Process for conversion of UF.sub.6 to UO.sub.2

A process for conversion of gaseous UF.sub.6 to UO.sub.2 powders by using a fluidized bed reaction apparatus comprising pyrohydrolizing gaseous UF.sub.6 and steam to obtain UO.sub.2 F.sub.2 particles, hydrating and dehydrating the UO.sub.2 F.sub.2 particles to UO.sub.2 F.sub.2 anhydride and reducing the UO.sub.2 F.sub.2 anhydride to UO.sub.2 powders. The obtained UO.sub.2 powders are suitable for production of nuclear fuels in power plant owing to its good ceramic properties, low fluorine contents and free flowability.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for conversion of gaseous 
UF.sub.6 to UO.sub.2 powders which is suitable for production of nuclear 
fuels in power plant owing to its good ceramic properties, small fluorine 
contents and free flowability. 
As a process for converting UF.sub.6 into UO.sub.2 powders for nuclear 
fuels in a power plant, gaseous UF.sub.6 has been conventionally converted 
to be UO.sub.2 powder in a industrial scale by two different process, that 
is, a wet process and a dry process. The wet process is defective in that 
many steps required make the operation complex and a large quantity of 
waste solution is produced. 
On the other hand, the dry process has defects of having poor ceramic 
properties of UO.sub.2 powders as a product and a large fluorine contents 
of the product, but it has advantages of having simple steps and also a 
small quantity of waste solution produced. Therefore, adoption of the dry 
process has been recently increased by overcoming the defects above 
mentioned. As the dry process, there are a process using a rotary kiln, a 
process using a fluidized bed reaction apparatus and a process using a 
flame combustion reaction apparatus. Of these processes, the process using 
a fluidized bed reaction apparatus produces UO powders as a product which 
has a free flowability, thus making handling of the UO.sub.2 powders in 
following steps very much easier, as compared with that of the other 
processes. 
In the process using a fluidized bed reaction apparatus which has the 
advantages mentioned, the ceramic properties of the UO.sub.2 powders 
produced become poorer and also the fluorine contents thereof become 
larger, as compared with those of the other processes. The poor ceramic 
properties of the UO.sub.2 powders is due to a formation of UO.sub.2 
F.sub.2 in fine particles by gas phase reaction of gaseous UF.sub.6 with 
steam as shown in the following equation (1) and a formation of UF.sub.4 
in converting of UO.sub.2 F.sub.2 to UO.sub.2 with hydrogen gas as shown 
in the following equations (2) and (3). In the conventional dry processes, 
especially in the process using a fluidized bed reaction apparatus, the 
reaction is mostly composed of the following two stage reactions. 
EQU UF.sub.6 +2H.sub.2 O.fwdarw.UO.sub.2 F.sub.2 +4HF (1) 
EQU UO.sub.2 F.sub.2 +H.sub.2 .fwdarw.UO.sub.2 +2HF (2) 
In this process, UF.sub.4 is apt to be formed by a reverse reaction as 
shown in the equation (3). Namely, the UO powder may possibly be 
hydrofluorinated to UF powder. 
EQU UO.sub.2 +4HF.fwdarw.UF.sub.4 +2H.sub.2 O (3) 
UF.sub.4 is a substance which is apt to sinter at a relatively low 
temperature (about 1000.degree. C.) and begins to sinter at the operating 
temperature of the equation (2) to hinder a defluorinating reaction which 
is important for lowering fluorine contents of UO.sub.2 powder as a 
product. Therefore, it was formerly required to add an excess of steam in 
the equation (2) to suppress hydrofluorination of the UO.sub.2 powder. As 
a result, the fluidized bed operation became more complex and at the same 
time the excessively added steam increased a quantity of waste solution 
substantially. Further, as a long time was required for defluorinating 
UO.sub.2 powder as a product, it was exposed to a high temperature for a 
long time. Consequently the ceramic properties of UO.sub.2 powders were 
apt to be substantially reduced. 
Further, another defect of the case in which the fluidized bed reaction 
apparatus is used relates to a stability of the operation of the fluidized 
bed which converts UF.sub.6 into UO.sub.2 F.sub.2. 
Namely, the UO.sub.2 F.sub.2 particles form the fluidized bed, but gaseous 
UF.sub.6 blown into the fluidized bed reacts with steam as a fluidizing 
gas introduced through the bottom of the fluidized bed to form UO.sub.2 
F.sub.2 particles which deposit on the surface of UO.sub.2 F.sub.2 
particles already existing. As the result, the thus deposited UO.sub.2 
F.sub.2 cause growth of the UO.sub.2 F.sub.2 particles. On the other hand, 
a part of the UO.sub.2 F.sub.2 particles are pulverized by abrasion owing 
to collison with each other. The mean particle size of the UO.sub.2 
F.sub.2 particles is determined by these balances, but in the conventional 
fluidized bed reaction apparatus, the thus obtained UO.sub.2 F.sub.2 
particles are apt to grow substantially. Therefore, it was required to 
supply new UO.sub.2 F.sub.2 particles into the fluidized bed to maintain a 
stable fluidized bed operation. Consequently, the process system became 
complicated and the operation also became troublesome. 
BRIEF SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a process for 
conversion of gaseous UF.sub.6 to UO.sub.2 powders which is suitable for 
production of nuclear fuels in power plant owing to its good ceramic 
properties, low fluorine contents and free flowability. 
According to the present invention, there is provided: 
1. A process for conversion of UF.sub.6 to UO.sub.2 by using a fluidized 
bed reaction apparatus comprising 
(a) a first step of pyrohydrolizing gaseous UF.sub.6 and steam in said 
fluidized bed to obtain UO.sub.2 F.sub.2 particles, 
(b) a second step of reacting said UO.sub.2 F.sub.2 particles with water to 
obtain UO.sub.2 F.sub.2 hydrate, 
(c) a third step of dehydrating said UO.sub.2 F.sub.2 hydrate by heating to 
UO.sub.2 F.sub.2 anhydride, 
(d) a fourth step of reducing said UO.sub.2 F.sub.2 anhydride with hydrogen 
gas or hydrogen gas and steam to convert to UO.sub.2 powders. 
2. A process for conversion of UF.sub.6 to UO.sub.2 by using a fluidized 
bed reaction apparatus comprising 
(a) a first step of pyrohydrolizing gaseous UF.sub.6 and steam in said 
fluidized bed to obtain UO.sub.2 F.sub.2 particles, 
(b) a second step of hydrating said UO.sub.2 F.sub.2 particles with water 
to UO.sub.2 F.sub.2 hydrate, 
(c) a third step of dehydrating said UO.sub.2 F.sub.2 hydrate by heating to 
UO.sub.2 F.sub.2 anhydride, 
(d) a fourth step of calcining said UO.sub.2 F.sub.2 anhydride with steam 
to convert to UO.sub.3 or a mixture of UO.sub.3 and U.sub.3 O.sub.8 
particles 
(e) a fifth step of reducing said UO.sub.3 or said mixture of UO.sub.3 and 
U.sub.3 O.sub.8 with hydrogen gas or hydrogen gas and steam to convert to 
UO.sub.2 powders. 
Based on the above processes 1 and 2, following processes 3 and 4 are also 
within the scope of the present invention. 
3. A process for conversion of UF.sub.6 to UO.sub.2 by using a fluidized 
bed reaction apparatus comprising 
(a) a first step of pyrohydrolizing gaseous UF.sub.6 and steam in said 
fluidized bed to obtain UO.sub.2 F.sub.2 particles, 
(b) a second step of hydrating said UO.sub.2 F.sub.2 particles with water 
solution added with ammonia containing water, oxalic acid or hydrogen 
peroxide to UO.sub.2 F.sub.2 hydrate, 
(c) a third step of dehydrating said UO.sub.2 F.sub.2 hydrate by heating to 
UO.sub.2 F.sub.2 anhydride, 
(d) a fourth step of reducing said UO.sub.2 F.sub.2 anhydride with hydrogen 
gas or a mixture of hydrogen gas and steam to convert to UO.sub.2 powders 
4. A process for conversion of UF.sub.6 to UO.sub.2 by using a fluidized 
bed reaction apparatus comprising 
(a) a first step of pyrohydrolizing gaseous UF.sub.6 and steam in said 
fluidized bed to obtain UO.sub.2 F.sub.2 particles, 
(b) a second step of hydrating said UO.sub.2 F.sub.2 particles with a 
solution added with ammonia containing water, oxalic acid or hydrogen 
peroxide to UO.sub.2 F.sub.2 hydrate, 
(c) a third step of dehydrating said UO.sub.2 F.sub.2 hydrate by heating to 
UO.sub.2 F.sub.2 anhydride, 
(d) a fourth step of reducing said UO.sub.2 F.sub.2 anhydride with steam to 
convert to UO.sub.3 or mixture of UO.sub.3 and U.sub.3 O.sub.8 partices, 
(e) a fifth step of reducing said UO.sub.3 or said mixture of UO.sub.3 and 
U.sub.3 O.sub.8 with hydrogen gas or a mixture of hydrogen gas and steam 
to UO.sub.2 powders. 
Further, in the present invention, hydration of the UO.sub.2 F.sub.2 
particles to UO.sub.2 F.sub.2 hydrate can be carried out with steam as a 
water source. Further more, in the present invention, supply of gaseous 
UF.sub.6 and steam into the fluidized bed is carried out more effectively 
by using a twin fluid atomizer.

DETAILED DESCRIPTION OF THE INVENTION 
In the present invention, the steps of converting gaseous UF.sub.6 to 
UO.sub.2 powders are as flollows: 
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the first step 
UF.sub.6 + 2H.sub.2 O .fwdarw. UO.sub.2 F.sub.2 
(1)HF 
the second step 
UO.sub.2 F.sub.2 + nH.sub.2 O .fwdarw. UO.sub.2 F.sub.2.nH.sub. 
2 O (2) 
the third step 
UO.sub.2 F.sub.2.nH.sub.2 O .fwdarw. UO.sub.2 F.sub.2 + 
nH.sub.2 O (3) 
the fourth step 
UO.sub.2 F.sub.2 + H.sub.2 .fwdarw. UO.sub.2 
(4)HF 
or 
the first step 
UF.sub.6 + 2H.sub.2 O .fwdarw. UO.sub.2 F.sub.2 
(1)HF 
the second step 
UO.sub.2 F.sub.2 + nH.sub.2 O .fwdarw. UO.sub.2 F.sub.2.nH.sub. 
2 O (2) 
the third step 
UO.sub.2 F.sub.2.nH.sub.2 O .fwdarw. UO.sub.2 F.sub.2 + 
nH.sub.2 O (3) 
the fourth step 
UO.sub.2 F.sub.2 + H.sub.2 O .fwdarw. UO.sub.3 
(4)HF 
UO.sub.2 F.sub.2 + H.sub.2 O .fwdarw. U.sub. 3 O.sub.8 + 2HF + 
1/6 O.sub.2 
the fifth step 
UO.sub.3 + H.sub.2 .fwdarw. UO.sub.2 + H.sub.2 O 
(5) 
U.sub.3 O.sub.8 + 2H.sub.2 .fwdarw. 3UO.sub.2 + 2H 
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O 
Of these steps, the most important point involves the fact that in the 
second step, UO.sub.2 F.sub.2 particles formed in the fluidized bed of the 
first step are hydrated and then in the third step, the hydrated UO.sub.2 
F.sub.2 particles are dehydrated by heating. 
It is known that the UO.sub.2 powders obtained by conversion of the 
dehydrated UO.sub.2 F.sub.2 particles have, a remarkably larger specific 
surface area and improved ceramic properties, but combination of the 
operation with the process of converting UF.sub.6 to UO.sub.2 using the 
fluidized bed can overcome the large defect of the conventional process 
that the UO.sub.2 powders formed are unsuitable for fabrication of nuclear 
fuels in power plant owing to its low ceramic properties and slow 
defluorinating velocity can utilize also effectively the large advantage 
of the conventional process in which the UO.sub.2 powders formed have an 
extremely good flowability. 
Further, in the steps of converting UO.sub.2 F.sub.2 particles to UO.sub.2 
powders of the present invention, the UO.sub.2 F.sub.2 particles are 
calcined with only steam to UO.sub.3 /U.sub.3 O.sub.8 to suppress a 
formation of HF.sub.4 which hinders a defluorinating reaction of UO.sub.2 
F.sub.2 and then the UO.sub.3 /U.sub.3 O.sub.8 are reduced with hydrogen 
gas to UO.sub.2 powders. Combination of these operations with the above 
stated conventional process can improve the defect thereof moreover. 
The easily defluorinated UO.sub.2 powders obtained by the present invention 
posses a fundamental condition suitable for fabrication of nuclear fuels 
in power plant owing to its good ceramic properties and at the same time 
have an extremely good flowability which makes handling thereof in 
following steps very easy and can omit a granulating operation which is 
usually carried out in fabrication of pellet for nuclear fuels. 
Further, when a solution added with such reagent as ammonia, oxalic acid or 
hydrogen peroxide is used instead of water in hydration of the UO.sub.2 
F.sub.2 particles, these reagents form uranate, uranyl salt. Therefore, 
the added quantity of these reagents can control the ceramic properties of 
UO.sub.2 powders formed. Further, when steam is used as a water source 
instead of water, homogeneous hydration can be easily obtained due to its 
good dispersion and at the same time choice of apparatus can be more free. 
In the fluidized bed of the first step, it is effective to use a twin 
fluid atomizer for controlling the particle size of the formed UO.sub.2 
F.sub.2 particles. This is due to the phenomenon that when the twin fluid 
atomizer is used, the gaseous UF atomized from central nozzle of the 
atomizer is pyrohydrolized with steam atomized from periphery of the 
nozzle to form fine particles of UO.sub.2 F.sub.2 which become nucleus of 
following granulation and average particle size of UO.sub.2 F.sub.2 
particles which form the fluidized bed is decreased. Further, when the 
twin fluid atomizer is used, the formed UO.sub.2 F.sub.2 particles become 
highly reactive owing to its fine granule form to promote proceeding of 
following reactions. 
The drawing is a schematic flow diagram of the process in accordance with 
the present invention. In the drawing, UF.sub.6 is vaporized in a 
vaporizing chamber 1, and the gaseous UF.sub.6 is atomized. At the same 
time, steam as a fluidizing gas is introduced into the bottom of the 
apparatus 2 through pipe 10. A part of gaseous UF.sub.6 reacts immediately 
with steam as an atomizing gas near the atomizer to form UO.sub.2 F.sub.2 
particle. A part of the newly fomred UO.sub.2 F.sub.2 particles are 
deposited on the surface of UO.sub.2 F.sub.2 as seed material, and the 
UO.sub.2 F.sub.2 particles are growing. Further, a part of these UO.sub.2 
F.sub.2 particles is pulverized by collison with each other to be fine 
particles. The UO.sub.2 F.sub.2 partices are controlled in particle size 
by these phenomena and form the fluidized bed. 
The operating temperature of the first fluidized bed 2 is less than 
400.degree. C., preferably in the range of 200.degree.-300.degree. C. in 
consideration of ceramic properties of the particles and control of the 
particle size. HF produced in the first fluidized bed 2 is recovered as HF 
solution by HF condenser 2a to be stored in HF receiver 2b. The newly 
formed UO.sub.2 F.sub.2 particles are discharged out of the first 
fluidized bed 2 through the overflow pipe which is located at the upper 
part of the fluidized bed 2 and are sent to the second reactor 3. 
In the reactor 3, the UO.sub.2 F.sub.2 particles are hydrated with water 
introduced into the reactor 3 through pipe 11. The operating temperature 
is less than 100.degree. C., preferably in the range of 10.degree. 
C.-50.degree. C. in consideration of hydration velocity. 
The UO.sub.2 F.sub.2 hydrate is passed to chamber 4 from which it is sent 
to a third fluidized bed 5 where it is dehydrated by heating. Air as a 
fluidizing gas is introduced into the bottom of the apparatus 5 through 
pipe 13. The operating temperature is less than 200.degree. C., preferably 
in the range of 120.degree. C.-150.degree. C. The formed UO.sub.2 F.sub.2 
anhydride is discharged out of the fluidized bed 5 through the overflow 
pipe which is located at the upper part of the fluidized bed 5 and ssent 
to a fourth fluidized bed 6 where it is calcined with steam as reaction 
fluidizing gas introduced to the bottom of the apparatus 6 through pipe 14 
to convert to UO.sub.3 or U.sub.3 O.sub.8 particles. The operating 
temperature is less than 700.degree. C., preferably in the range of 
450.degree. C.-600.degree. C. at which UO.sub.2 powders are formed. 
Further, when the operating temperature is in the range of 500.degree. 
C.-600.degree. C., a mixture of UO.sub.3 and U.sub.3 O.sub.8 is formed. 
The UO.sub.3 or U.sub.3 O.sub.8 particles are discharged out of the 
apparatus 6 through the overflow pipe and sent to a fifth fluidized bed 7 
where they are reduced with a mixture of steam and hydrogen gas introduced 
to the bottom of the apparatus 7 through pipe 15 to convert UO.sub.2 
powders. The UO.sub.2 powder is received as a product by a container 8. 
This operating temperature is less than 700.degree. C., preferably in the 
range of 500.degree. C.-600.degree. C. in consideration of ceramic 
properties of the UO.sub.2 powder. 
The waste gas from the reaction apparatus 2, 5, 6, 7 is led to a waste gas 
treatment apparatus through a pipe 16. 
The features of the present invention are as follows: 
(1) The UO.sub.2 powder formed has good ceramic properties, low fluorine 
contents and good flowability. Therefore, it is suitable for nuclear fuels 
in power plant. Such UO.sub.2 powder as that of the present invention can 
not be obtained by conventional process. 
(2) The good flowability of the UO.sub.2 powder can make its handling in 
following steps very easy and can omit granulating operation before 
pelletizing which is generally carried out for fabricating nuclear fuels. 
The following examples are illustractive of the present invention. However, 
it is understood that these examples are merely examplary and do not limit 
the scope the present invention. 
Example 
Three pilot runs were made using the processes by the present invention. 
In case 1 corresponding to process 1, UO.sub.2 F.sub.2 particles formed by 
the first fluidized bed are hydrated, dehydrated and directly reduced to 
UO.sub.2 powders. 
In case 2 corresponding to process 2, UO.sub.2 F.sub.2 particles formed by 
the first fluidized bed are hydrated, dehydrated, calcined with steam to 
convert to UO.sub.3 or U.sub.3 O.sub.8 and the UO.sub.3 or U.sub.3 O.sub.8 
are reduced to UO.sub.2 powders. 
In case 3 corresponding to process 3, UO.sub.2 F.sub.2 particles formed by 
the first fluidized bed are hydrated with ammonia containing water, 
dehydrated and reduced to UO.sub.2 powders. 
As a comparative example, one pilot run was made using the conventional dry 
process. In the comparative case, UO.sub.2 F.sub.2 particles formed by the 
first fluidized bed were directly reduced to UO.sub.2 powders. 
The fluidized beds used in these runs are 83 mm in diameter. The operating 
conditions are shown in Table 1. The characteristics of the UO.sub.2 
powders obtained in these runs are shown in Table 2. 
Table 2 shows that the UO.sub.2 powders of the process by the present 
invention are smaller in bulk density and in mean particle size, larger in 
specific surface area and lower in residual fluorine contents than those 
of the conventional dry process. Therefore, the UO.sub.2 powder obtained 
by the process of the present invention is suitable for fabrication of 
nuclear fuels. 
TABLE 1 
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com- 
present invention 
parative 
case 1 
case 2 case 3 case 
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1st fluidized bed 
bed temperature (.degree.C.) 
280 280 280 280 
UF.sub.6 rate (g/min) 
90 90 90 90 
atomizer steam rate 
14 14 14 0 
(g/min) 
fluidizing gas velocity 
25 25 25 25 
(cm/s) 
UO.sub.2 F.sub.2 production rate 
79 79 79 79 
(g/min) 
use of twin fluid atomizer 
used used used not used 
2nd reactor not used 
reaction temperature (.degree.C.) 
20 20 20 
UO.sub.2 F.sub.2 rate (g/min) 
79 79 79 
water rate (g/min) 
9 9 9* 
formed UO.sub.2 F.sub.2 hydrate 
86 86 86 
(g/min) 
3rd fluidized bed not used 
bed temperature (.degree.C.) 
170 170 170 
UO.sub.2 F.sub.2 hydrate (g/min) 
86 86 86 
fluidizing gas velocity 
20 20 20 
(cm/s) 
formed UO.sub.2 F.sub.2 anhydride 
79 79 79 
(g/min) 
4th fluidized bed not used 
bed temperature (.degree.C.) 
not 500 500 
used 
UO.sub.2 F.sub.2 rate (g/min) 
79 79 
fluidizing gas velocity 20 20 
(cm/s) 
formed UO.sub.3 /U.sub.3 O.sub.8 (g/min) 
69/4 69/4 79 
5th fluidized bed 
bed temperature (.degree.C.) 
600 600 560 600 
UO.sub.3 /U.sub.3 O.sub.8 rate 
76 73 79 
fluidizing gas velocity 
20 20 20 20 
(cm/s) 
hydrogen/steam (mol. ratio) 
1/1 1/1 1/1 1/1 
formed UO.sub.2 (g/min) 
69 69 69 69 
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*water added with ammonia 1% 
TABLE 2 
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present invention 
case case case comparative 
Characteristics of UO.sub.2 powder 
1 2 3 case 
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bulk density (g/cm.sup.3) 
2.1 2.0 1.9 2.8 
specific surface area (m.sup.2 /g) 
2.6 2.9 3.1 1.0 
mean partice size (.mu.m) 
96 98 145 
U content (% U) 87.9 87.8 87.7 87.8 
residual fluorine contents 
58 41 38 278.0 
(ppm) 
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