UO.sub.2 pellet fabrication process

Making uranium dioxide pellets of controlled grain size by treating 50-500 g/l UO.sub.2 F.sub.2 with NH.sub.3 in a first and a second stages to form (NH.sub.4).sub.2 U.sub.2 O.sub.7 precipitate, wherein the NH.sub.3 /U molar ratio is between 3-5 in the first stage and between 6-12 in the second stage. The precipitate is then formed into UO.sub.2 pellets having grain size within the range from 10 to 100 .mu.m.

BACKGROUND OF THE INVENTION 
The present invention relates to a UO.sub.2 fabrication process on the 
basis of an ADU (ammonium diuranate) process, and more particularly, to an 
improvement in a method of controlling the grain size of UO.sub.2 pellets 
made from UO.sub.2 powder produced by the ADU method, by adjustment of the 
precipitation conditions in the ADU method. 
RELATED ART 
As is well known, the ADU method is widely utilized as a process for 
fabricating UO.sub.2 powder from UF.sub.6. 
The ADU method is such that an aqueous solution of UO.sub.2 F.sub.2 
obtained by reaction of UF.sub.6 gas with water is reacted with NH.sub.4 
OH produced by passing NH.sub.3 through the solution to precipitate the 
ADU. The ADU is then filtered and dried and, thereafter, calcined and 
reduced to form UO.sub.2 powder. The reaction by which the ADU is formed 
is represented by the following chemical equations: 
EQU UF.sub.6 +2H.sub.2 O.fwdarw.UO.sub.2 F.sub.2 +4HF (1) 
EQU UO.sub.2 F.sub.2 +4HF+7NH.sub.4 OH.fwdarw.(1/2)(NH.sub.4).sub.2 U.sub.2 
O.sub.7 +6NH.sub.4 F+(11/2)H.sub.2 O (2) 
The UF.sub.6 is first transformed to an aqueous solution of UO.sub.2 
F.sub.2 by the hydrolytic reaction represented by chemical equation (1). 
Then, since four moles of HF exists in the aqueous solution compared to 
one mole of uranium, a neutralizing reaction of HF takes place 
simultaneously in the second equation with the formation of ADU 
precipitate in the form of inactive particles of relatively large size. If 
the UO.sub.2 powder obtained through the processes of calcination and 
reduction of the inactive ADU is employed as a raw material to form 
pellets, the grain size of the pellets is usually made to be approximately 
10 .mu.m. 
In order to burn the UO.sub.2 pellets in a nuclear reactor in a stable 
manner, it is desirable to decrease the fission-product gas (FP gas) 
release from the pellets as low as possible. It has been found that, if 
the grain size of the pellets is increased, the retention of the FP gas is 
enhanced. However, there is concern that excessively large grain in the 
pellets may result in a reduction of the mechanical strength thereof. 
Although an optimum grain size has not yet been determined, it is 
considered appropriate to aim for an upper limit of 100 .mu.m. 
In view of the above, the assignee of this invention has previously, in 
Japanese Patent Application No. 61-190079, proposed a process for 
fabricating UO.sub.2 pellets having a large grain size. 
The method disclosed in the above patent application is characterized in 
that NH.sub.3 is reacted with a UO.sub.2 F.sub.2 aqueous solution 
containing U but no HF, to form ADU, with the U concentration in the 
UO.sub.2 F.sub.2 aqueous solution being within the range from 50 to 1000 
g/l, and the rate at which the NH.sub.3 is added to the UO.sub.2 F.sub.2 
aqueous solution being set to a value equal to or higher than two moles of 
NH.sub.3 /min for every one mole of U. 
According to the above method, the lower the U concentration of the 
UO.sub.2 F.sub.2 aqueous solution, and the higher the rate of adding the 
NH.sub.3 to the UO.sub.2 F.sub.2 aqueous solution, the smaller the size of 
the primary particles of ADU formed so that the particles are highly 
active when burned to form FP gas. As a result of this, the growth of the 
UO.sub.2 grains of the UO.sub.2 pellet obtained by way of UO.sub.2 powder 
from ADU is accelerated so that the grain size becomes larger. This is 
done by appropriately setting the precipitating conditions within the 
aforesaid range making it possible to fabricate the UO.sub.2 pellets with 
a grain size within the range from 10 to 100 .mu.m. 
In the above method, however, the rate of reaction between the U and the 
NH.sub.3, and the NH.sub.3 /U molar ratio must be brought to a 
sufficiently high value, because of the necessity to maintain sufficient 
productivity as the ADU precipitates out. The reason for this is that, if 
the NH.sub.3 /U ratio is less than 6 (less than pH 10), all the U is not 
consumed in the reaction to form ADU precipitate so that some U remains in 
the waste liquid. The reason for the above is also that the low reaction 
rate results in a reduction in the productivity of ADU. Accordingly, by 
controlling the U concentration the size of the primary particles of the 
ADU can be adjusted. If UO.sub.2 pellets are to be fabricated with a 
relatively small grain size near the low value of the range from 10 to 100 
.mu.m, the U concentration in the UO.sub.2 F.sub.2 aqueous solution must 
be equal to or higher than 500 g/l. This inevitably raises the viscosity 
of the UO.sub.2 F.sub.2 aqueous solution considerably, affecting the 
conditions of the precipitate, resulting in the disadvantage that the 
final UO.sub.2 pellets becomes heterogeneous. 
SUMMARY OF THE INVENTION 
The invention has been done in order to solve the above-discussed problems 
related to optimum grain size, and it is the object of the invention to 
provide a UO.sub.2 fabrication process which can control the grain size of 
UO.sub.2 pellets to an optimal value within a range of from 10 to 100 
.mu.m, and which can fabricate pellets with homogeneous properties of any 
particle size.

DETAILED DESCRIPTION 
A process for fabricating UO.sub.2 pellets, according to the invention, 
will specifically be described below. 
The process is characterized in that when ADU is precipitated, the U 
concentration in UO.sub.2 F.sub.2 aqueous solution is brought to a value 
within the range from 50 to 500 g/l, that the reaction of the UO.sub.2 
F.sub.2 aqueous solution with NH.sub.3 is divided into two stages, and 
that the NH.sub.3 /U molar ratio is set in the first step to a value 
within the range from 3 to 6, and in the second stage to a value within 
the range from 6 to 12. 
According to the above-described process, the properties of the ADU formed 
are substantially determined by the precipitating reaction in the first 
stage. In this connection, if conditions are such that the NH.sub.3 /U 
molar ratio is equal to or less than 6, it is possible to form ADU as 
primary particles which are relatively large in size, even if the U 
concentration in the UO.sub.2 F.sub.2 aqueous solution is equal to or less 
than 500 g/l. It is considered that the reason for this is that in the 
case where the NH.sub.3 /U molar ratio is equal to or less than 6, as the 
NH.sub.4 F concentration increases on the basis of the reaction 
represented by the following chemical equation (3) in which the ADU is 
precipitated out of the UO.sub.2 F.sub.2 aqueous solution, the NH.sub.4 F 
causes a reaction to occur whereby the ADU is formed by way of ammonium 
uranyl fluoride (AUF), as represented by the equations (4) and (5): 
EQU UO.sub.2 F+3NH.sub.4 OH.fwdarw.(1/2)(NH.sub.4).sub.2 U.sub.2 O.sub.7 
+2NH.sub.4 F+(3/2)H.sub.2 O (3) 
EQU UO.sub.2 F.sub.2 +3NH.sub.4 F.fwdarw.(NH.sub.4).sub.3 UO.sub.2 F.sub.5 (4) 
EQU (NH.sub.4).sub.3 UO.sub.2 F.sub.5 +3NH.sub.4 
OH.fwdarw.(1/2)(NH.sub.4).sub.2 U.sub.2 O.sub.7 +5NH.sub.4 F+(3/2)H.sub.2 
O (5) 
Since the AUF is a crystalline material inert under normal conditions, the 
ADU formed by way of the AUF is also inert and has relatively large 
primary particles. The lower the NH.sub.3 /U molar ratio set in the first 
stage reaction, the greater the tendency for the ADU to be formed by way 
of the AUF, so that ADU which is inert and has larger primary particles is 
obtained. It is not desirable for the NH.sub.3 /U molar ratio to be lower 
than 3 in the first stage precipitating reaction, because this will lower 
the ratio at which the U is precipitated. On the other hand, if the 
NH.sub.3 /U molar ratio is equal to or higher than 6, the conventional 
problems cannot be solved. In this connection, although the U remains in 
the aqueous solution even if the NH.sub.3 /U molar ratio is within the 
range of from 3 to 6, it is possible to react the U sufficiently if the 
NH.sub.3 /U molar ratio in the second stage reaction is brought to a value 
within the range of from 6 to 12. Further, if the NH.sub.3 /U molar ratio 
is lower than 6 in the second stage reaction, the U is not sufficiently 
precipitated. On the other hand, there is no value in having a NH.sub.3 /U 
molar ratio above 12, because this merely increases the amount of water 
used and an amount of waste liquid. Also in the case where the NH.sub.3 /U 
molar ratio is brought to a value within the range from 6 to 12 in the 
second stage in order to precipitate the ADU sufficiently in the manner 
mentioned above, the properties of the resulting ADU are no different than 
those of the ADU formed in the standard method equation (2). 
Thus, according to the process of this invention, it is possible to easily 
fabricate pellets with an optimal particle size within the range from 10 
to 100 um and which are homogeneous in properties, without wasting the U 
even under for the condition where the U concentration in the UO.sub.2 
F.sub.2 aqueous solution is equal to or less than 500 g/l. 
The advantages of the invention will next be expounded with reference to an 
embodiment. 
The UO.sub.2 F.sub.2 powder was dissolved in demineralized water to form an 
aqueous solution whose U concentration within the range from 40 to 600 
g/l. The aqueous solution and NH.sub.3 water were first fed continuously 
to a first-stage settling chamber, with a 2.5 to 6.5 NH.sub.3 /U molar 
ratio, to carry out the first stage ADU precipitation. Subsequently, the 
ADU slurry formed in the first-stage settling chamber, and the aqueous 
NH.sub.3 were fed continuously to a second stage settling chamber, and the 
NH.sub.3 /U molar ratio brought to a value within the range from 5 to 15. 
The resulting second-stage ADU slurry was filtered and dried and, 
thereafter, calcined and reduced at 650.degree. C. under a H.sub.2 
atmosphere, to transform the slurry into UO.sub.2 powder. The UO.sub.2 
powder was compacted at a pressure of 5 t/cm.sup.2, and then sintered for 
four hours at 1750.degree. C. in an H.sub.2 atmosphere, to form pellets. 
The following table indicates the relationship between the pellet grain 
size and the ADU precipitating conditions at each of the first-stage and 
second-stage settling chambers. 
TABLE 
______________________________________ 
U CONCENTRATION FIRST SECOND PELLET 
UO.sub.2 F.sub.2 
STAGE STAGE GRAIN 
AQUEOUS SOLUTION 
NH.sub.3 /U 
NH.sub.3 /U 
SIZE 
(g/l) RATIO RATIO (.mu.m) 
______________________________________ 
50 2.5 9.0 7 
50 3.0 9.0 10 
50 4.3 9.0 46 
50 6.0 9.0 98 
50 6.5 9.0 110 
40 6.0 9.0 105 
50 6.0 9.0 96 
300 6.0 9.0 42 
500 6.0 9.0 23 
600 6.0 9.0 9 
100 5.0 5.0 34 
100 5.0 6.0 36 
100 5.0 9.0 33 
100 5.0 12.0 35 
100 5.0 15.0 34 
______________________________________ 
As will be clear from the above table, in case where the U concentration in 
the UO.sub.2 F.sub.2 aqueous solution was 50 g/l, pellets having a grain 
size within the range from 10 to 100 .mu.m were obtained when the NH.sub.3 
/U molar ratio in the first stage was within the range from 3 to 6. 
Further, if the NH.sub.3 /U molar ratio in the first stage was brought to 
6, pellets having a grain size within the range from 10 to 100 .mu.m were 
obtained when the U concentration in the UO.sub.2 F.sub.2 aqueous solution 
was within the range from 50 to 500 g/l. Moreover, if the U concentration 
was 100 g/l and the NH.sub.3 /U molar ratio in the first stage was brought 
to 5, the grain size of the pellets remained practically unchanged, even 
if the NH.sub.3 /U molar ratio in the second stage varied within the range 
from 5 to 15. If, however, the NH.sub.3 /U molar ratio in the second stage 
was 5, the loss of the U was so great that approximately 20% of the U 
remained in the waste liquid. On the other hand, even if the NH.sub.3 /U 
molar ratio in the second stage was 15, the U loss remained the same as 
for when the ratio ranged from 6 to 12, and a sufficiently high collecting 
ratio was obtained even if the NH.sub.3 /U molar ratio in the second stage 
was within the range from 6 to 12. 
As described above, according to the UO.sub.2 pellet fabrication process of 
the invention, it is possible to easily fabricate pellets which have their 
optional grain size within the range from 10 to 100 .mu.m and which are 
homogeneous in properties, without wasting the U even under conditions 
where the U concentration in the UO.sub.2 F.sub.2 is equal to or less than 
500 g/l. Thus, the amount of the pellets restrict the rate of release of 
fission product gas can be set to a desired value, making it possible to 
enhance the combustion stability of the pellets.