STUDY OF TRANSVERSE CRACK FORMATION ON SURFACE AREA OF UO 2 PELLET CIRCUMFERENCE.

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1 2013 International Nuclear Atlantic Conference INAC 2013 Recife, PE, Brazil, November 24-29, 2013 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR ABEN ISBN: STUDY OF TRANSVERSE CRACK FORMATION ON SURFACE AREA OF UO 2 PELLET CIRCUMFERENCE. Dionísia S. Oliveira 1, Lélia F. P. C. Paneto 1 and Patrícia O. de Souza 1 1 Gerência de Análise Técnica do Combustível Nuclear Indústrias Nucleares do Brasil S. A. Rodovia Presidente Dutra, km 330 Engenheiro Passos Resende, RJ dionísia@inb.gov.br lelia@inb.gov.br patriciasouza@inb.gov.br ABSTRACT Microstructure of a polycrystalline material has a considerable influence on particular properties, such as mechanical strength, electrical conductivity, optical transmission and magnetic susceptibility. The uranium dioxide (UO 2 ) is used in water-cooled nuclear reactors, due to its desirable ceramics characteristics as a nuclear fuel. The UO 2 is used in the form of pellets manufactured by wet route by INB, where they are loaded into fuel rods to build the fuel assemblies used in pressurized water reactors of Angra 1, Angra 2 and future Angra 3 nuclear power plants, for electric energy generated from nuclear power in Brazil. The geometric and structural integrity of these pellets cause direct influence on their performance during the reactor core operating cycle,so pellets presenting surface cracks leading to the phenomenon of pellet cladding interaction-pci, resulting in failures in the fuel rod and subsequently release of fission products in the reactor coolant. Transverse cracks on surface area of pellet circumference are detected by visual inspection during the manufacturing process. This paper presents the study of these cracks formation by content analysis conducted with the support of electron microscopy. These results here are analyzed from the point of view of materials science through observation of the microstructure, and the pressing process where the defect was probably generated. 1. INTRODUCTION The fuel pellets of uranium dioxide are produced in Brazil through a sequence of manufacturing steps with control measures designed to ensure that the product complies with the design requirements [1]. In the manufacturing process, after mixing the UO 2 powder with U 3 O 8 originating from recycled material and lubricant, the blended mixture is transported to the automatic rotary press, where a metal matrix by pressure applied by the upper and lower punches in order to form products called green pellets, since they aren t still sintered. The green pellets are then inserted into a sintering furnace and heated around to 1750 C to increase hardness and acquire the necessary resistance to operating conditions that they are submitted within the reactor of the nuclear power plants Angra 1 and Angra 2 [2].

2 Figure 1: Manufacturing process of fuel pellets UO 2 [2]. After the processes of pressing, sintering and grinding (figure 1), the visual inspection step allows to verify whether cracks occurred on the surface of UO 2 fuel pellets, as shown in figure 2, where it s presented two transverse cracks in surface area UO 2 pellet circumferences. So this current paper aims to study the formation of these cracks through metallographic examinations by optical and scanning microscopies, and further presenting analysis of the parameters that influence the pressing process. Figure 2: UO 2 design pellets presenting transverse crack in the surface area of its circumference. 2. MATERIALS AND METHODS UO 2 fuel pellet manufactured by wet route with nominal density of g/cm 3 and 4.25% enriched uranium-235 having transverse crack in the circumference area was used for microscopy analysis. The cross section of the UO 2 fuel pellet sample was obtained in a manner that it clearly observes the extent of cracking, from surface until interior of the pellet. For this purpose, the pellet was embedded in cold resin, followed of thermal attack at 1400 C for 4 hours. The metallographic analyses were obtained by optical and scanning microscopies with 100X and 400X magnification, respectively

3 The homogenization and pressing process parameters appropriated to the pellet design are shown in table 1 and then compared with the press process parameters generated in the production of the pellets that presented cracks. Analyses were done by consulting the data of the control program of the press as shown in table 2: lower pressure and higher pressure of the punch, lubrification pressure, green density, filling depth and pressing speed. Table 1: Process parameters of homogenization and pressing. Nominal operating values Limits Target value (set-point) Unit Homogenization time mín minutes Green density limits value 5.55 a g/cm 3 Pressing pressure limits limit 1.5 a t/cm 2 Speed press limit 101 a pellets/minute Source: Data process of INB. Table 2: Parameters of production press Run Presure (bar) Upper Bottom Lubrication Filling depth Table height To target density of 5.75 g/cm 3 (1) Number punch pellet which is above the tolerance Pellet density above the tolerance Source: Control system of the press. (1) Green density 5.75 g/cm 3 ± 0.05g/cm RESULTS As can be observed from optical metallography at a 100X magnification (figure 3a), the grains show straight edges and glossy surface. There is the presence of a crack with appearance predominantly intergranular in the grain boundaries. As can also be shown from metallography scanning at a 400X magnification (figure 3b) the microstructure confirms microvoids located in grain boundaries and the presence of a crack with characteristic intergranular in certain regions and transgranular, through the grain, in other. No metallographic analysis realizes the presence of second phase precipitates in the grain boundaries [3].

4 a) b) Figure 3a: Optical metallography with 100X magnification showing the crack propagation in grain boundaries; Figure 3b: Scanning metallography with 400X magnification showing the crack propagation and pores. Alignment of pores and stress line on pellets surface is shown in figures 4a and 4b through optical micrographs with 50X magnification, respectively. Probably the causative agent evidenced at the pellets is owing to these defects. a) b) Figure 4a: Optical metallography with 50X magnification showing an alignment of pores; Figure 4b: Optical metallography with 50X magnification showing a stress line. The press machine is programmed in accordance with the suitable parameters to achieve an acceptable product (table 1). However due the aging of the machine, adjustments were made during the process to ensure the desired density values (table 2). The process target density is 5.75 g/cm3 with tolerance of ± 0.05 g/cm3, a value above the predicted would result in a diversion of waist in pellet (figure 6) during the sintering process, this situation also occurs with density values below the minimum tolerance. According to table 2, pellets from

5 punctures 13 and 15 showed density above the allowable tolerance, however its influence was not observed in the crack formation. The deficiency of the lubricant in lower punch was detected with evidence of enhanced brightness on lateral surface pellet. The lack of lubricant has been corrected by adjusting valves and regulating micrometric adjusting the lubrication pressure, regulating the amount of lubricant to each matrix [4]. Taking into account that in the pressing process in analysis, the enrichment uranium 235 information is recorded at the pellet top by the top punch printing, was found easily that the pellet analyzed showed crack in the top to the surface center of the circle (figure 2) giving its proximity to the upper punch. 4. DISCUSSION The phase diagram of the uranium-oxygen system (figure 5) shows that in the equilibrium condition and stoichiometric 2.01 for oxygen/uranium ratio at temperatures up to about 337ºC, two phases are present UO 2+X and U 4 O 9-y [5], but according to figure 3a and 3b is not observed inclusions and precipitates along the grain boundaries or second phase particles retained in microstructure as a result of abrupt cooling after the sintering process causing weakening. The grains are cohesive and impurities are not presented in the grain boundaries [6]. Figure 5: Uranium-oxygen system phase diagram [5]. In the analysis of microstructure represented in figures 3a and 3b it is noted that the crack originated in the grain boundary by microvoid nucleation and the formation of the cavity

6 (figure 3b) occurred in stress-rupture condition at high temperature. Figures 4a and 4b respectively show micrographs showing stress-line and aligned pores. The presence of these defects is derived, in most cases, of anomalies in the pressing process causing the appearance of cracks and in many cases leading to fracture the pellet. Regarding the type of crack, some considerations are made: the presence of intergranular cracks in the grain boundaries, is an indication that the crack may have propagated in the early sintering as a cause of the density gradient and, by the presence of transgranular crack, cracks passing through the grain, its propagation may have occurred during cooling influenced by the temperature gradient or phase changing. In the pressing, stage of the manufacturing process, the friction force between the powder particles and the matrix may reduce the applied pressure causing uniform density gradients as shown in figure 6 [7], so the appearance of transverse cracks in the area of the pellet circumference can happen due to inhomogeneity in the green pellets influenced by the difference in pressing force of the upper and the lower punches often arises from the lack lubrication, whose function is to reduce the frictional force between the powder and the matrix wall during the compression and to avoid retention force during the green pellet output of the matrix to the flat-boat [8]. Compression occurs by simultaneous displacement of the upper and lower punches, if there is difference of these forces can occur displacement of the neutral zone which is the region where the powders particles suffer less influence of mechanical compression, i.e., lower strength compression which will generate final pellet heterogeneous zones with different properties (figure 6). Around the neutral zone there is a degree of compaction incompatible with the part of the other regions and cracks may occur on the pellet surface. An increase in the pressing pressure influences positively increasing density, but otherwise appear cracks at the pellets ends faces [9]. However, the pressing machine does not issue report with the actual values, it is not possible to show the actual pressure values of the upper and lower punches, i.e., the difference in compression was not measurable. In the data analysis pressing machine (table 2) revealed that the values of lubricant, upper and lower pressure parameters have been introduced by the operator and adjusted as happened target value deviations in the density. The green pellets density is highly heterogeneous and depends on the pressing parameters: applied pressure, residence time, lubrication of matrix wall, as well as the pellets geometry: diameter, length/diameter ratio. If all parameters are not specified, it will happen in the density distribution of green pellets different than expected. Figure 6: Distribution of the density green pellets after the pressing process.

7 5. CONCLUSION Evidences of metallographic analysis indicates the cause of the crack formation, the combination of porosities alignment and the appearance of internal stresses at high temperature caused by inhomogeneity of the Green pellets by atypical density gradient at sintering. These abnormalities were consequences of the difference in strength between the upper and lower punches by lack of lubrication through the lower punch in the pressing step. The difference in compression force between the punches shifted the neutral zone immediately to above the center of the pellet causing a asymmetric distribution density of the green pellet and providing opening of the crack by difference in density between the regions. Acknowledgements We thank the pellets expert Georg Maier for having shared their valuable knowledge that contributed greatly to this work, our manager Márcio Adriano Coelho da Silva for his encouragement and guidance in our activities and laboratory technical Ticiana Aparecida Vieira Gomes and Luiz Arnaldo Rodrigues Olegário by their talent and dedication in sample preparation and processing of the images. REFERÊNCIAS 1 SANTOS, A. M. Fundamentos para especificação de pastilhas de UO 2 Parte 1: Considerações gerais. Nota Técnica DETS.CN DITCO.CN, Programa integrado NUCLEBRAS-NUCLEN na área do combustível nuclear, Resende, p. 1-17, ago FCN Pastilhas. Disponível em: < Acesso em: 06 mar CALLISTER, D. W. Ciência e engenharia de materiais uma introdução. 5 a edição. Editora LTC, Rio de Janeiro, Yi, H.; PURI V. M. Stress gradient within powder en masse during hydrostatic compression. Elsevier: Powder technology 239, p KIM, Y. S. A thermodynamic evaluation of the U-O system from UO 2 to U 3 O 8. Elsevier: Journal of nuclear materials 279, p ALBERO, J. L. A. A operação de prensagem: Considerações técnicas e sua aplicação industrial. Instituto de tecnologia cerâmica, Universidade de Valência. Set. out YANAI, K.; ISHIMOTO, S.; KUBO T.; ITO, K.; ISHIKAWA T.; HAYASHI H. Characterization of ceramic powder compacts. Elsevier: Journal of nuclear materials 224. p BAYLE, J. O.; DELETTE, F. J.; GOBIN, F.; MOYER P. Finite element modeling and experiments for shaping nuclear powder pellets, application in technological tool developments to minimize damage during ejection, and geometrical tolerances. Elsevier: Procedia chemistry 7,

8 9 ANDREEV, E. I.; GLAVIN, K. V.; IVANOV, A. V.; et al. Some results uranium dioxide powder structure investigation. Russian Journal of Non-ferrous Metals, 2009, Vol.50, N 3, pp