ASPECTS ON SINTERING AND GRAIN GROWTH IN PURE AND Al 2 O 3 SiO 2 DOPED UO 2 PELLETS

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1 2009 International Nuclear Atlantic Conference - INAC 2009 Rio de Janeiro,RJ, Brazil, Septemer27 to Octoer 2, 2009 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: ASPECTS ON SINTERING AND GRAIN GROWTH IN PURE AND Al 2 O 3 SiO 2 DOPED UO 2 PELLETS Gino de Assis 1, Wilmar Barosa Ferraz 1 and Elias Basile Tamourgi 2 1 Núcleo de Tecnologia do Comustível Centro de Desenvolvimento da Tecnologia Nuclear Av. Presidente Antônio Carlos Belo Horizonte, MG assisg@cdtn.r ferrazw@cdtn.r 2 Faculdade de Engenharia Química Universidade Estadual de Campinas Av. Alert Einstein 500 Bloco A Campinas, SP eliastam@feq.unicamp.r ABSTRACT The sintering ehavior of UO 2 pellets was investigated at 1700 C/2h/H 2 and 1750 C/4h/H 2, using fresh and storage UO 2 powders. Moreover, the sintering ehavior of Al 2 O 3 -SiO 2 doped UO 2 pellets otained from storage powder, in the same sintering conditions was also investigated. Pellets otained from fresh UO 2 powder have presented heterogeneous grain microstructures as they were sintered at 1700 C/2h/H 2 and homogeneous grain microstructures when they were sintered at 1750 C/4h/H 2. On the contrary, pellets otained from storage powder have presented homogeneous grain microstructures when they were sintered at 1700 C/2h/H 2 and heterogeneous grain microstructures when they were sintered at 1750 C/4h/H 2.On the other hand, the pellets with additions of Al 2 O 3 -SiO 2 presented homogeneous grain microstructures, regardless the sintering conditions were. The mechanisms y which the homogeneous and the heterogeneous grain microstructures were developed will e discussed in this paper concerning the fresh powder, the storage powder, the sintering conditions and the Al 2 O 3 -SiO 2 additions. 1. INTRODUCTION In order to e used in power reactors, uranium dioxide pellets with strict requirements of density and microstructure are employed in fuel rods manufacturing. Density and pore microstructure govern the in-service fuel dimensional ehavior, while grain microstructures affect fuel mechanical properties and kinetics of fission gas release [1]. Nowadays, with increasing goals to the discharge urnups of the nuclear fuel, the grain microstructure acquires a larger importance since larger grain sizes represent the larger aility of the fuel to hold the fission gases [2]. The powder characteristics (e.g. surface energy) and the sintering process parameters (e.g. temperature) strongly influence these pellets properties, y changing the predominant diffusion mechanism involved in the densification and grain growth processes. Other important factors affecting pellets properties are the different present impurities, which are ale to act in three ways. First, the impurity cation sustitutes the U cation in the UO 2 crystalline network, increasing then the uranium diffusion coefficient through Schottky equilirium [3-5]. Second, volatile impurities increase the oxygen pressure of the sintering atmosphere, changing the value of x of the UO 2+x, which also increase the defects

2 concentration in the UO 2 crystalline network (Frenkel and Schottky defects), promoting a considerale increase of the uranium diffusion coefficient in the UO 2 [3-9]. Third, the present impurities melt down and promote sintering in the presence of liquid phase. Each of these possiilities involves different paths of densification and grain growth [3, 9, 10]. This work reports the results of experiments on densification and grain growth in pellets otained from pure UO 2 and UO 2 with small contents of Al 2 O 3 -SiO 2, sintered in different sintering conditions. The results are discussed in relation to: the powders surface energy, the volatile impurities presence and phase liquid formed from Al 2 O 3 -SiO EXPERIMENTAL PROCEDURE The UO 2 powder used in this study was otained y decomposition of Ammonium Uranyl Caronate (AUC) at 600 C/20h/air in inconel trays followed y reduction in a inconel rotating chamer oven at 600 C/4h/H 2 (fresh powder) [11]. AUC and the oxides otained from its decomposition were characterized y X-ray diffraction (XRD). The UO 2 crystal morphology was revealed y Scanning Electronic Microscopy (SEM) and its surface area was determined y BET method. To determine the suitale pressing pressure, a test atch with five pellets was otained y fresh powder pressing in the range MPa and sintered at 1700 C/2h/H 2. Then, a atch with 39 pellets was pressed and sintered at 1700 C/2h/H 2 and another ath with 39 was pressed and sintered at 1750 C/4h/H 2. One year later, the same UO 2 powder (stored powder) was again characterized, and then, pellets were otained y pressing the stored powder at the same pressure previously used. Part of the pellets was otained with 0.1 wt% Al 2 O 3 -SiO 2 (0.04 wt% Al 2 O 3 and 0.06 wt% SiO 2 ) and another with 0.2 wt% Al 2 O 3 -SiO 2 (0.08 wt% Al 2 O 3 and 0.12 wt% SiO 2 ) additions. The addition of Al 2 O 3 -SiO 2 was carried out gradually in a homogenizer for two hours. First, it was added Al 2 O 3 -SiO 2 in a little part of UO 2 powder and then, the other parts were of UO 2 powder at intervals of 20 minutes. In each of these compositions, two green pellets were otained, one of the condition was sintered at 1700 C/2h/H 2 and the other one at 1750 C/4h/H 2, such that each atch contained 31 pellets. The green and the sintered pellets densities were measured y geometric and penetration-immersion (MPI) methods, respectively. A sample of each sintered pellet was cut longitudinally, sanded and polished. These samples were sumitted to a thermal etching at 1325 C/2h/CO 2 in order to reveal their grain structures. The grain structures images were taken y optical microscopy and quantified y the Quantikov software [12]. The average grain sizes were determined y the mean linear intercept method and the grain size distriutions were otained y the Saltykov method [12, 13]. 3. RESULTS The predominant crystallization phase of the precursor AUC and the oxides otained from its decomposition y calcination (U 3 O 8 ), reduction (UO 2 ) and storage (UO 2 ) can e evaluated y the X-ray spectra (Figure 1). The morphological aspects of the UO 2 powders particles are shown in Figure 2. The fresh powder particle (Figure 2a), due to calcination process has een coarsened. The coarsening can e oserved if compared with a reference powder particle (Figure 2c), otained y the conventional process [11]. The surface area of the fresh powder was reduced from 4.66 to 2.88 m 2 /g in relation to the reference powder. However, after eing stored for one year, the powder particles suffered a degradation process proaly due to the oxidation of its particles (Figure 2), increasing its surface area from 2.88 to 3.85 m 2 /g. This degradation process produced a fraction with very fine particles (Figure 2d).

3 STORED UO 2 FRESH UO 2 U 3 O 8 AUC Figure 1. X-ray spectra of precursor AUC and its decomposition products after calcination, reduction and storage (a) () (c) (d) Figure 2. Morphology of the UO 2 powder particles, (a) fresh powder (4000X), () storage powder (5000X), (c) reference powder (4000X), (d) storage powder (10000X).

4 The pressing of the fresh powder at 600 MPa produced green pellets, which after eing sintered at 1700 C/2h/H 2 in a small test atch (five pellets), reached ~95% TD and its grain microstructure was homogeneous (Figure 3). Therefore, the pressing pressure in this work was estalished at 600 MPa. a Figure 3. Grain microstructure of the pre-test pellet, (a) centre, () edge Tale 1 presents the values of green density ( V ), sintered density ( S ) and average grain size (GS) of pellets otained in the different conditions studied. Pellets otained from pure fresh and pure stored powders reached the same density (~95% TD TD = Theoretical Density) ut presented different grain microstructures after sintering at 1700 C/2h/H 2. The pellet otained from the pure fresh powder presented heterogeneous grain microstructure, suggesting that a grain growth occurred from the center to the edge of the pellet (Figure 4). On the other hand, the pellet otained from pure stored powder presented a small homogeneous grain microstructure (Figure 5). Pellets from fresh and stored powders and sintered at 1750 o C/4h/H2 reached densities of % and % TD, respectively. The grain size of the pellets otained from pure fresh powder increased after sintering at 1750 C/4h/H 2, ut the density increase was negligile. The fuel pellets from pure fresh powder presented the homogeneous microstructure of the grains (Figure 6), while the fuel pellets from stored powder presented the heterogeneous microstructure of grains (Figure 7). The homogeneity or heterogeneity of the grain microstructures can e etter evaluated through grain size distriution (Figures 8 and 9). Tale 1 Farication conditions and UO 2 pellets properties from large atches. UO 2 PELLETS SINTERED at 1700 C/2h/H 2 SINTERED at 1750 C/4h/H 2 NUM. V [%TD] S [%TD] GS ( m) NUM. V [%TD] S [%TD] GS ( m) 2782 (1) (1) (2) (2) (3) (3) (4) (4) (1) UO 2 fresh powder pure (2) UO 2 stored powder pure (3) UO 2 stored powder + 0.1% of Al 2O 3-SiO 2 (4) UO 2 stored powder + 0.2% of Al 2O 3-SiO 2.

5 a Figure 4. Grain microstructure of pellets sintered at 1700 C/2h/H 2 otained from fresh powder, (a) center, () edge a Figure 5. Grain microstructure of pellets sintered at 1700 C/2h/H 2 otained from stored powder, (a) center, () edge a Figure 6. Grain microstructure of pellets sintered at 1750 C/4h/H 2 otained from fresh powder, (a) center, () edge

6 a Figure 7. Grain microstructure of pellets sintered at 1750 C/4h/H 2 otained from stored powder, (a) center, () edge FRESH STORED 0,1% Al 2 O 3 -SiO 2 0,2% Al 2 O 3 -SiO FRESH STORED 0,1% Al 2 O 3 -SiO 2 0,2% Al 2 O 3 -SiO 2 V/V (%) V/V (%) GRAIN SIZE ( m) GRAIN SIZE ( m) Figure 8. Grain size distriutions of pellets sintered at 1700 C/2h/H 2 Figure 9. Grain size distriutions of pellets sintered at 1750 C/4h/H 2 The pellets otained from pure UO 2 stored powder sintered at 1700 C/2h/H 2 and 1750 C/4h/H 2 reached densities of 94.98% and 96.23% TD, respectively. The addition of Al 2 O 3 -SiO 2 (0.1 and 0.2 wt%), depending on their content and sintering conditions, presented different tendencies concerning the UO 2 pellets densification. In general, the addition of 0.1 wt% Al 2 O 3 -SiO 2 increased the pellets densities, although there was density suppression when this content was raised to 0.2 wt%. In relation to grain growth, on the sintering condition of 1700 C/2h/H 2, the average grain size increased approximately 173% and 230% due to the additions of 0.1 and 0.2% Al 2 O 3 -SiO 2, respectively. Even though they have allowed otaining average grain size almost three times igger than the pure UO 2 pellet, the difference of the average grain size etween them was too little. The average grain size was almost the same in the pure (20.5 m) or with 0.2 wt% Al 2 O 3 -SiO 2 (19.8 m) UO 2 pellets, sintered at 1750 C/4h/H 2, even if the first and the last ones have heterogeneous (Figure 7) and homogeneous (Figure 10) grain microstructures, respectively. The average grain size in the pellet with 0.1 %wt Al 2 O 3 -SiO 2 was reduced. All

7 pellets with Al 2 O 3 -SiO 2 show homogenous grain microstructure regardless the additive content or sintering conditions (e.g. Figures 10 and 11). a Figure 10. Grain microstructure of pellets sintered at 1700 C/2h/H 2 otained from stored powder 0.1% Al 2 O 3 -SiO 2, (a) center, () edge a Figure 11. Grain microstructure of pellets sintered at 1750 C/4h/H 2 otained from stored powder 0.1% Al 2 O 3 -SiO 2, (a) center, () edge. The difference etween the grain size distriutions of the pure UO 2 pellets sintered at 1700 C/2h/H 2 (Figure 8) and 1750 C/4h/H 2 (Figure 9) are remarkale, due mainly to their quite distinct grain microstructures. These figures also show the grains size distriutions of the pellets doped with Al 2 O 3 -SiO 2 sintered at oth conditions. Although these two plots lie almost on each other, one can note that the grain size distriution of the pellet with 0.2 wt% Al 2 O 3 -SiO 2 is slightly narrower, suggesting that the grain microstructure of this pellet is discreetly more homogeneous. This sutle difference seems to e responsile for its larger average grain size. 4. DISCUSSION The AUC usually decomposed in a different condition from that practiced in this work, results in UO 2 powder particles with gas-solid interfaces (opened porosity) similar to the reference powder. In the present work, the calcination conditions applied to the powder promoted its particles growth (Ostwald ripening), and eliminated a part of its opened porosity. This particle growth resulted in a powder with lower surface energy, and possily

8 greater mechanical strength. However, during the storage period of this powder, the outside layers of its particles possily underwent an oxidation process, resulting in cracked or roken particles. It recovered part of the surface energy eliminated during the calcination process. This analysis is ased on studies aout the increase of the UO 2+x powder surface area and its surface energy during the storage, with the value of x [14-16]. According to Bannister [15], the oxidation of the UO 2 particles powder due to the storage occurs only on its more external layers, i.e., the percentage of higher oxides may e negligile. It explains the asence of highest uranium oxides in the stored powder X-ray diffraction spectrum. All these changes also impact the particles packing in the green pellet; which influences the sinteraility and the microestrutural evolution of the pellet during the sintering process [9]. So, it can e assumed that in the green pellets otained from the fresh powder there are large pores and low surface energy, and in the in the pellets otained from the stored powder there are a fraction of large pores and the remaining fraction consisting of very small pores and higher surface energy. After sintering at 1700 C/2h/H 2, oth pellets otained from the fresh and stored powder, reached ~95% DT, despite having started from different green densities. Due to sintering at 1750 C/4h/H 2 the density of the pellet otained from fresh powder increased only 0.32% TD in relation to the pellet otained at 1700 C/2h/H 2, while etween the ones otained from the stored powder the difference was 1.25% TD. This difference can e explained y the higher surface energy of the green pellet otained from the stored powder, where it is assumed the predominance of the surface diffusion mechanism, that does not promote densification, during a major part of the sintering cycle. The heating rate has een set at 600 C/h in all experiments. So, the densification process should egin earlier in the fresh powder pellets, and it also occurred in relation to the grain growth process. These results are in agreement to those of Balakrishina et al [8], who estalished that there is a transition temperature etween densification and microstructure coarsening regimes. They also found out that in UO 2 pellets sintered and resintered at the same conditions, once estalished that the microstructure coarsening, there is no additional densification due to the resintering. So, it seems reasonale the density difference of 0.32% TD etween the pellets otained from the fresh powder since their sintering were carried out at different temperatures and times. The grain growth occurred from the center to the edge of the pure UO 2 pellet, producing an anisotropic structure, except in the stored powder pellet and in the fresh powder pellet of the sintering test, oth sintered at 1700 C/2h/H 2. The development of this type of microstructure is typical of sintering in the presence of volatile impurities, presumaly emerged from the pellets of the large atches, which increase the oxygen pressure in the pellet center, increasing the sinteraility in this region. Particularly, the sinteraility of UO 2 is very sensitive to small changes in the oxygen pressure of the sintering atmosphere. In fact, studies indicate the presence of volatile impurities as the more plausile condition to explain this microstructure development [7, 8]. Parameters like heating rate, gas flow rate, initial content of impurities and others should also e considered together. Balakrishina et al [8], e.g, have reported that keeping gas flow rate constant and increasing the numer of pellets in the sintering furnace, difficulties may arise in the impurities and humidity removing through the sintering atmosphere. Consequently, activated sintering can occur at temperatures as low as 1200 C. The pellet otained in the sintering test (Figure 3) developed an isotropic grain microstructure. This test was carried out with just 5 fresh powder pellets, while in the experiment, also involving pellets otained from fresh powder, was carried out with 39 pellets, and the anisotropic grain microstructure was developed. So, as the other sintering conditions were the same, only the different quantities of pellets present on the tests may

9 explain these different grain microstructures. Even though, in the stored powder pellet sintered at 1700 C/2h/H 2, the increasing of the oxygen pressure in the sintering atmosphere may have increased the sinteraility of the material, its high surface energy was responsile for the predominance of the surface diffusion mechanism in a significant part of this sintering cycle, avoiding the development of the anisotropic grain microstructure. The same must have occurred during the sintering at 1750 C/4h/H 2, where the surface energy may have promoted a delay in the grain growth process, revealing a fraction of small grains close to the edge of the pellet (Figures 7 and 9). Although these arguments are plausile, specific tests would e required to attriute, unquestionaly, this type of microstructure to the volatile impurities. The higher surface energy of the stored powder produced different impacts on the densification and grain growth of the pure UO 2 pellets sintered at 1700 C/2h/H 2 and 1750 C/4h/H 2. In relation to the UO 2 pellets with additives, the system Al 2 O 3 -SiO 2 forms an eutectic etween mullite and cristoalite around 1587 C, containing approximately 95 mol% SiO 2 [3]. In these pellets there are typical signs of sintering in liquid phase presence, since the density, the grain shape and size were extremely changed. The strongest evidence that this kind of sintering has occured is the contrast etween the pellets microstructures sintered at 1750 C/4h/H 2, with and without Al 2 O 3 -SiO 2 (Figures 7 and 10). The pure UO 2 pellet heterogeneous microstructure (Figure 7) has een attriuted to different oxygen pressures in the sintering atmosphere in its different regions. It occurs due to the defects concentration in the UO 2 crystalline lattice(schotky defects) increases when the oxygen pressure rises increasing the U cation diffusivity in the UO 2 crystallinelattice. In the pellets with addition of Al 2 O 3 -SiO 2 after the liquid phase formation, the wetting of UO 2 crystals surface occurs, and the gas-solid interface ecomes disaled and it s replaced y liquid-vapor interface (pores), then it ecomes the driving force operating the system [9,10]. Until the additives are molten, the solid state sintering of the UO 2 pellets occurs normally. Based on the grain microstructure of the pure stored powder pellet sintered at 1700 C/2h/H 2 (Figure 5), it can e assumed that the presence of the liquid phase has occurred while this grain microstructure was still homogeneous and composed of very small grains. Since then, there were changes over the mechanisms governing the densification and grain growth, i.e., the typical liquid phase sintering mechanisms prevail: rearrangement at the first stage, solution reprecipitation at the intermediate stage and diffusion in solid state at the final stage of sintering. The solution reprecipitation mechanism, which is responsile for the grain growth at the second stage, promotes a fast growth of the largest grains at the expense of the smallest ones, up to the disappearance of the smallest (Ostwald ripening). This process, ased on the dissolution of the smallest particles, diffusion of the ions through the liquid phase and reprecipitation of these ions on the surface of the largest particles, is a process faster than the processes of diffusion in solid phase [9,10], explaining why the grain size distriution plots of these pellets are on the right of the pure UO 2 pellet curves, sintered at 1700 C/2h/H 2 (Figure 8). Moreover, the high surface energy of the pure UO 2 pellet inhiits its grain growth, increasing this difference. During the sintering at 1750 C/4h/H 2, despite the high-surface energy of the pure UO 2 pellet, a part of the grain size distriution plot of this pellet is on the right of the grain size distriution plot of the pellets with addition of Al 2 O 3 -SiO 2, which represents the largest grains in the center of this pellet (Figure 9). Again, the narrow grain size distriution of the

10 pellets with addition of Al 2 O 3 -SiO 2 occurred due to the replacement of the gas-solid for the solid-liquid interface, as discussed efore [9,10]. This fact reinforces the hypothesis that the volatile impurities must have changed the oxygen pressure in the sintering atmosphere in the center of the pure UO 2 pellets, and the liquid phase sintering in the pellets with addition of Al 2 O 3 -SiO 2 occurred. Both sintering conditions promoted appreciale difference on the grain size distriution, it may not e concerned to the different contents of Al 2 O 3 -SiO 2 (Figures 8 and 9). Matsuda et al [17] have presented similar results, in which they showed that a small amount of Al 2 O 3 -SiO 2 (~0.04 wt%) was sufficient to grow grains in the UO 2 pellets, and higher contents decreased the density and rought only a little grain growth. The density decreasing may e related to the increasing pores size due to trapped gases, resulted from the volatilization of some additives into the pores. According to Randall [9], this reverse of the densification process may e attriuted to a too long final stage of sintering. This is a possiility in the case of the pellets sintered at 1750 C/4h/H 2, in which due to the igger cycle, certainly the third sintering stage was longer. Matsuda et al [17] also have sintered at 1750 C/4h/H 2, UO 2 pellets doped with Al 2 O 3 -SiO 2, up to 0.6 wt%, and they have oserved density decreasing in these UO 2 pellets with the Al 2 O 3 -SiO 2 content increase, which was attriuted to the volatilization of these additives. 5. CONCLUSIONS Densification and grain growth studies on pellets otained from pure UO 2 and with addition of Al 2 O 3 -SiO 2 have een performed in two sintering conditions. The following main results have een otained: The UO 2 powder energy surface removal showed great potential on the pellets grain growth promotion at expense to its densification; Undesirale heterogeneous grains microstructures were otained in some pellets, depending on the powder characteristics, the sintering conditions and volatile impurities presence; Addition of small contents of Al 2 O 3 -SiO 2 proved to e an important technical acquisition of homogeneous large grains microstructure, regardless their sintering conditions and volatile impurities presence. REFERENCES 1. H. Assmann, W. Dörr, Microstructure and Density of UO 2 Pellets for Light Water Reactors as Related to Powders Properties, Proceedings of Ceramic Powders, Netherlands, pp (1983). 2. IAEA. Vienna, 1999, p.334. TECDOC W. D. Kingery, H. K. Bowen, D. R. Uhlmann, Introduction to Ceramics, John Wiley & Sons, New York & USA (1975). 4. H. Matzke, On the effect of TiO 2 additions on sintering of UO 2, Journal of Nuclear Materials, v.20, pp (1966). 5. K. C. Radford, J. M. Pope, UO 2 fuel pellet microstructure modification through impurity additions, Journal of Nuclear Materials, v. 116, pp (1983). 6. Hj. Matzke, Diffusion processes and surface effects in non-stoichiometric nuclear fuel oxides UO 2+x and (U, Pu)O 2±x, Journal of Nuclear Materials, v. 114, pp (1983).

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