Preparation and Characterization of Large Grain UO2 for Accident Tolerant Fuel

Large grain UO2 is considered as an accident tolerant fuel with great application potential due to its competitive advantage of good fission gas retention. In this paper, the influence of preparation parameters such as sintering atmosphere, mixing process, powder pretreatment and grain growth additives on the grain size of UO2 is systematically studied. The result shows that the factors mentioned above have different effects on the grain size of UO2. The grain growth of UO2 pellet sintered in oxidizing atmosphere is better than those in reducing atmosphere. The wet mixing process has a significant advantage over the dry mixing process. In addition, the powder pretreatment has little effect on grain growth while the influence of additives plays the main role. Large grain UO2 pellets with uniform grain size up to 150 μm are successfully prepared. Finally, the thermo-physical properties of the pellets are investigated.


INTRODUCTION
After the Fukushima accident in 2011, the Accident Tolerant Fuels (ATFs) has been paid much attention by researchers. As a kind of ATF fuels, large grain UO 2 fuel is characterized as the significantly increased UO 2 grain size in the fuel pellet, by adding trace additives and optimized preparation process. Grain boundaries and open pores are the main means of fission gas release (FGR). The specific surface area of grain decreases with the grain size increasing, and thus the pores will be distributed in the grain as far as possible. Moreover, the diffusion distance of fission gases from the grain interior to the grain boundary is increased accordingly. This means a higher fission gas retention rate for large grain UO 2 . On the other hand, the plasticity and thermal creep properties of large grain UO 2 are higher than normal UO 2 (Kang et al., 2010). Then the radiation swelling, which is of great significance to reduce the Pellet-Cladding Interaction (PCI), could be reduced. Thus, the large grain UO 2 fuel is capable to achieve higher burn-up than the normal UO 2 fuel (Turnbull, 1974;Hastings, 1983;Une et al., 1993). Meanwhile, the large grain UO 2 fuel is almost composed of pure UO 2 . It can completely inherit the advantages of normal UO 2 fuel, such as high melting point, good thermal stability, good compatibility with coolant and cladding material, excellent irradiation stability and water corrosion resistance. Besides, the large grain UO 2 fuel owns perfect compatibility to the present Light-Water Reactor plants (LWRs) fuel manufacturing and operating system. This is much favorable for nuclear power plants and nuclear fuel plants. Based on the advantages in terms of safety, economy, and compatibility, the large grain UO 2 fuel is considered as one of the most promising ATFs.
Many studies have been carried out in the past to investigate the factors that contribute to the grain growth of UO 2 pellets. In general, sintering temperature, holding time and sintering aids were reported to be important for increasing the grain size of UO 2 pellets (Singh, 1977). However, much higher temperature could lead to abnormal growth of UO 2 grains (MacEwan and Lawson, 1962). Long-term heat preservation had little effect on grain growth (Glodeanu et al., 1987), but would increase the cost of fuel production. The most effective way was adding additives to the UO 2 matrix. The common additives included Cr 2 O 3 (Bourgeois et al., 2001;Arborelius et al., 2006;Yang et al., 2012), TiO 2 (Amato et al., 1966;Ainscough et al., 1974;Yao et al., 2016), Nb 2 O 5 (Killeen, 1975;Song et al., 1994), Al 2 O 3 (Oh et al., 2014;Une et al.,2000) and MgO (Sawbridge et al., 1980). Recently, Yang et al. (2012) (Che et al., 2018). Satisfactory simulation results have been obtained. However, few of the above literatures reported the influence of the preparation process on the grain size of UO 2 .
In this work, the effects of different additives, mixing methods, powder pretreatment processes, forming pressure, and sintering atmosphere on the grain growth of UO 2 were systematically studied. Additionally, the thermo-physical properties of the large grain UO 2 were also discussed.

Starting Powder and Preparation Process
UO 2 powders were obtained from CNNC Jianzhong Nuclear Fuel Co., Ltd. The powders purity was 99.9%, and its sizes ranged from 10 to 20 μm. The O/U ratio of the UO 2 powders was 2.17 detected by thermos-gravimetric analysis. The zinc stearate (99% purity, 50 μm particle size, Macklin Scientific Co., LTD., China) was used as binder and lubricant to enhance the strength of the green body, and the addition amount in the pellets is 0.1 wt%. Six types of additive powders were used, as listed in Table 1.
Firstly, the raw materials were mixed. During the mixing process, three different mixing methods were studied. The difference between the two dry mixing methods was with or without grinding media. The Dry mixing was performed on a planetary mixer using a WC container, while the wet mixing was performed on a horizontal mixer using a nylon tank. Both mixing processes run at a speed of 300 r/min for 24 h. The powder of wet mixing was obtained by heating at 100°C for 5 h. Secondly, the mixed powders were granulated. Two kinds of granulation processed were adopted. Spark Plasma Sintering (SPS) can activate the powder and realize the densification of high melting point materials in a short time. In this experiment, the difference between SPS pre-sintering granulation and traditional cold press granulation was investigated. The SPS pre-sintering granulation process was heating the powder at 800°C for 1 min. A pressure of 25 MPa was applied on the powders during the whole sintering process. After the SPS pre-sintering process, the density of pellets reached 68%. The Cold press granulation was to hold the powder under 300 MPa for 5 min. Then, the green body was crushed and sieved with a 50mesh screen. Thirdly, the mixed particles were introduced into a cemented carbide mold for biaxial pressing with 100 MPa, 200 MPa, and 300 MPa respectively. The green density of pellets formed under different processes were distributed within the range of 5.7-6.5 g/cm 3 . Finally, the green body was sintered at temperature of 1680 and 1700°C. The holding times were 1.5 and 4 h. For comparison, pure UO 2 pellets were prepared by the same technological process. H 2 , H 2 -H 2 O, and H 2 -CO 2 were used as sintering atmosphere. The content of H 2 O or CO 2 in H 2 were 1 vol% and 2 vol%, respectively. The details of the above process conditions and parameters were listed in Table 2.

Microstructure Characterization
The pellets were successively grinded by 500, 1000, 2000 mesh SiC sandpaper, then polished with 1 μm diamond suspension, and finally eroded with H 2 O 2 /HCl etching solution. The erosion time is 1-3 min. The grain morphology was analyzed by laser confocal microscopy (LSCM, LEXT OLS4000, Olympus). Grain size was determined by the linear intercept method, and more than 200 intercepts were counted for each sample.

Thermal Physical Performance
The thermal diffusivity (TD) of the pellet was determined by a laser testing equipment (LFA 427, Netzsch, Germany). The sintered pellet was processed into the final size of Φ8 × 3 mm for the thermal diffusivity test. The testing temperature range was 25-1000°C, with an interval of 200°C. The density (ρ p ) of UO 2 pellet was measured by the Archimedes method. The thermal expansion coefficient of the pellet was measured by a thermal expansion tester (DIL 402, Netzsch, Germany). The testing temperature range was 25-1200°C. The sintered pellet was processed into the final size of Φ8 × 13 mm for the thermal expansion test.
The thermal conductivity (TC) λ c (W m −1 K −1 ) was determined from the following formula: where ρ p (g cm −3 ) is the density of pellet, D d (mm 2 s −1 ) is the thermal diffusivity of pellet, C p (J g −1 K −1 ) is the specific heat capacity of pellet. The value of C p is dominated by UO 2 where the additives is ignored due to its low mass fraction (≤0.5 wt%). The specific heat capacity of UO 2 can be derived from the following equation (Fink, 2000): where C UO2 refers to the heat capacity of UO 2 in J mol −1 K −1 , and a (a T/1000) refers to the temperature in K. The coefficient of thermal expansion (CTE) was calculated from the following computational formula: where E T (K −1 ) is the CTE of UO 2 pellet at temperature T, T L UO2 (T) L (273) × 9.9734 × 10 −1 + 9.802 × 10 −6 T − 2.705 × 10 −10 T 2 + 4.391 × 10 −13 T 3 (4) and for 923-3120 K: where L UO2 (T) and L (273) are lengths of UO 2 at temperatures of T(K) and 273 K, respectively.

RESULTS AND DISCUSSIONS
The Effect of Different Additives on UO 2 Grain Growth Figure 1 shows the metallographic photos of UO 2 doped with different additives. The sintering process is performed at 1700°C for 1.5 h. The average grain size of doped UO 2 with different additives is listed in Table 3. In this experiment, the average grain size of pure UO 2 sintered under H 2 -CO 2 atmosphere is about 16 μm. It can be found that Al 2 O 3 has almost no effect on the growth of UO 2 grains. This is consistent with the calculation results reported by Cooper et al. (2018). CaO has a similar promoting effect with MgO. Cr 2 O 3 shows the best promotion effect on grain growth of UO 2 . This can be attributed to the higher concentration of uranium vacancies produced by doped Cr 2 O 3 and the liquid phase behavior of Cr 2 O 3 in the sintering process. Therefore, Cr and Cr 2 O 3 are using as the additives in the following experiments. The effect of doped NiO is reported for the first time. However, the effect is contrary to expectations. The grain size of UO 2 doped with NiO is smaller than pure UO 2 under the same condition. This indicates that NiO can inhibit the grain growth of UO 2 . According to the defect chemistry theory by Cooper, additives can increase the concentration of negatively charged uranium vacancies and thus promote UO 2 grain growth (Cooper et al., 2018). Therefore, the doping of NiO may cause a decrease of the uranium vacancy concentration during the sintering process. On the other hand, NiO can be reduced by H 2 above 400°C (Janković et al., 2008). So, there is the possibility of interaction between Ni and UO 2 , and thereby reducing the effect of oxygen potential on UO 2 . At the same time, the dispersion of liquid Ni on the grain edge may pin the migration of UO 2 grain boundaries. In summary, the doping effect of NiO needs further research and analysis.
The Effect of Mixing Method on UO 2 Grain Growth Figure 2 shows the metallographic photos of Cr doped UO 2 pellets fabricated by different mixing processes. The sintering process is performed at 1700°C for 4 h. As shown in Figure 2, the grain morphologies obtained by the three different mixing processes are uniform, but the grain growth effect is quite different. Table 4 shows the average grain size of the Cr-doped UO 2 pellets in Figure 2. The average grain size of pellets from dry mixing with balls and without balls are 28 and 57 μm, respectively. The grain size of pellets from wet mixing with balls reaches 75 μm. It seems that the additive dispersion  in the matrix is an important factor affecting the grain growth. Due to the difference in the density of raw material density and the agglomeration characteristics of fine powders, the dry mixing process without any mixing medium leads to the worst dispersion effect of additive. As a result, the grain size of pellets from such a mixing process was small. In comparison, the introduction of a stirring medium promotes the powders to be mixed. So the grain size obtained by dry mixing with balls is much larger than that of dry mixing   samples without balls. The wet mixing process always leads to the most uniform dispersion of the additive in the UO 2 matrix, which is beneficial to the dissolution and diffusion of Cr additive. Thus, the pellets from the wet mixing process own the largest grain size.
The Effect of Powder Pretreatment on UO 2 Grain Growth Figure 3 shows the metallographic photos of Cr and Cr 2 O 3 doped UO 2 under different powder pretreatment processes. The sintering process is performed at 1700°C for 4 h. The UO 2 grain morphologies shown in Figure 3 are uniform. This indicates that the additive achieved a good mixing effect with UO 2 during the process of dry mixing with balls. Table 5 shows the average grain size of the doped UO 2 after different powder treatments. Obviously, with the same additive, the average grain size of the doped UO 2 pellet obtained by cold press granulation is larger than that obtained by the SPS pre-sintering method. The reason may be that the UO 2 is slightly sintered at 800°C (Ge et al., 2013). Therefore, the densification of the pellets becomes more difficult, and the diffusion of additives during the sintering process is inhibited. Under the same fabrication condition, the grain size of Cr 2 O 3 doped UO 2 is larger than that of Cr doped UO 2 . The reason for this difference should be related to the oxidation state of Cr in UO 2 . Martial found that the solubility of Cr in UO 2 increased with the growth of the oxidation state of Cr in the solid phase (Riglet-Martial et al., 2014). This indicates that the solubility of Cr 2 O 3 in UO 2 is much higher than that of metallic Cr. Johnson et al. studied the binary phase diagram of Cr 2 O 3 and Cr, and reported the existence of a eutectic at 1665°C (Johnson and Muan, 1968). So, the possible reason for the difference between the two additives may be caused by the easier formation of Cr-O liquid phase with Cr 2 O 3 than Cr, which is more conducive to the growth of UO 2 grains. Figure 4 shows the metallographic photos of Cr and Cr 2 O 3 doped UO 2 under different forming pressures. The sintering process is performed at 1700°C for 1.5 h. The forming parameters and average grain size of the pellets are listed in Table 6. As shown in Figure 4, the morphology of UO 2 grains is clear and uniform. However, due to the similar grain size, it is difficult to distinguish the influence of different forming pressures on the grain size of the UO 2 pellet. The grain size distribution of two kinds of doped UO 2 by statistical method is calculated and shown in Figure 5. It is found that the ratio of 150-200 μm grain size increases rapidly with the increasing of forming pressure. For example, from 100 to 300 MPa, the percentage of grain size between 150 and 200 μm for the Cr-doped UO 2 increases from 6.57 to 14.28%. In general, the average grain size of UO 2 increases with the forming pressure. Since the additives content in the pellet is very small, the green density of the UO 2 pellet doped with Cr and Cr 2 O 3 is nearly the same. As shown in Table 6, the green density of UO 2 pellets varies with different forming pressures, but the density tends to be similar after sintering. This shows that the forming pressure has little effect on the sintered density with certain strength. However, the average grain size of the pellet increases with the density of the green body. When the binder content is low, the UO 2 grain size increases with the density of the compacted body (Amato and Colombo, 1964). Besides, higher body density means less porosity. Pores are an important microstructural feature of powder compacts and have been considered to have an impediment to grain boundary migration (Nichols, 1968). During the sintering process, the pores will be located at the intersection of grain boundaries or distributed along individual boundaries (Coble, 1961). Thereby, the pores will hinder the growth of grains. Therefore, under the same sintering process, the grain sizes of pellets with higher green density are larger.

The Effect of Different Sintering
Atmosphere on UO 2 Grain Growth Figures 6 and 7 are the metallographic morphologies of normal UO 2 and doped UO 2 in different sintering atmospheres, respectively. The sintering process is performed at 1680°C for 4 h. As shown in Figures 6 and 7, the morphology of UO 2 grains is clear and uniform in size. The grain size data are listed in Table 7. It can be seen that the grain size of pure UO 2 sintered in H 2 -CO 2 is larger than that in H 2 . In addition, the grain growth of Cr and Cr 2 O 3 doped UO 2 under H 2 -CO 2 is obvious. The grain size of pellets under H 2 -H 2 O is equivalent to that under pure H 2 . By comparing the result in Table 7, the effect of Cr 2 O 3 on grain growth is still better than that of Cr. Moreover, when comparing the results of Cr doping in Figures 6 and 7, it indicates that metallic Cr is easier to form oxidation state at 1700°C than at 1680°C. In the past, the importance of oxygen potential for grain growth during UO 2 sintering has been confirmed (Assmann et al., 1986;Harada, 1997). Additionally, the results shown in Figure 7 also prove that the grain growth of UO 2 is directly related to the oxygen potential during the sintering process. The essence of grain  growth is a process of grain boundary displacement. Grain boundary displacement is a process in which atoms adjacent to the grain boundary are activated, and diffuse to the vacancies or dislocations in the grain boundary. These are mainly affected by the lattice diffusion rate and the vacancy concentration. Matzke pointed out that the main defects in  UO 2 were oxygen interstitials and uranium vacancies, and the self-diffusion rate of uranium was squared with the x of UO 2+x (Matzke, 1969;Matzke, 1983). Assuming that UO 2+x is related to the oxygen potential, then the value of x will be in equilibrium with the oxygen partial pressure (Ohse et al., 1985). The results obtained in this work are also consistent with these theories. This indicates that the grain growth of UO 2 is enhanced by increasing the oxygen partial pressure.
The Effect of Additive Content on UO 2 Grain Growth Figure 8 shows the metallographic photos of UO 2 doped with different contents of Cr and Cr 2 O 3 . The sintering process is performed at 1700°C for 4 h. As shown in the Figures, the morphology of UO 2 grains is visible after erosion and the size  is relatively uniform. The corresponding average grain size results are listed in Table 8. It can be seen that the grain sizes of Cr and Cr 2 O 3 doped UO 2 both increase with the growth of the additive amount. In general, the promotion effect of Cr 2 O 3 on the grain growth of UO 2 is better than that of metallic Cr under the same content. The increase of Cr content under the same oxygen potential will increase the number of Cr ions and thus create more uranium vacancies. So, the grain size of Cr doped UO 2 increases with the increase of Cr content in the range of 0.1 wt% to 0.5 wt%. By contrast, the increase of Cr 2 O 3 content above 0.25 wt% has a gentler effect on grain growth promotion. Regarding this, Kuri et al. combined the quantitative analysis of micro-beam XRD and EPMA data to determine that the average concentration of dissolved Cr in the UO 2 matrix was 0.07 ± 0.01 wt% (Kuri et al., 2014). However, there are still many differences between the results of this experiment and those of others. Firstly, unlike the results by Bourgeois, there is no obvious pinning phenomenon with the increase of Cr 2 O 3 content in this experiment. As shown in Table 7, the grain growth effect of UO 2 doped with Cr and Cr 2 O 3 in H 2 -CO 2 atmosphere is much better than that in H 2 -H 2 O. This indicates that the oxygen potential of CO 2 is much higher than H 2 O. Therefore, the difference between the results of this experiment and those obtained by Bourgeois was probably related to the different atmospheres in the sintering process. Secondly, Yang et al. believed that the continuous introduction of CO 2 suppressed the grain growth of Cr 2 O 3 doped UO 2 , and claimed that the UO 2 grain size was only 12.2 μm when the doping amount was 0.103 wt% (Yang et al., 2012). By contrast, in this work, with a similar Cr 2 O 3 doping amount, the grain size obtained by continuously introducing CO 2 can reach 117 μm.

Thermal-Physical Properties of the Large Grain UO 2
According to formulas (1, 2), the TD and TC of large grain UO 2 doped with Cr are presented in Figure 10. The TD and TC of normal UO 2 and values according to the empirical formula recommended by Fink (Fink, 2000) are also plotted for comparison. The calculated density of the sintered pellets in Figure 10 is about 97%. The average grain size of the 0.25 wt% Cr doped UO 2 pellet used in this TC test is about 102 μm and metallographic photo is shown in Figure 9. The preparation process was dry mixing with balls and cold press granulation. The sintering process was performed at 1700°C for 4 h under H 2 -CO 2 . It is worth noting that the TD and TC of large grain UO 2 are slightly lower than those of the normal UO 2 at lower temperatures. This can be attributed to the fact that impurities can reduce the thermal conductivity of UO 2 (Mei et al., 2014). However, As the temperature rises, the TD and TC of the large grain UO 2 turn higher than those of the normal UO 2 above 600°C. The explanation for this may be that UO 2 crystals are not isotropic, and the presence of grain boundaries will reduce TC to a certain extent (Gofryk et al., 2014). In addition, the TC increase may also relate to the higher specific heat capacity of Cr.
The CTE of large grain UO 2 doped with 0.5 wt% Cr 2 O 3 and normal UO 2 sintered in this experiment can be calculated by formulas (3-5), and the results are shown in Figure 11. The preparation and sintering processes of the samples for CTE were consistent with those for the TC testing. For comparison, the CTE of UO 2 is calculated according to the recommendation of Martin (1988). The average grain size of the Cr 2 O 3 -doped UO 2 pellet used in this CTE test is about 159 μm and the metallographic photo is shown in Figure 9. The CTE of large grain UO 2 and normal UO 2 grows with temperature. However, the CTE of large grain UO 2 is lower than that of the normal UO 2 at the same temperature. The decrease range is maintained at 11.8-16.9% in the temperature range of 200-1200°C. Thermal expansion is the result of volume expansion caused by the aggravation of the lattice vibration of the solid material during heating process. For the effect of grain size on the thermal expansion, it can be explained by the reduction of the interface which leads to the weakening of the anharmonic effect. There are larger anharmonic atomic vibrations at the grain boundaries, and the thermal expansion of the grain boundaries is 2.5-5 times that of the crystalline state (Klam et al., 1987). In addition, two experiments both show that the thermal expansion coefficient will decrease when the grain size   increases (Parker andRice, 1989, Lu andSui, 1995). In summary, the reduction in CTE is beneficial to alleviate PCI at high temperatures, and also is of great significance to improving fuel safety.

CONCLUSION
In the current work, the influence of mixing technology, sintering atmosphere, forming pressure, powder pretreatment, additives, and content on the grain growth of UO 2 was systematically studied. The results showed that the influence of additives on the grain growth of UO 2 is the most obvious. Cr 2 O 3 shows the best promotion effect among the selected additives. On the contrary, NiO shows an inhibitory effect on the grain growth of UO 2 . The preparation process and the sintering atmosphere also play an important role in the grain growth of UO 2 . Different mixing and powder pretreatments processes can affect the dispersion effect of UO 2 powder and additives. The best promotion effect is achieved by the wet mixing process. The introduction of CO 2 into the sintering atmosphere can effectively promote the grain growth of UO 2 . The average grain size of UO 2 increases with the growth of forming pressure and additive content. However, due to the low solubility of Cr in UO 2 , the grain growth effect of Cr 2 O 3 tends to be saturated above 0.25 wt%. In this work, UO 2 doped with 0.5 wt% Cr 2 O 3 shows the largest grain size, and the average grain size reaches up to 159 μm. Compared with normal UO 2 , large grain UO 2 exhibits obvious advantages in CTE and also shows higher TC at temperatures higher than 600°C. Such enhancement effect has unique advantages in improving the properties of UO 2 fuel and reducing the swelling effect of the pellets at high temperatures, which can remarkably improve the service performance and the safety of the fuel system.

DATA AVAILABILITY STATEMENT
The raw data supporting the conclusion of this article will be made available by the authors, without undue reservation.