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Fabrication of high density UO2 fuel pellets involving sol-gel microsphere pelletisation and low temperature sintering
179
Journal ofNuclear Materials 178 (1991) 179-183 North-Holland
Fabrication of high density U02 fuel pellets involving sol-gel microsphere pelletisation and low temperature sintering C. Ganguly and U. Basak Radiometallurgy Division, Bhabha Atomic Research Centre, Bombay 400 085, India
Powder-free sol-gel microsphere pelletisation and low temperature ( 1473 K) oxidative sintering processes were used in combination for fabrication of high density ( 2 96% TD) U02 fuel pellets for pressurised heavy water reactors. The “internal gelation of uranium” process of BARC was modified for preparation of hydrated gel-microspheres of U03 containing “carbon black” pore former. The gel-microspheres were subjected to controlled air-calcination at 973 K, followed by hydrogen reduction to obtain porous, dust-free and free-flowing UO1 microspheres suitable for direct pelletisation at 225 MPa. Oxidative sintering of these pellets at 1473 K in COz atmosphere followed by Ar+ H2 treatment led to high density ( 2 96% TD) UOz pellets having equiaxed grains of I 10pm and uniformly distributed “closed” spherical pores in the diameter range of 2-5 pm. Resintering of these pellets at high temperature ( 1973 K) for 8 hours in Ar+8% Hz atmosphere did not show any significant change in pellet dimension or grain size. The UOz pellets prepared by sol-gel microsphere pelletisation route had higher thermal conductivity compared to pellets of equivalent density prepared by the “powder-pellet” route. U02 pellets of large grain size (45-55 pm) and high density could be obtained with TiOz dopant and high temperature sintering in Ar+ H2atmosphere. Ti02 dopant was not effectivefor low
temperature oxidative sintering.
1. Introduction
UOZ fuel pellets for light water reactors (LWR) and pressurised heavy water reactors (PHWR) have the specification of high density (94-96% TD for LWR and 2 96% TD for PHWR). These pellets should preferably have large grain size ( 240 pm) and uniformly distributed “closed” spherical pores in the diameter range of 2-5 pm for minimising fission gas release, accommodating fuel swelling, avoiding in-pile densitication and improving thermal conductivity. The industrial methods of fabrication of high density UOz pellets all over the world are based on the “powderpellet” route and consist of cold-pelletisation of UOz powder, obtained from uranyl nitrate or UF6 feed materials by ammonium diuranate (ADU), ammonium urany1 carbonate (AUC) or the integrated dry route (IDR) processes, followed by high temperature ( 2 1973 K) sintering in hydrogen atmosphere (HZ, cracked NH3, Nz+H2 or Ar+ H,). The “powder-pellet” route involves generation and handling of large quantities of radioactive U02 powder and is hence associated with the problem of radiotoxic dust hazard. Further, the ADU and IDR derived U02 powders are extremely tine ( I 1 pm), not free-flowing and require granulation before final compaction. In
addition, high temperature ( 2 1973 K) sintering of UOZ pellets involves high energy consumption. The alternative sol-gel microsphere pelletisation (SGMP) technique, also known as “sphere-Cal” process, is based on preparation of dust-free and free-flowing hydrated gel-microspheres of UO1 by ammonia gelation (external or internal) of uranyl nitrate broth. These microspheres are dried and reduced to UOZ before pelletisation. Thus, in the SGMP route, fine UOz powder is avoided and in turn the radiotoxic dust hazard is minimised. The earlier investigations [ l-21 on SGMP process were not encouraging and led to the conclusions that sol-gel derived microspheres are hard, difficult to pelletise and lead to “blackberry” microstructure in sintered pellet because the microspheres retain their identity even after pelletisation and sintering. However, recently [ 37] with the use of “carbon black” pore former in the gelmicrospheres, the SGMP technique has been successfully adapted for fabrication of high density UOZ, (U,Pu)O*, U02-Gd203, Th02 and (Th,U)02 pellets devoid of “blackberry” microstructure. For obtaining high density UOZ pellets, a low temperature ( I 1473 K) oxidative sintering technique known as the NIKUSI process [ 8.91 was developed at Kraftwerk Union (KWU), FRG. In NIKUSI, the enhancement of
0022-3 115/9l/%03.50 0 199 1- Elsevier Science Publishers B.V. (North-Holland)
ofhigh density UO,fuel pellets
C. Ganguly, C! Basak /Fabrication
180
U-diffusion (D”xx’) in UOZ+X (xLO.25) is utilisedfor rapid densitication of UOZ pellets by sintering in oxidizing atmosphere like CO2 at relatively low temperature ( $1473 K). After densification, the hyperstoichiometric UOZ+X pellets are reduced to UOZ+, (~50.05) by hydrogen treatment. Thus, the NIKUSI process ensures lower energy consumption. With the objective of reducing the radiotoxic dust hazard and minimising the energy requirement during sintering, in the present investigation, a fabrication flowsheet combining the powder-free SGMP process and the
low temperature ( I 1473 K) oxidative sintering technique has been developed. Internal gelation of uranium (IGU ), described in detail elsewhere [ IO], was utilised for preparation of hydrated gel-microspheres of UO, containing carbon black pore former.
2. Experimental Fig. 1 summarises the essential process steps followed for fabrication of high density U02 pellets by the SGMP
1’
MIXING 8 COOLING I
I
273 K
L
DROPLET
Vibratory
FORMATION
50Hz.
1
INTERNAL
Nozzle lmm
diameter
SIllconeOil
GELATION
363:l
K
1 BATCH
WASHING
CCIk IINH,,OH
I
c BATCH
I
AIR-DRYING
573
GEL-HICROSPHERES I uo3*c
)
i
I
A
CHA9ACTERISATION
Fig. 1. Fabrication
K.12h
flowsheet for high density UO, pellets by SGMP route involving
oenrtty,
rllcrostrutture
Thermal
Conduclivlty
low and high temperature
sintering.
C. Ganguly, U. Basak /Fabrication ofhigh density UOzfuel pellets
route. Acid deficient uranyl nitrate solution of low concentration was mixed with carbon dispersed hexa methylene tetra amine (HMTA) and urea solution at 273 Kin the following proportions to form the “feed solution”: Uranyl nitrate HMTA + urea Carbon black pore former (type Flamruss 100 from M/s. Degussa, FRG)
: I 1.2 molar, : 1.50-1.65 molar,
: 30 g/molar U.
In a few batches, small amount of TiOz powder (0.050.1 wt%) was doped in the “feed solution” with the objective of enhancing grain growth in U02 pellets during sintering. The gelation of hexavalent uranium was achieved internally via the ammonia generator “HMTA”, in presence of the complexing agent “urea” according to the following reaction: UOz(N0X)2 +2NH40H +UOX .xHzO.yNH,
(1) + 2NH4N03.
Droplets of the “feed solution” were formed by forcing it through a stainless steel capillary attached to a vibrator (frequency: 50 Hz). The droplets entered the top of the gelation column containing hot immiscible silicone oil (type DC 200 100 CST) maintained at 363 ? 1 K. The hot gelation bath caused decomposition of HMTA and release of NH3. The droplets gelled within the period of contact with the hot silicone oil (30 s). The gelled microspheres were transferred to a moving filter bed for separation of silicone oil and were washed in batches several times with CC& and 2M NH40H for removal of silicone oil and NH4N03 respectively. The washed gel-microspheres were air-dried in batches at 573 K for 12 h. The dried UO, + C microspheres ( 1200 pm in diameter) were subjected to controlled air-calcination in batches of 5 kg at 973 K for 24 h for removal of carbon black particles and treated with Ar+8% H2 at 973 K for 8 h for reduction to UOz. The calcined microspheres ( N 600 pm) were cooled and given a stabilisation treatment in a CO2 atmosphere after which they were characterised in terms of crushing strength, specific surface area, tap density and oxygen to uranium ratio. The dust-free, free-flowing and porous microspheres were directly pelletised at 225 MPa in a double action hydraulic press using 1% stearic acid in acetone solution for die wall lubrication. The pellets were sintered in the following modes: - low temperature ( 1473 K) sintering in CO2 atmosphere for 1 h, followed by reduction in Ar + 8% Hz at 1473 K for 1 h and cooling;
181
-
low temperature (1473 K) sintering in CO2 atmosphere for 1 h, followed by high temperature ( 2 1923 K) sintering in Ar+ 8% H2 for 8 h; - high temperature ( 1923 K) sintering in Ar+ 8% Hz for 8 h. The sintered pellets were subjected to density measurement (geometrical and liquid displacement methods), microstructural examination and thermal conductivity evaluation. The thermal conductivity (k) of UOz pellets was calculated from the following relation using the measured thermal diffusivity (a) and density (p) values and the literature data of specific heat (C,) [ 111. b=
(apt,).
(2)
The thermal diffusivity (a) of sintered pellets were measured at different temperatures ( T) upto 1800 K by the laser flash method [ 12 1.
3. Results and discussion Table 1 summarises the characteristics of representative batches of calcined UOz microspheres. The average Table 1 Characteristics of undoped and TiOz-doped porous UOz microspheres calcined at 973 K Characteristics
Undoped UO1 microspheres
TiOz doped UO2 microspheres
Oxygen to uranium ratio Tap density (g/cm’) Specific surface area (m’/g ) Average crushing strength (N/microsphere)
2.03 k 0.01 1.85kO.05 4.25 k 0.15
2.03+0.01 1.95t0.03 5.25f0.15
2.00
2.80
C Canguly, Cf.Basak /Fabrication ofhigh density UOzjiiei pellets
182
Table 2 Characteristics of representative batches of undoped and TiO, doped UOz pellets sintered under different conditions Material
Undoped U02 pellet
TiOz doped UOz pellet
Green density in g/cm’ (%TD)
Sintering conditions
5.37 (49.00)
5.47 (50.00)
Sintered pellets Geometrical density in g/cm-’ (% TD)
Density by liquid displacement in g/cm3 f% TD)
Open porosity (%)
Closed porosity (%)
1473 K, 1h, C02, followed by Ar+8%H,at 1473Kfor 1h
10.63 (96.98)
10.73 (97.91 )
0.02
2.07
1473 K, 1h, CO2 followed by 1923 K, 8 h, At-+81 H,
10.65 (97.20)
10.92 (99.63)
0.02
0.35
1923K,Sh,Ar+8%H,
10.64 (97.09)
10.72 (97.81 f
0.42
1.77
1473 K, I h, COz, followed by Ar+8% HZ
9.75 (88.95)
9.95 (90.81)
1.28
7.9L
1923 K, 8 h, Ar+8% H2
10.53 (96.12)
10.78 (98.38)
0.33
1.29
$ ‘s2” !i! 3
2
:,\.
0,
lb
20
3.0
4.0
50
60
70
PORE SIZE IN MICRON ----)
PORE SIZE (micron ) -
Fig. 3. Microstructures of high density UOz pellets prepared by SGMP route. (a) Undoped, sintered at 1473 Kin CO,. (b) TiO, doped, sintered at 1923 K in Ar + 8% Hz.
Fig. 4. Pore size distribution in UOz pellets prepared by SGMP route. (a) Undoped, sintered at 1923 Kin Arf8% Hz. (b) TiOz doped, sintered at 1923 K in At+ 8% HZ.
diameter of the microspheres was around 600 pm as shown in fig. 2. The microspheres were porous, had low crushing
strength (2-3 N/microsphere) and disintegrated easily during the pelletisation operation. As a result, the indi-
C. Gang&y, U. Basak /Fabrication ofhigh densityUO,fuel pellets
"E 81
800
1000
1200
1400
TEMPERATURE
1600
I600
2000
( K) -
Fig. 5. Thermal conductivity of UOZpellets prepared by SGMP and powder route. vidual identity of the microspheres was lost and the “blackbe~” structure in sintered pellets was avoided. Table 2 hi~li~ts the density, grain size and porosity of UO:! pellets sintered under different conditions. High ( 2 96% TD) pellet density was obtained in case of undoped UOZ pellets both by low temperature oxidative sintering in COZ and high tempemture sintering in Ar + 8Ok,11,. The grain size of undoped UOz pellets was in the range 8-10 ,um for both oxidative and reductive sinterings. In case of TiOz doped UOZ pellets, high sintered density and large grain size (45-55 pm) could be obtained only after sintering at high temperature in Ar+8% Hz atmosphere. Fig. 3 shows the microstructure of undoped and TiOz doped U02 pellets. Pig. 4 shows the pore size dist~bution in these pellets. Most of the pores were in the ideal diameter range of 1S-3.0 m, High density undoped UOZ pellets sintered at 1473 K in COZ atmosphere did not show any significant change in pellet dimension or grain size when resintered at 1973 K for 8 h in Ar + 8% H2 atmosphere. Fig. 5 shows the relative thermal conductivity of high density UOZ pellet prepared by SGMP and the conventional “powder-pellet” route. The higher thermal conductivity of UOZ pellet prepared by SGMP process is attributed to the uniform distribution of spherical pores of diameter 1.5-3,Ogm.
4. Conclusions - SGMP and low tempemture oxidative sinte~ng pro-
183
cesses could be combined to produce high density UOZ pellets, conforming to the specification of PHWR fuel; thus, radiotoxic dust hazard and energy consumption during U02 fuel fabrication could be minimised; - ‘“Porous” UOZ microspheres are the appropriate feed materials for direct pelletisation because they crushed easily and lost their individual identity at relatively low compaction pressure (225 MPa), thus avoiding the “blackberry” structure and open porosity in sintered pellets. Porous UOZ microspheres are prepared by adding carbon black pore former to the broth prior to gelation and later removing the same by controlled air ~lcination; - The UOZ pellets fabricated by the SGMP route had uniformly distributed closed spherical pores in the diameter range of 1.5-3.0 hum which is desirable for PHWR. TiOz dopant was found to produce UOZ pellets of large grain size (45-55 pm) only if the sintering is carried out at high temperature ( 2 1923 K) in hydrogen atmosphere.
The authors are grateful to Fuel Chemistry Division, BARC for preparation of gel-microspheres.
References [ I] SM. Tiegs, P.A. Haas and R.D. Spencer, ORNL/TM-6906 (1979). [2] R.B. Matthews and P.E. Hart, J. Nucl. Mater. 92 ( 1980) 207. [ 31 C. Ganguly, H. Langen, E. Zimmer and E. Merz, Nucl. Technol., 73 (1986) 84. [4] M. Peehs, W. D&r and H. Hrovat, IAEA-TECDGC 352 (IAEA, Vienna, 1985) p. 175. [ 51E. Zimmer, C. Ganguly, J. Borchardt and H. Langen, J. Nucl. Mater. 152 (1988) 169. [ 61 C. Gang&y, U. Basak, V.N. Vaidya, D.D. Sood and P.R. Roy, Proc. Second Int. Conf. on CANDU Fuel, Pembroke, Canada, October i-5,1989. [ 71 C. Ganguly, Metals, Mater. and Proces. 1 ( 1990) 253. [8] H. Assmann, W. DSrrand M. Peehs, J. Am. Ceram. Sot. 67 (1984) 631. [9] W. D&r, M. Peehs and H. Assmann, Transaction ENC ‘86, Geneva, June l-6, 1986, Vol. 4 ( 1986) 233. [ 101 V.N. Vaidya, SK. Mu~e~ee, J.K. Joshi, R.V. Kamat and D.D. Sood, J. Nucl. Mater. 148 (1987) 324. [ II] J.K. Fink, M.G. Chasanov and L. Leibowitz, J. Nucl, Mater. 1.02(1981) 17. f12] W.J. Parker, R.J. Jenkins, C.P. Butler and G.L. Abott, J. Appl. Phy. 34 ( 196 1) 1679.
Journal ofNuclear Materials 178 (1991) 179-183 North-Holland
Fabrication of high density U02 fuel pellets involving sol-gel microsphere pelletisation and low temperature sintering C. Ganguly and U. Basak Radiometallurgy Division, Bhabha Atomic Research Centre, Bombay 400 085, India
Powder-free sol-gel microsphere pelletisation and low temperature ( 1473 K) oxidative sintering processes were used in combination for fabrication of high density ( 2 96% TD) U02 fuel pellets for pressurised heavy water reactors. The “internal gelation of uranium” process of BARC was modified for preparation of hydrated gel-microspheres of U03 containing “carbon black” pore former. The gel-microspheres were subjected to controlled air-calcination at 973 K, followed by hydrogen reduction to obtain porous, dust-free and free-flowing UO1 microspheres suitable for direct pelletisation at 225 MPa. Oxidative sintering of these pellets at 1473 K in COz atmosphere followed by Ar+ H2 treatment led to high density ( 2 96% TD) UOz pellets having equiaxed grains of I 10pm and uniformly distributed “closed” spherical pores in the diameter range of 2-5 pm. Resintering of these pellets at high temperature ( 1973 K) for 8 hours in Ar+8% Hz atmosphere did not show any significant change in pellet dimension or grain size. The UOz pellets prepared by sol-gel microsphere pelletisation route had higher thermal conductivity compared to pellets of equivalent density prepared by the “powder-pellet” route. U02 pellets of large grain size (45-55 pm) and high density could be obtained with TiOz dopant and high temperature sintering in Ar+ H2atmosphere. Ti02 dopant was not effectivefor low
temperature oxidative sintering.
1. Introduction
UOZ fuel pellets for light water reactors (LWR) and pressurised heavy water reactors (PHWR) have the specification of high density (94-96% TD for LWR and 2 96% TD for PHWR). These pellets should preferably have large grain size ( 240 pm) and uniformly distributed “closed” spherical pores in the diameter range of 2-5 pm for minimising fission gas release, accommodating fuel swelling, avoiding in-pile densitication and improving thermal conductivity. The industrial methods of fabrication of high density UOz pellets all over the world are based on the “powderpellet” route and consist of cold-pelletisation of UOz powder, obtained from uranyl nitrate or UF6 feed materials by ammonium diuranate (ADU), ammonium urany1 carbonate (AUC) or the integrated dry route (IDR) processes, followed by high temperature ( 2 1973 K) sintering in hydrogen atmosphere (HZ, cracked NH3, Nz+H2 or Ar+ H,). The “powder-pellet” route involves generation and handling of large quantities of radioactive U02 powder and is hence associated with the problem of radiotoxic dust hazard. Further, the ADU and IDR derived U02 powders are extremely tine ( I 1 pm), not free-flowing and require granulation before final compaction. In
addition, high temperature ( 2 1973 K) sintering of UOZ pellets involves high energy consumption. The alternative sol-gel microsphere pelletisation (SGMP) technique, also known as “sphere-Cal” process, is based on preparation of dust-free and free-flowing hydrated gel-microspheres of UO1 by ammonia gelation (external or internal) of uranyl nitrate broth. These microspheres are dried and reduced to UOZ before pelletisation. Thus, in the SGMP route, fine UOz powder is avoided and in turn the radiotoxic dust hazard is minimised. The earlier investigations [ l-21 on SGMP process were not encouraging and led to the conclusions that sol-gel derived microspheres are hard, difficult to pelletise and lead to “blackberry” microstructure in sintered pellet because the microspheres retain their identity even after pelletisation and sintering. However, recently [ 37] with the use of “carbon black” pore former in the gelmicrospheres, the SGMP technique has been successfully adapted for fabrication of high density UOZ, (U,Pu)O*, U02-Gd203, Th02 and (Th,U)02 pellets devoid of “blackberry” microstructure. For obtaining high density UOZ pellets, a low temperature ( I 1473 K) oxidative sintering technique known as the NIKUSI process [ 8.91 was developed at Kraftwerk Union (KWU), FRG. In NIKUSI, the enhancement of
0022-3 115/9l/%03.50 0 199 1- Elsevier Science Publishers B.V. (North-Holland)
ofhigh density UO,fuel pellets
C. Ganguly, C! Basak /Fabrication
180
U-diffusion (D”xx’) in UOZ+X (xLO.25) is utilisedfor rapid densitication of UOZ pellets by sintering in oxidizing atmosphere like CO2 at relatively low temperature ( $1473 K). After densification, the hyperstoichiometric UOZ+X pellets are reduced to UOZ+, (~50.05) by hydrogen treatment. Thus, the NIKUSI process ensures lower energy consumption. With the objective of reducing the radiotoxic dust hazard and minimising the energy requirement during sintering, in the present investigation, a fabrication flowsheet combining the powder-free SGMP process and the
low temperature ( I 1473 K) oxidative sintering technique has been developed. Internal gelation of uranium (IGU ), described in detail elsewhere [ IO], was utilised for preparation of hydrated gel-microspheres of UO, containing carbon black pore former.
2. Experimental Fig. 1 summarises the essential process steps followed for fabrication of high density U02 pellets by the SGMP
1’
MIXING 8 COOLING I
I
273 K
L
DROPLET
Vibratory
FORMATION
50Hz.
1
INTERNAL
Nozzle lmm
diameter
SIllconeOil
GELATION
363:l
K
1 BATCH
WASHING
CCIk IINH,,OH
I
c BATCH
I
AIR-DRYING
573
GEL-HICROSPHERES I uo3*c
)
i
I
A
CHA9ACTERISATION
Fig. 1. Fabrication
K.12h
flowsheet for high density UO, pellets by SGMP route involving
oenrtty,
rllcrostrutture
Thermal
Conduclivlty
low and high temperature
sintering.
C. Ganguly, U. Basak /Fabrication ofhigh density UOzfuel pellets
route. Acid deficient uranyl nitrate solution of low concentration was mixed with carbon dispersed hexa methylene tetra amine (HMTA) and urea solution at 273 Kin the following proportions to form the “feed solution”: Uranyl nitrate HMTA + urea Carbon black pore former (type Flamruss 100 from M/s. Degussa, FRG)
: I 1.2 molar, : 1.50-1.65 molar,
: 30 g/molar U.
In a few batches, small amount of TiOz powder (0.050.1 wt%) was doped in the “feed solution” with the objective of enhancing grain growth in U02 pellets during sintering. The gelation of hexavalent uranium was achieved internally via the ammonia generator “HMTA”, in presence of the complexing agent “urea” according to the following reaction: UOz(N0X)2 +2NH40H +UOX .xHzO.yNH,
(1) + 2NH4N03.
Droplets of the “feed solution” were formed by forcing it through a stainless steel capillary attached to a vibrator (frequency: 50 Hz). The droplets entered the top of the gelation column containing hot immiscible silicone oil (type DC 200 100 CST) maintained at 363 ? 1 K. The hot gelation bath caused decomposition of HMTA and release of NH3. The droplets gelled within the period of contact with the hot silicone oil (30 s). The gelled microspheres were transferred to a moving filter bed for separation of silicone oil and were washed in batches several times with CC& and 2M NH40H for removal of silicone oil and NH4N03 respectively. The washed gel-microspheres were air-dried in batches at 573 K for 12 h. The dried UO, + C microspheres ( 1200 pm in diameter) were subjected to controlled air-calcination in batches of 5 kg at 973 K for 24 h for removal of carbon black particles and treated with Ar+8% H2 at 973 K for 8 h for reduction to UOz. The calcined microspheres ( N 600 pm) were cooled and given a stabilisation treatment in a CO2 atmosphere after which they were characterised in terms of crushing strength, specific surface area, tap density and oxygen to uranium ratio. The dust-free, free-flowing and porous microspheres were directly pelletised at 225 MPa in a double action hydraulic press using 1% stearic acid in acetone solution for die wall lubrication. The pellets were sintered in the following modes: - low temperature ( 1473 K) sintering in CO2 atmosphere for 1 h, followed by reduction in Ar + 8% Hz at 1473 K for 1 h and cooling;
181
-
low temperature (1473 K) sintering in CO2 atmosphere for 1 h, followed by high temperature ( 2 1923 K) sintering in Ar+ 8% H2 for 8 h; - high temperature ( 1923 K) sintering in Ar+ 8% Hz for 8 h. The sintered pellets were subjected to density measurement (geometrical and liquid displacement methods), microstructural examination and thermal conductivity evaluation. The thermal conductivity (k) of UOz pellets was calculated from the following relation using the measured thermal diffusivity (a) and density (p) values and the literature data of specific heat (C,) [ 111. b=
(apt,).
(2)
The thermal diffusivity (a) of sintered pellets were measured at different temperatures ( T) upto 1800 K by the laser flash method [ 12 1.
3. Results and discussion Table 1 summarises the characteristics of representative batches of calcined UOz microspheres. The average Table 1 Characteristics of undoped and TiOz-doped porous UOz microspheres calcined at 973 K Characteristics
Undoped UO1 microspheres
TiOz doped UO2 microspheres
Oxygen to uranium ratio Tap density (g/cm’) Specific surface area (m’/g ) Average crushing strength (N/microsphere)
2.03 k 0.01 1.85kO.05 4.25 k 0.15
2.03+0.01 1.95t0.03 5.25f0.15
2.00
2.80
C Canguly, Cf.Basak /Fabrication ofhigh density UOzjiiei pellets
182
Table 2 Characteristics of representative batches of undoped and TiO, doped UOz pellets sintered under different conditions Material
Undoped U02 pellet
TiOz doped UOz pellet
Green density in g/cm’ (%TD)
Sintering conditions
5.37 (49.00)
5.47 (50.00)
Sintered pellets Geometrical density in g/cm-’ (% TD)
Density by liquid displacement in g/cm3 f% TD)
Open porosity (%)
Closed porosity (%)
1473 K, 1h, C02, followed by Ar+8%H,at 1473Kfor 1h
10.63 (96.98)
10.73 (97.91 )
0.02
2.07
1473 K, 1h, CO2 followed by 1923 K, 8 h, At-+81 H,
10.65 (97.20)
10.92 (99.63)
0.02
0.35
1923K,Sh,Ar+8%H,
10.64 (97.09)
10.72 (97.81 f
0.42
1.77
1473 K, I h, COz, followed by Ar+8% HZ
9.75 (88.95)
9.95 (90.81)
1.28
7.9L
1923 K, 8 h, Ar+8% H2
10.53 (96.12)
10.78 (98.38)
0.33
1.29
$ ‘s2” !i! 3
2
:,\.
0,
lb
20
3.0
4.0
50
60
70
PORE SIZE IN MICRON ----)
PORE SIZE (micron ) -
Fig. 3. Microstructures of high density UOz pellets prepared by SGMP route. (a) Undoped, sintered at 1473 Kin CO,. (b) TiO, doped, sintered at 1923 K in Ar + 8% Hz.
Fig. 4. Pore size distribution in UOz pellets prepared by SGMP route. (a) Undoped, sintered at 1923 Kin Arf8% Hz. (b) TiOz doped, sintered at 1923 K in At+ 8% HZ.
diameter of the microspheres was around 600 pm as shown in fig. 2. The microspheres were porous, had low crushing
strength (2-3 N/microsphere) and disintegrated easily during the pelletisation operation. As a result, the indi-
C. Gang&y, U. Basak /Fabrication ofhigh densityUO,fuel pellets
"E 81
800
1000
1200
1400
TEMPERATURE
1600
I600
2000
( K) -
Fig. 5. Thermal conductivity of UOZpellets prepared by SGMP and powder route. vidual identity of the microspheres was lost and the “blackbe~” structure in sintered pellets was avoided. Table 2 hi~li~ts the density, grain size and porosity of UO:! pellets sintered under different conditions. High ( 2 96% TD) pellet density was obtained in case of undoped UOZ pellets both by low temperature oxidative sintering in COZ and high tempemture sintering in Ar + 8Ok,11,. The grain size of undoped UOz pellets was in the range 8-10 ,um for both oxidative and reductive sinterings. In case of TiOz doped UOZ pellets, high sintered density and large grain size (45-55 pm) could be obtained only after sintering at high temperature in Ar+8% Hz atmosphere. Fig. 3 shows the microstructure of undoped and TiOz doped U02 pellets. Pig. 4 shows the pore size dist~bution in these pellets. Most of the pores were in the ideal diameter range of 1S-3.0 m, High density undoped UOZ pellets sintered at 1473 K in COZ atmosphere did not show any significant change in pellet dimension or grain size when resintered at 1973 K for 8 h in Ar + 8% H2 atmosphere. Fig. 5 shows the relative thermal conductivity of high density UOZ pellet prepared by SGMP and the conventional “powder-pellet” route. The higher thermal conductivity of UOZ pellet prepared by SGMP process is attributed to the uniform distribution of spherical pores of diameter 1.5-3,Ogm.
4. Conclusions - SGMP and low tempemture oxidative sinte~ng pro-
183
cesses could be combined to produce high density UOZ pellets, conforming to the specification of PHWR fuel; thus, radiotoxic dust hazard and energy consumption during U02 fuel fabrication could be minimised; - ‘“Porous” UOZ microspheres are the appropriate feed materials for direct pelletisation because they crushed easily and lost their individual identity at relatively low compaction pressure (225 MPa), thus avoiding the “blackberry” structure and open porosity in sintered pellets. Porous UOZ microspheres are prepared by adding carbon black pore former to the broth prior to gelation and later removing the same by controlled air ~lcination; - The UOZ pellets fabricated by the SGMP route had uniformly distributed closed spherical pores in the diameter range of 1.5-3.0 hum which is desirable for PHWR. TiOz dopant was found to produce UOZ pellets of large grain size (45-55 pm) only if the sintering is carried out at high temperature ( 2 1923 K) in hydrogen atmosphere.
The authors are grateful to Fuel Chemistry Division, BARC for preparation of gel-microspheres.
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