satisfactory thermal conductivity (12 W/(m-deg) at 900"C in nonporous samples).The objective of the present work was to develop a technology for producing PuO 2-MgO fuel pellets. It was determined by means of a physical calculation that the actual volume fraction of plutonium dioxide in an inert matrix equals approximately 15%. Two technological schemes for preparing fuel pellets with the composition 15% uranium dioxide -85 % magnesium oxide were compared in advance using uranium dioxide as a simulator: by means of mechanical mixing of powders and coprecipitation of magnesium hydroxide and ammonium polyuranate from a nitrate solution [4]. Preference was given to the technology for preparing pellets by coprecipitation from solutions, since this gives the most uniform distribution of plutonium dioxide, a high density, and a higher thermal conductivity. Furthermore, the technology contains the standard operations and equipment used in nuclear fuel production plants. The samples were investigated by metallographic, x-ray, and other forms of analysis, their thermal conductivity was determined, and the possibility of chemical reprocessing of the samples was assessed.Preparation of PuOz-MgO Pellets (Fig. 1). The work was performed in hermetically sealed glove boxes at the hot laboratory at the Physics and Power-Engineering Institute. The pellets were prepared by a technological regime chosen on the basis of experiments with uranium dioxide simulator.A solution of plutonium and magnesium nitrates was prepared with 18.1 g of plutonium and 36.3 g of magnesium oxide. The solution was dissolved in nitric acid, yielding 460 ml of the solution with the concentrations (g/liter): Mg 51.5, Pu 42, and HNO 3 -30. The hydroxides PU(OH)4-Mg(OH)2 were precipitated using a 25% solution of ammonia by the method of simultaneous decantation in the precipitation apparatus. To achieve a pH of 10.5, 1030 ml of ammonia were consumed in order to precipitate plutonium and magnesium hydroxides from a 460 ml of the nitric-acid solution. The precipitation process lasted for 3 h, and the temperature of the pulp during the precipitation process was maintained equal to 40-42~ When the decantation process was finished, the pulp was mixed for 1 h at 40-42~ and the...
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