Stress, MPa
2. EXPERIMENTAL PROCEDURES 1. Manufacturing of mixed powder
UO2 powders produced by Dry Conversion process were used in this experiment. The impurities and characteristics of undoped UO2 powder are shown in Table I. In order to simulate the commercial UO2 pellet, U3O8, AZB as a pore former and ACROWAX as a lubricant were added as much as 12, 0.3 and 0.2wt%, respectively.
Table I. Characteristics of UO2 powder
Properties Value
O/U ratio 2.046
Particle Size <400µm 100%
Specific Surface Area (m2/g) 2.34
Apparent Density (g/cm3) 1.1
Moisture Content (ppm) 1200
Impurity Value
(ppm)
Impurity Value
(ppm)
Al 0.8 Cr 1.3 B 0.2 Cu <10 Bi 0.3 Fe <20 Ca <5 In 0.8 Cd 0.06 Mg 2.3
C 32 Mn 0.2 Cl 2 Mo 0.3
F 7 Ni 1.0 N 25 Pb <5 Co <5 Si <20
Mixing was carried out for 1.5 hrs in “Turbula Mixer”. The mixed powders including U3O8, pore former and lubricant were used as standard powders. When the standard powder was mixed with TiO2 and Al2Si5O5(OH)4 powders, two-step mixing was performed to enhance the homogeneity.
The standard powder was mixed with additives with the ratio of 9 to 1 for 1.5 hrs in the first mixing, and in the second mixing step, the mixture was diluted with additional standard powder to get a final composition and then mixed again for 1.5 hrs. In the mixed powder, the contents of TiO2 and Al2Si5O5(OH)4 were in the range of 0.06 to 0.2% by weight.
2.2. Pelletizing
The prepared powders were pressed to have green densities of 5.95±0.05 g/cm3. The green pellets were sintered in a tube furnace at 1740°C for 4 hrs. These sintering conditions were the same as those of commercial plant. The furnace was heated at the rate of 5°C/min and held at 700°C for 1 h to burn the lubricant out. Pure H2 gas was blown to keep the interior of the furnace in a reduced atmosphere. The flowing rate of H2 gas was 20 ml/min.
2.3. Annealing
After sintering, all the sintered pellets were annealed at 1700°C up to 48 hrs. Annealing process was performed at the same condition as the sintering process except temperature and time.
2.4. Measuring properties
Sintered density and open porosity were measured using immersion method. In all cases the densities were reported as a percentage of the theoretical density of UO2(10.96 g/cm3). Pellets were sectioned longitudinally and polished to observe microstructure. For measuring grain size, thermal etching was performed at 1250°C for 1.5 hrs. Grain size was determined by a linear intercept method.
3. RESULTS AND DISCUSSION
The UO2 pellets containing TiO2 or Al2Si5O5(OH)4 showed lower density than the standard pellet as shown in Figure 1. The density tended to slightly decrease with additive content and the effect of Al2Si5O5(OH)4 was greater than that of TiO2at the same amount.
Both of these additives had an influence on reducing open porosity of UO2 pellet (Figure 2).
A large amount of open porosity was decreased by doping additives but there was no significant difference in the amount of open porosity with additive content and types.
The grain size increased with the addition of additive as shown in Figure 3. Apparently, substantial increases in grain size can be obtained with the addition of additives. TiO2 was more effective to increase grain than Al2Si5O5(OH)4. The grain was coarsened up to 48 µm when 0.2 wt % TiO2 was added while Al2Si5O5(OH)4doped pellet increases its grain only to 20µm at the same amount.
It is widely known that liquid sintering is the main cause of grain growth of TiO2 and Al-Si compound doped pellets. From the grain morphology it appears that a liquid phase was
present during sintering(Figure 4). It is correspondent with the phase diagram of Ti-O-U in which the eutectic phase begins to appear at the temperature of 1645oC.
The formation of eutectic phase in Al-Si-U-O system commences at the similar temperature as that of Ti-O-U system. As all the test specimens were sintered at 1740oC, temperature is high enough to form liquid phase in both UO2 pellets including TiO2 and Al2Si5O5(OH)4. It is evident that uranium ions can increase their mobility in the liquid phase.
The enhanced diffusion rate of uranium ion accelerates sintering process resulting in the coarsening of grain. In the case of TiO2, charge compensation upon the addition of Ti2+ to UO2 matrix can occurs by creating the uranium vacancies. Since diffusion of uranium ions is the rate controlling step in the process of UO2 sintering, sinterability of the matrix can be increased by forming these uranium vacancies. The difference of grain size between TiO2and Al2Si5O5(OH)4 doped UO2 pellet shown in Figure 3. can be explained by the increased uranium vacancies.
The enhanced uranium ion diffusion rate also affects the pore morphology in the matrix as shown in Figure 5. The standard UO2 pellet has pores with irregular shape while pores of pellet doped with TiO2 are round. The pore size of Al2Si5O5(OH)4 doped pellet are larger than those of the standard pellet and had a slightly round shape, but the degree of roundness is not completed.
Pores change their shape in order to reduce free energy. The free energy of a substance is determined by the ratio of surface area to volume and spherical shape is the most stable morphology since its ratio is lower than any other shapes. Therefore, a shape of pore would be transformed to round one if there is a sufficient driving force. The addition of TiO2 may give enough driving force for changing of pore from irregular to round shape ascribing to the enhanced diffusion rate resulting from the formation of liquid phase and uranium vacancies. On the contrary, UO2 pellet containing Al2Si5O5(OH)4has oblong shaped pores and their size is small compared with those of TiO2doped UO2 pellet.
Figure 6 shows the effect of additives on the pore size distribution of pellet. The pore size shown in Figure 6 represents the average values of certain range, i.e.,>1, 2~5, 5~10, and
<10 µm. The addition of TiO2 and Al2Si5O5(OH)4reduce the amount of small pores with diameter under 1 µm and increase large pores with diameter over 10 µm. Both additives show a similar trend but the fraction of pore over 10 µm is greater in TiO2doped pellet. It is thought that these large and spherical shaped pores led to swelling rather than densification after annealing for 48 hrs. while the standard pellet and Al2Si5O5(OH)4doped UO2 pellet were densified(Figure 7).
Densification process that is a typical phenomenon for pellet with pore if sufficient thermal energy is provided occurs by elimination of pores. Accordingly, the amount of open porosity as well as the size and morphology of pore existing in the matrix are major factors in determining the degree of densification. Grain size of pellet is also important because it affects the diffusion rate of pore.
In comparison with small pore with irregular shape, large pore with spherical one is difficult to move and hard to dissolve out. Instead of being removed, they tend to grow by consuming neighboring small pores. This is confirmed by Figure 8 that shows the difference of pore size distribution of 0.2% TiO2 doped UO2 pellet before and after annealing for 48 hrs. It can be clearly seen that pores having the size greater than 10 µm are definitely increased by annealing while the amount of pore with diameter less than 10 µm is decreased. The additive
content seems no profound effect on density change after annealing in case of TiO2 is added.
On the other hand, there was an inverse linear relationship between additive content and density change in the UO2 pellet containing Al2Si5O5(OH)4.
4. CONCLUSIONS
The microstructural changes of TiO2 and Al2Si5O5(OH)4 doped UO2 pellets have been examined. These two additives are known to increase grain size of pellet by forming liquid phase. The typical features of liquid phase sintering can be found in both test specimens.
The sintered density and open porosity decrease by the addition of additive. TiO2 is more effective for grain growth than Al2Si5O5(OH)4 and the largest grain size was 48 µm when 0.2 wt% TiO2 was added. These two additives also change the morphology of pore from irregular to spherical shapes.
The amount of pores with diameter greater than 10 µm increases while that of small pores less than 1 µm is significantly reduced by the additive. Pore size and morphology are somewhat different between TiO2 and Al2Si5O5(OH)4doped UO2 pellets and the difference of temperature at which liquid phase begin to emerge is believed to be the main reason of difference in pore size and morphology.
TiO2 doped UO2 pellets have a sufficient mobility for grain growth and round pore because its liquid phase begins to appear at the temperature below 100oC from sintering temperature. The density decrease shown in TiO2 doped UO2 pellet after annealing for 48 hrs is attributed to these microstructural characteristics.
It can be found that fraction of pores with diameter over 10 µm increase while that of small pores decreases. There was no tangible difference in density change depending on the amount of additives.
REFERENCES
[1] W. CHUBB AND A.C. HOTT, “The influence of fuel microstructure on in-pile densification,”Nucl. Technol.,26, 486–495 (1975).
[2] H. ASSMANN, W. DOERR, G. GRADEL AND M. PEECHS, “Doping UO2 with Niobia-beneficial or not,” J. Nucl. Mater.,98, 216–218 (1981).
[3] P.T. SAWBRIDGE, C. BAKER AND R.M. CORNELL, ”The irradiation performance of Magnesia-doped UO2 fuel,” J. Nucl. Mater.,95, 119–128 (1980)
[4] J.B. AINSCOUGH, F. RIGBY AND S.C. OSBORN, “The effect of titania on grain growth and densification on sintered UO2,”J. Nucl. Mater.,52, 191–202 (1974).
[5] K.W. LAY, “Grain growth in UO2-Al2O3 in the presence of a liquid phase,” J. Am.
Ceram. Soc.,51, 373–376 (1968).
[6] G. MAIER, H. ASSMANN, “Resinter testing in relation to in-pile densification,” J. Nucl.
Mater., 153, 213–220 (1988).
0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 94.0
94.5 95.0 95.5 96.0 96.5
Standard Pellet:95.72%T.D.
Sintered Density(%T.D.)
A d d itiv e C on ten t(w t% ) UO2-TiO2
UO2-Al-Si
FIG. 1. Variation of sintered density due to additives.
94.0 94.5 95.0 95.5 96.0 96.5
0.00 0.05 0.10 0.15 0.20 0.25 0.30
Standard Pellet:0.25%
Amount of open porosity(%)
Sintered Density(%T.D.) UO2-TiO2 UO2-Al-Si
FIG. 2. Variation of open porosity due to additives.
0.00 0.05 0.10 0.15 0.20 0.25 0
10 20 30 40 50 60
Grain Size(µm)
A d d itiv e C o n te n t(w t% )
UO2-TiO2 UO2-Al-Si
FIG. 3. Variation of grain size due to additives.
(b)
(
Liquid Phase
10 µm
(c) (a)
FIG. 4. Metallography showing grain growth due to additives.
(a) Standard pellet (b) Al2Si2O5(OH)4 (c) TiO2
10 µm
Liquid Phase
10 µm
(a)
(b)
(c)
FIG. 5. Variation of pore morphology due to additives.
(a) Standard pellet (b) Al2Si2O5(OH)4 (c)TiO2
10 µm
10 µm
10 µm
0.0 2.5 5.0 7.5 10.0 12.5 0.0
0.5 1.0 1.5 2.0 2.5 3.0
Volume(%)
Pore Size(µm) UO2
UO2-TiO2 UO2-Al-Si
FIG. 6. Variation of pore size distribution.
0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 -0.5
0.0 0.5 1.0 1.5
Density Change(%T.D.)
A d d itiv e C o n te n t(w t% )
UO2-TiO2 UO2-Al-Si
FIG. 7. Density change of pellet after being annealed for 48 hrs.
0.0 2.5 5.0 7.5 10.0 12.5 0.0
0.5 1.0 1.5 2.0 2.5 3.0
Volume(%)
Pore Size(µm) UO2-0.2% TiO2
UO2-0.2% TiO2(annealed for 48 hrs)
FIG. 8. Pore size distribution of TiO2 doped UO2 pellet.
EFFECT OF SINTERING GAS ON THE GRAIN SIZE OF UO2 PELLETS DERIVED FROM DIFFERENT POWDER ROUTES*
Keon Sik Kim, Kun Woo Song**, Jae Ho Yang, Youn Ho Jung Advanced LWR Fuel Development,
Korea Atomic Energy Research Institute, Daejon, Republic of Korea
Abstract
UO2 pellets were sintered at 1600-1700°C using three different UO2 powders - the ADU, AUC and DC routes.
The oxygen partial pressure of sintering gas was controlled using the method of adding CO2 gas to H2 gas, and the CO2-to-H2 ratio ranged from 0.1 to 0.4. The grain size of UO2 pellets is nearly independent of the CO2-to-H2 ratio when the AUC- or DC-UO2 powder is used, but it is increased up to 25 µm for the ADU-UO2 powder if the CO2-to-H2 ratio is higher than 0.25. Impurity analysis of the three UO2 powders reveals that phosphorus is high in the ADU-UO2 powder but not in the other powders. Phosphorus was added to the AUC-UO2 powder, and then UO2 pellets were sintered in the mixed gas of CO2 and H2 (CO2/H2=0.25). It has been found that the grain size of about 25 µm is achieved with a phosphorus content as small as 30 ppm. So a large-grained UO2 pellet can be produced if the P-doped UO2 powder is sintered in the slightly reducing gas.
1. INTRODUCTION
Uranium dioxide (UO2) fuel pellets which are widely used in light water reactors are manufactured by a conventional powder processing method [1]; UO2 powder is pressed into green pellets and then sintered at 1700-1780 in hydrogen-containing gas. The UO2 pellet properties — density, pore structure, and grain size — greatly influence the in-reactor performance of the UO2 pellet. Especially, the amount of fission gas released during irradiation decreases as the grain size of UO2 pellets increases [2], and thus large-grained UO2 pellets are desirable at high burnup.
The fabrication of large-grained UO2 pellets has been investigated extensively, so that many fabrication methods have been developed. The most prevailing method is simply to add sintering agents to UO2 powder. Many sintering agents are known such as niobia[3,4], titania[5], chromia[6], and magnesia[7]. It is also known that the grain size of UO2 pellets can be increased, without sintering agents, by using the UO2 powder with a high sintering activity [8]. Wood and Perkins [9] have reported that the addition of UO2 seeds to UO2 powder enhances grain growth, and Song et al. [10]
have found that the addition of U3O8 seeds to UO2 powder increases the grain size.
In addition, it has been reported that the grain size of UO2 pellets can be increased by a high oxygen partial pressure in the sintering gas [11]. This technique is based on the fact that an oxidizing gas provides excess oxygen for UO2 and the uranium diffusion is accelerated in this hyperstoichiometric state. According to Assmann et al [11], the UO2 pellet that was sintered at 1300°C in CO2 gas using the UO2 powder derived from the AUC (Ammonium Uranyl Carbonate) route, has a bimodal grain structure; a mixture of large grains and small grains. The bimodal grain structure is not formed when the UO2 powder derived from the ADU (Ammonium Diuranate) or DC (Dry Conversion) route is used [12]. However, the grain size of UO2 pellets is, generally, not different from each other between the powder routes when each UO2 powder is sintered in a reducing gas. These results suggest that UO2 powder properties, which are dependent on the powder routes, might affect the grain size of UO2 pellets if UO2 powder is sintered in non-reducing gas.
The sintering gas can be classified into three types depending on its oxygen partial pressure; reducing, slightly reducing, and oxidizing. UO2 pellets are usually fabricated in a reducing gas, which is a GGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGG
QGThis work has been carried out under the nuclear R&D program supported by the Ministry of Science and Technology in Korea.G
QQGcorresponding author, e-mail: kwsong@kaeri.re.kr telephone: +82-42-868-2579 fax: +82-42-861-7340G
hydrogen gas containing additionally H2O or CO2 gas up to about 3 vol%. But a slightly reducing gas is a gas mixture made by adding the larger amount of H2O or CO2 to H2. The sintering of UO2 pellets under this slightly reducing gas has not been much investigated. This paper deals with the sintering of UO2 pellets in a slightly reducing gas and especially describes the effect of powder routes on the grain size.
2. EXPERIMENTAL PROCEDURES
The UO2 powders used in this work were produced through three different powder routes; AUC, DC and ADU. The former two powders were made by KNFC (Kepco Nuclear Fuel Co.), and the last powder was purchased from Canada. The UO2 powders were characterized in impurity, O/U ratio, apparent density, BET surface area, and shape. Zinc stearate, a lubricant, was added to the three UO2 powders. The UO2 powder from the ADU or DC route was granulated before pressing into green pellets, but the UO2 powder from the AUC route was directly pressed into green pellets because it is has a good flow ability. The pressing pressures were fixed to be 310 MPa.
The green pellets were heated up to 1600 or 1700 and then held for 2h, followed by cooling. The sintering gases were mixtures of H2 and CO2 gases, and the ratios of CO2 to H2 gas were controlled to be 0.1, 0.15, 0.25, and 0.4, respectively. In parallel, the green pellets were sintered in pure hydrogen gas. The ratio of CO2 to H2 gas was fixed throughout the whole sintering (heating, holding and cooling) period. The oxygen partial pressure of the mixed H2 and CO2 gas is determined by the thermodynamic equilibrium of the following reactions:
CO2 + H2 → CO + H2O 2H2O → 2H2 + O2 2CO + O2 → 2CO2
The oxygen partial pressure was calculated with the aid of the SOLGASMIX program [13]. Figure 1 shows the relation between oxygen partial pressure and temperature for the mixed CO2 and H2 gases.
The oxygen partial pressure tends to increase with the ratio of CO2 to H2.
The density of sintered pellets was measured by the water immersion method (Archimedes method).
The internal structure of sintered pellets was examined by microscopy, and the grain size was
mIG. 1. Oxygen partial pressures as a function of temperature for various sintering gases.
measured by the linear intercept method. In addition to the pellet fabrication, the shrinkage of green pellets during sintering was analyzed by dilatometry.
3. RESULTS AND DISCUSSION
The SEM micrographs are shown in Fig. 2 for the three kinds of UO2 powders. The UO2 powder derived from the AUC process is characterized as a round shape with no agglomeration. But the UO2 powder derived from the ADU or DC process is softly agglomerated, so the size appears larger than the real size. The physical properties of the three UO2 powders are listed in Table I. The oxygen to uranium ratio and BET surface area are different depending on the powder routes. The impurity content is shown for the three UO2 powders in Table II. Impurities are also a little dependent on the powder routes. However, the physical properties and impurities satisfy with the technical specification, in spite of the slight differences in properties between the powder routes.
Impurity DC AUC ADU Al 1.2 1.3
C 15 71 33
Cl 3 4
F 12 68 8
N 24 6
V 0.21 5.2
P - - 22
Na - - 7
K - - 12
Properties DC AUC ADU
O/U ratio 2.03 2.10 2.15
Moisture Content (ppm) 750 1540 1030
Specific Surface Area(m2/g-UO2) 2.29 5.45 2.30
particle size (µm) 4.8 15 2.5
U content (wt%) 87.896 87.446 87.235
10µm 10µm 10µm
(a) (b) (c)
FIG. 2. SEM micrographs showing powder morphology. (a)AUC route, (b)DC route, (c)ADU route.
Table I. Properties of the three UO2 powders
Table II. Impurity in the three types of UO2powders
Figure 3 shows the relation between grain size and CO2-to-H2 ratio when the three UO2 powders are sintered for 2 h at 1600°C. Fig. 4 shows the same relation for the 1700°C sintering. The dependency of grain size on the CO2-to-H2 ratio seems to be very similar between 1600 and 1700°C. The grain size of UO2 pellets remains nearly constant over the whole range of gas ratios for both the AUC- and DC-UO2 powders, but it rises greatly for the ADU-UO2 powder when the CO2-to-H2 ratio is increased from 0.15 to 0.25. The grain size can be increased up to 25 µm at 1600°C and 30 µm at 1700°C. A large grain results form fast grain growth during sintering and thereby means that the uranium diffusion was fast in the sintering of only the UO2 powder from the ADU. This increase in uranium diffusion might not be associated with the initial O/U ratio of powder since the three powders are sintered in the same gas atmosphere.
Figure 5 shows the microstructure of UO2 pellets sintered at 1600°C for the CO2-to-H2 ratio of 0.25.
Figures 5(a), Fig. 5(b), and Fig. 5(c) stand for the AUC route, the DC route and the ADU route, respectively. The UO2 pellet derived from the ADU route seems to be distinct from the other pellets in
FIG. 3. Dependence of grain size on the CO2/H2 ratio for the UO2 pellets sintered at 1600oC using
FIG. 3. Dependence of grain size on the CO2/H2 ratio for the UO2 pellets sintered at 1600oC using