Abstract:
A process for converting UF 6  gas into UO 2  powder comprising blowing UF 6  gas and steam into a fluid bed to produce UO 2  F 2  particle, hydrating and dehydrating the UO 2  F 2  particle to UO 2  F 2  powder, and defluorinating and reducing the UO 2  F 2  powder to UO 2  powder. The UO 2  powder is suitable for manufacturing a reactor fuel owing to its high activity, low remaining fluorine amount and excellent fluidity.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a process for converting UF 6  gas into UO 2  powder which is suitable for manufacturing a reactor fuel owing to its high activity, low remaining fluorine amount and excellent fluidity. 
     As a process for converting UF 6  gas into UO 2  powder for a reactor fuel, there have been conventionally two processes, that is, a wet process and a dry process. The wet process is a process in which an uranyl ion containing solution is obtained by hydrolyzing UF 6  in gas-liquid reaction, then the solution is added with reagent to be precipitated, and the precipitate is filtered, dried roasted, reduced to be UO 2  powder. The UO 2  powder obtained by the wet process is high in activity and low in remaining fluorine amount, but it is defective in that there are many steps which are complex and the generated volume of waste liquid is large. Especially, large load in the filtering step, low filterbility of the precipitate and uranium loss into the filtrate have been already pointed out. 
     On the other hand, in the case of the dry process, there is a process using a rotary kiln, a process using a fluid bed reaction apparatus and a process using a flame combustion reaction apparatus. Of these processes, the process using a fluid bed reaction apparatus forms UO 2  powder as a product which has a very excellent fluidity, thus making the handling of UO 2  powder in following steps very easy, as compared with the other processes. The UO 2  powders obtained by almost all the conventional processes are bad in fluidity, so their handlings in the following steps are not easy. 
     As above described, in the process using a fluid bed reaction apparatus, there is obtained UO 2  powder having an excellent fluidity thus making handling of the UO 2  powder in the following steps very easy, but the activity of the UO 2  powder becomes smaller and the remaining fluorine amount thereof becomes larger, as compared with these of the other processes. This lowering of the activity of the UO 2  powder is due to formation of UO 2  F 2  by gas phase reaction of UF 6  gas with steam as shown in the following equation (1) and formation of UF 4  by conversion into UO 2  of UO 2  F 2  with hydrogen gas as shown in the following equation (2). 
     In the conventional dry process, especially in the process using a fluid bed reaction apparatus, the conversion of UF 6  gas into UO 2  powder is following two stage reactions. 
     
         UF.sub.6 +2H.sub.2 O→UO.sub.2 F.sub.2 +4HF          (1) 
    
     
         UO.sub.2 F.sub.2 +H.sub.2→UO.sub.2 +2HF             (2) 
    
     In this process, a reverse reaction of equation (2) is apt to form UF 4 . 
     
         UO.sub.2 +4HF→UF.sub.4 +2H.sub.2 O                  (3) 
    
     UF 4  is a substance which is apt to sinter at relatively low temperature (about 1000° C.) and it begins to sinter at the operating temperature of equation (2) to hinder a deflorinating reaction which is important for lowering a remaining fluorine amount of UO 2  powder as a product. Therefore, it was formerely required to add in excess of steam in equation (2) to make equation (3) not occur. As a result, the operation became more complex and at the same time the excess added steam increased the amount of waste liquid. Further, as a long time was required for deflourinating the UO 2  powder as a product, it was exposed to the high temperature for a long time. Consequently, activity of the UO 2  powder is apt to be largely reduced. 
     Further, another defect of the case in which the fluid bed reaction apparatus is used relates to the operational stability of the fluid bed. Namely, the UO 2  F 2  particles form the fluid bed, but UF 6  gas blown into the fluid bed reacts with steam as a fluidizing gas introduced through the bottom of the fluid bed to form UO 2  F 2  which sticks on the surface of UO 2  F 2  particles already existing. As the result, the thus sticky UO 2  F 2  causes growth of the UO 2  F 2  particles. On the other hand, a part of the UO 2  F 2  particles are pulverized by abrasion owing to collison with other UO 2  F 2  particles. The mean particle size of the UO 2  F 2  particle is determined by these balances, but in the conventional fluid bed reaction apparatus, the resultant UO 2  F 2  particles are apt to grow very large. Therefore, it was required to supply new UO 2  F 2  powder into the fluid bed to maintain a stable operation of the fluid bed. Consequently, the apparatus system became complex and the operation was complicated. 
     BRIEF SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a process for converting UF 6  gas into UO 2  powder in dry phase in which the resultant UO 2  powder is suitable for a reactor fuel owing to its high activity, low remaining fluorine amount and excellent fluidity. 
     According to the present invention, there is provided a process for converting UF 6  gas into UO 2  particle comprising combining 
     (a) a first step of blowing UF 6  gas and steam into a fluid bed reaction apparatus to convert said UF 6  gas into UO 2  F 2  particle. 
     (b) a second step of reacting said UO 2  F 2  particle from the first step with ammonia water solution to change said UO 2  F 2  particle to ADU (ammonium diuranate). 
     (c) a third step of drying and dehydrating said product of the second step, 
     (d) a fourth step of reacting said product of the third step with hydrogen gas or hydrogen gas and steam to convert said product into UO 2  powder. 
     Further, according to the present invention, there is also provided a process for converting UF 6  gas into UO 2  powder comprising combining 
     (a) a first step of blowing UF 6  gas and steam into a fluid bed reaction apparatus to convert said UF 6  gas into UO 2  F 2  particle, 
     (b) a second step of reacting said UO 2  F 2  particle from the first step with ammonia water solution to convert said UO 2  F 2  particle to ADU, 
     (c) a third step of drying and dehydrating said product of the second step, 
     (d) a fourth step of reacting said product of the third step to convert said product into UO 3  and/or U 3  O 8   
     (e) a fifth step of reacting said UO 3  and/or U 3  O 8  with hydrogen gas and/or hydrogen gas and steam to convert said UO 3  and/or U 3  O 8  into UO 2  powder. 
     In the present invention, blowing of UF 6  gas and steam into the fluid bed reaction apparatus can be carried out more effectively by using a binary fluid nozzle. 
     In the present invention, the third step of drying and dehydrating said product of the second step and the fourth step of reacting said product of the third step with hydrogen gas or hydrogen gas and steam to convert said product into UO 2  powder can be carried out favorably by using a relatively corresponding fluid bed reaction apparatus. 
     Further, in the present invention, the third step of drying and dehydrating said product of the second stepk, the fourth step of reacting said product of the third step with steam to convert said product into UO 3  and/or U 3  O 8 , and the fifth of reacting said UO 3  and/or U 3  O 8  with hydrogen gas or hydrogen gas and steam to convert said UO 3  and/or U 3  O 8  into UO 2  can be also similarly carried out favorably by using a relatively corresponding fluid bed reaction apparatus. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The features of the present invention are first, in the fluid bed reaction apparatus, UO 2  F 2  particle is obtained by gas phase reaction of UF 6  gas with steam, and secondly ADU is obtained by reacting the UO 2  F 2  particle with ammonia water solution, thereby avoiding the defects of the conventional wet process, that is, a bad filterability of the ADU precipitate, a large quantity of uranium loss into the filtrate and a vast volume of waste liquid generated improved. 
     This is due to the fact that in the present invention, as the resultant UO 2  powder inherits the same shape as that of the UO 2  F 2  particle formed by the first step, the filtering operation becomes very easy and the greater part of HF of which the whole quantity transfered into the filtrate in the conventional wet process can be recovered as a HF water solution which can be reutilized in the first step. 
     Further, in the present invention, by carrying out the reaction of the second step by atomizing ammonia water solution, the filtering step can be omitted. 
     Further more, the present invention can solve the laarge defect of the conventional wet case in which UF 6  is converted into UO 2  by using the fluid bed reaction apparatus, that is, the low activity of the UO 2  powder formed and the slow defluorinating velocity thereof which make the UO 2  powder not suitable for manufacturing a reactor fuel. It can also utilize effectively the great merit of the wet process in which UO 2  powder having an excellent fluidity is obtained. 
     Namely, the present invention possesses a fundamental condition suitable for manufacturing a reactor fuel, that is, an easy defluorination of UO 2  powder and a high activity thereof and at the same time makes the fluidity of UO 2  powder very excellent by solving the defect of the wet process. This makes handling of UO 2  powder in the following steps very easy and further eliminates a pelletizing operation which is usually carried out in the manufacturing of UO 2  pellet for a rector fuel. 
     Further, in the present invention, the remaining fluorine content of UO 2  powder can be easily lowered by converting the dehydrated product of ADU obtained in the third step into UO 3  or U 3  O 8  by reaction with only steam and then reducing the UO 3  or U 3  O 8  to UO 2 . These steps are carried out to check the formation of UF 4  which hinders the defluorinating reaction by making UO 2  not coexisit at removal of the remaining fluorine. 
     In the fluid bed reaction apparatus of the first step, it is effective for controlling of the particle size of UO 2  F 2  to use a binary fluid nozzle as an atomizing nozzle of UF 6  gas to atmozie UF 6  gas from the central part of the nozzle and steam from the periphery part respectively. This is to lower a mean particle size of UO 2  F 2  particle which is produced by reaction of UF 6  gas with steam near the outlet of the nozzle and becomes neucleas of pellet to form a fluid bed. The use of the binary fluid nozzle forms UO 2  F 2  which is a fine pellet having a high reactivity to make reactions of following steps proceed quickly. 
    
    
     The annexed drawing is an apparatus diagram used in one following example of the present invention and corresponds to claims 1 and 3. 
    
    
     In the drawing, UF 6  is vaporized in a vaporizing chamber 1, and the UF 6  gas and steam are atomized into a first fluid reaction apparatus 2 through a binary fluid nozzle of the apparatus 2. At the same time, steam as a reaction gas and a fludizing gas is introduced into the bottom of the apparatus 2 from pipe 10. A part of UF 6  gas atomized from the center part of the binary fluid nozzle reacts immediately with steam atomized from the periphery part of the nozzle to form UO 2  F 2  particles. A part of the formed UO 2  F 2  particles cover the surface of UO 2  F 2  already forming the fluid bed to make the UO 2  F 2  grow. Further, a part of these UO 2  F 2  particles is worn away by collison with other particles to be fine particles. The particle size of UO 2  F 2  particle is controlled by these steps and the particles form the fluid bed. 
     The operating temperature of the first fluid bed reaction apparatus 2 is less than 400° C., preferably in the range of 220°-300° C. in consideration of activity of the particle and control of the particle size. Further, HF gas secondarily produced in the first fluid bed reaction apparatus 2 is recovered as a HF water solution by HF condenser 2a to be accepted by HF receiver 2b. The thus formed UO 2  F 2  particles are overflowed from the upper part of the fluid bed to be discharged out of the apparatus 2 and are sent to a reaction apparatus 3. In the reaction apparatus 3, the UO 2  F 2  particles are converted into ADU with ammonia water solution introduced through pipe 11. A part of these particles and the ammonia water solution are discharged from pipe 12 and is filtered by a filtering apparatus 4 to be separated to the particles and the liquid. The operating temperature of the reaction apparatus 3 is less than 90° C., preferably in the range of 10° C.-40° C. The thus obtained particles are sent to a second fluid bed reaction apparatus 5 where the particles are heated to be dried and dehydrated. Air as a fluidizing gas is introduced to the bottom of the apparatus 5 through a pipe 13. The operating temperature is less than 200° C., preferably in the range of 100° C.-180° C. The dried and dehydrated particles are overflown from the upper part and sent to a third fluid bed reaction apparatus 6, where the particles react with steam as a reaction gas and a fluidizing gas introduced to the bottom of the apparatus 6 through a pipe 14, to form UO 3  or U 3  O 8 . This UO 3  or U 3  O 8  is similarly overflown from the upper part of the fluid bed to be sent to a fourth fluid bed reaction apparatus 7, where the UO 3  or U 3  O 8  reacts with a mixed gas of steam and hydrogen gas as a reacting gas and a fluidizing gas introduced into the bottom of the apparatus 7 from 15, to form UO 2 . The UO 2  powder is received as a product by a container 8. This operating temperature is less than 700° C., preferably in the range of 500° C.-600° C. in consideration of activity of the UO 2  powder. 16 is a waste gas treatment line. 
     The effects of the present invention are as follows: 
     (1) the UO 2  powder obtained has a high activity, a small remaining fluorine quantity and excellent fluidity. Such UO 2  powder as that of the present invention can not be obtained by conventional process. 
     (2) The excellent fluidity of the UO 2  powder can make handling thereof in following steps very easy and can omit pelletizing operation generally carried out in manufacturing a reactor fuel. 
     The present invention will be understood more readily with reference to the following examples. The examples, however, are intended to illustrate the present invention and are not to be construed to limit the scope of the present invention. 
     EXAMPLE 
     This example contains a case corresponding to a first process 1 in which UO 2  F 2  particles formed by the first fluid bed reaction apparatus react with ammonia water solution to form ADU and the ADU is reduced to UO 2  powder and another case corresponding to a second process in which UO 2  F 2  particles formed by the first fluid bed reaction apparatus react with ammonia water solution to form ADU and the ADU reacts with steam to form UO 3  /U 3  O 8  powder and the UO 3  /U 3  O 8  is reduced to UO 2  powder. For comparisob, a comparative example in which UO 2  F 2  particles formed by the conventional first fluid bed reaction apparatus are reduced to UO 2  powder is shown together in Tables 1-6. 
     
                       TABLE 1______________________________________                         comparative        Process 1               Process 2 example______________________________________first fluid bedreaction apparatusreaction temperature (°C.)          280      280       280UF.sub.6 supplying velocity          90       90        90(g/min)nozzle steam supplying          14       14         0velocity (g/min)fluidizing gas velocity          25       25        25(cm/sec)formed UO.sub.2 F.sub.2 (g/min)          79       79        79use of binary fluid nozzle          used     used      not used______________________________________ 
    
     
                       TABLE 2______________________________________                         comparative        Process 1               Process 2 example______________________________________second fluid bedreaction apparatusreaction temperature (°C.)          10       10        (not used)UO.sub.2 F.sub.2 supplying velocity          79       79(g/min)product (ADU) (g/min)          82       82______________________________________ 
    
     
                       TABLE 3______________________________________                         comparative        Process 1               Process 2 example______________________________________third fluid bedreaction apparatusreaction temperature (°C.)          170      170       (not used)formed UO.sub.2 F.sub.2 hydrate          82       82(g/min)fluidizing gas velocity          30       30(cm/sec)UO.sub.2 F.sub.2 anhydride (g/min)          76       76______________________________________ 
    
     
                       TABLE 4______________________________________                         comparative        Process 1                Process 2                         example______________________________________fourth fluid bedreaction apparatusreaction temperature (°C.)          (not used)                    500      (not used)UO.sub.2 F.sub.2 particle supplying                    76velocity (g/min)fluidizing gas velocity  20(cm/sec)formed UO.sub.3 /U.sub.3 O.sub.8 (g/min)                    73______________________________________ 
    
     
                       TABLE 5______________________________________                         comparative         Process 1                Process 2                         example______________________________________fifth fluid bedreaction apparatusreaction temperature (°C.)           600      600      660UO.sub.3 /U.sub.3 O.sub.8 particle           76       73       79supplying velocity (g/min)fluidizing gas velocity           20       20       20(cm/sec)hydrogen/steam (mol ratio)           1/1      1/1      1/1formed UO.sub.2 (g/min)           69       69       69______________________________________ 
    
     
                       TABLE 6______________________________________                         comparative         Process 1                Process 2                         example______________________________________property of UO.sub.2 powderbulk specific gravity (g/m.sup.3)           1.9      1.8      2.8specific surface area (m.sup.2 /g)           2.9      3.1      1.0mean particle size (m)           98       96       145U%              87.6     88.0     87.8remaining fluorine quantity           43       38       276(ppm)______________________________________ 
    
     Thus, as UO 2  powder obtained by the present invention is smaller in bulk specific gravity and particle size, larger in specific surface area, higheer in activity and smaller in remaining fluorine quantity than those of conventional process, it is suitable for manufacturing UO 2  pellet for a reactor fuel.