Photothermal microscopy applied to the characterization of UO 2 -Gd 2 O 3 nuclear fuel pellets
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Photothermal microscopy applied to the characterization of UO2-Gd2O3 nuclear fuel pellets Oscar Martínez2,3,5 , Facundo Zaldivar1, Nélida Mingolo1, Rodolfo Kempf4 1
Fisica, Facultad de Ingeniería, Universidad de Buenos Aires, Buenos Aires, CABA, Argentina. Fisica, FCEN, Universidad de Buenos Aires, Buenos Aires, CABA, Argentina. 3 Toket SRL, Buenos Aires, CABA, Argentina. 4 Unidad Actividad Combustibles Nucleares. División Caracterización, CNEA, San Martin, Buenos Aires, Argentina. 5 CONICET, Buenos Aires, CABA, Argentina 2
ABSTRACT The photothermal photodeflection technique is shown to provide information on the homogeneity of fuel pellets, pore distribution, clustering detection of pure urania and gadolinea and to provide a two-dimensional mapping of the thermal diffusivity correlated to the composition of the interdiffused Gadolinium and Uranium oxide. Histograms of the thermal diffusivity distribution become a reliable quantitative way of quantifying the degree of homogeneity and the width of the histogram can be used as a direct measure of the homogeneity. These quantitative measures of the homogeneity of the samples at microscopic levels provides a protocol that can be used as a reliable specification and quality control method for nuclear fuels, substituting with a single test a battery of expensive, time consuming and operator dependent techniques. INTRODUCTION The development of manufacturing processes compatible with production requirements and quality restrictions gives rise to the need for detection and characterization techniques that are reliable, fast and inexpensive capable of determining as many relevant parameters as possible in a single test. In particular the UO2-Gd2O3 to be used in new small frame power plants like the one under design by the CNEA (Atomic Energy Agency from Argentina) have stringent requirements regarding the homogeneity of the resulting solid solution, the size distribution of clusters of remaining pure oxides and pore sizes and distributions [1,2]. In this work a technique is presented that satisfies all these requirements, substituting with advantages in cost, time, accuracy and simplicity an entire battery of techniques otherwise necessary and currently in use such as X-ray diffraction and scanning electron microscopy employed to analyze the chemical homogeneity in UO2-Gd2O3 pellets [3] and micrographic techniques performed to produce an image of the pellet surface after the sample is cut, polished and color etched, so that in the image the free gadolinia, urania and solid solution grains reflect different colors [3].
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EXPERIMENT Samples Two different batches of (U, Gd)O2 pellets were produced and the samples obtained were named Type I and Type II. The UO2+Gd2O3 powders were blended by mixing the precursors in double cone blender, with an intensifier bar, rotating at 32 rpm, for 4 h. The UO2 used to prepare all samples was an ex-ammonium uranyl carbonate powder with impurity content less than 150 ppm, specific surface area (BET) of 5.4±0.1 m2/g, average particle size
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