Process of preparing sintered urandium dioxide pellets

Sintered uranium dioxide pellets suitable for power reactor use are described, and a process for their manufacture. This process involves incorporating a small amount of sulphur into the uranium dioxide before sintering, as a consequence large grain sizes are found in the pellets. The increase in grain size results in an improvement in overall efficiency when such pellets are used in a power reactor.

BACKGROUND OF THE INVENTION 
This invention relates to the preparation of uranium dioxide, particularly 
in the form of pellets as used in nuclear reactors for electrical power 
generation purposes. 
DESCRIPTION OF THE PRIOR ART 
Uranium dioxide, UO.sub.2, is the fuel most commonly used in present day 
nuclear power reactors. In its final form, as used in the fuel elements, 
the UO.sub.2 must meet stringent chemical and density specifications, 
which are set by the nuclear industry to allow efficient and economical 
operation of the power reactors. 
The most common method used to obtain the high densities required in the 
pellets used in reactor fuel elements, which desirably is greater than 95% 
of the theoretical density for UO.sub.2 in bulk, is by pressing a UO.sub.2 
powder into pellets, and then sintering these pellets in a hydrogen 
atmosphere at a temperature of at least 1600.degree. C. Even under these 
stringent conditions, the UO.sub.2 powder used generally has a very fine 
particle size if pellets which meet the desired density limits are to be 
met. 
There are a number of different methods in use for producing UO.sub.2 power 
of very fine particle size. The method most commonly used is by the 
hydrogen reduction of a material commonly called both ammonium diurante, 
and ammonium uranate, which is a solid having a formula approximating to 
(NH.sub.4).sub.2 U.sub.2 O.sub.7. It is also known by the acronym ADU. ADU 
for this purpose is generally obtained by precipitation from solution by 
reacting ammonia, or ammonium hydroxide with a solution of uranyl nitrate 
or uranyl fluoride. The ADU formed by this procedure has a very fine 
particle size which carries through into the final, sintered, UO.sub.2 
pellet. 
This process is not without its disadvantages. An improved process for 
obtaining UO.sub.2 capable of providing sintered pellets having a higher 
density is described in our South African Pat. No. 76.1302, issued August, 
1977. In this Patent is described a process for the preparation of fine 
particle size uranium dioxide from a uranium trioxide feed comprising the 
steps of: 
(a) reacting solid uranium trioxide with aqueous ammonium nitrate to form 
an insoluble ammonium uranate (it is to be noted that although described 
in the same language, this precipitated material is chemically different 
to that mentioned above: its formula is generally 
6UO.sub.3.2NH.sub.3.5H.sub.2 O); 
(b) neutralizing the thus formed slurry with ammonium hydroxide to 
precipitate out as an insoluble ammonium uranate the remaining dissolved 
uranium; 
(c) recovering the thus formed precipitates in a dry state; and 
(d) reducing the dried precipitate to uranium dioxide. 
The thus obtained dioxide can then be converted into pellets and sintered, 
to provide a pellet having density above 10.64 gm/cc, that is better than 
97% of the theoretical density of 10.96 gm/cc. 
SUMMARY OF THE INVENTION 
However, it is now known that producing a UO.sub.2 pellet having a high 
density is not the only criterion of relevance, although it is an 
important one. It has now been discovered that the size of the grains 
present in such pellets, which can be observed and measured, after 
applying appropriate sectioning techniques to a pellet, with an optical 
microscope, has an effect on their efficiency, in terms of reactor power 
output, when used as fuel in a nuclear power generating reactor. It has 
now been realized that the grains size present in the sintered pellets has 
an effect on the rate of loss of fission by-products from the pellet. 
Studies have shown that if the grains size of the UO.sub.2 present in the 
sintered pellets could be increased to a figure significantly higher than 
the presently larger known grains, which have a size of about 25 to 30 
microns, then an increase in overall power output efficiency should result 
of the order of 5 to 10%. 
We have now discovered a simple process whereby an acceptable sintered 
UO.sub.2 pellet may be made which not only has a suitably high density, 
and thus is acceptable as a reactor fuel, but also has an internal grains 
size considerably larger than has hitherto been possible.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Thus in a first aspect this invention comprises a sintered, high density 
uranium dioxide pellet composed of grains having a size in excess of 50 
microns. 
In a preferred aspect, this invention comprises a sintered, high density 
uranium dioxide pellet composed of grains having a size range of from 50 
microns to 1,000 microns. 
The process of this invention utilizes most of the process steps which are 
outlined above, and disclosed in our South African Pat. No. 76.1302, with 
the addition of one simple, but important step. This step is to provide as 
the initial feed a uranium trioxide material containing a known and 
controlled amount of sulphur. By this means, a uranium dioxide product is 
obtained which, before sintering exhibits a fine particle size which 
enables the preparation of a high density sintered product and which 
exhibits an increase in grains size during the sintering procedure. 
Thus in a second aspect this invention provides a process for the 
preparation of a sintered, high density, large grains size uranium dioxide 
pellet which comprises the steps of: 
(i) reacting a uranyl nitrate of formula UO.sub.2 (NO.sub.3).sub.2 6H.sub.2 
O with a sulphur source, at a temperature of about 300.degree. C. to about 
400.degree. C. to provide a sulphur-containing uranium trioxide; 
(ii) reacting the thus obtained modified uranium trioxide with ammonium 
nitrate to form an insoluble sulphur-containing ammonium uranate; 
(iii) neutralizing the thus formed slurry with ammonium hydroxide to 
precipitate out as an insoluble ammonium uranate the remaining dissolved 
uranium; 
(iv) recovering the thus formed precipitate in the dry state; 
(v) reducing the dry precipitate to UO.sub.2 and forming it into green 
pellets; and 
(iv) sintered the thus obtained pellets in a hydrogen atmosphere at an 
elevated temperature. 
In most of these steps, the conditions are not critical, and the manner in 
which the various operating parameters may be varied is fully discussed in 
our South African Pat. No. 76.1302. But in respect of steps (i), (v) and 
(vi) other considerations apply, since the amount of sulphur, expressed as 
elemental sulphur, present in the uranium dioxide at the green, unsintered 
pellet stage has a direct relationship to the grains size obtained in the 
pellet after sintering. As can be seen from the graphical representation 
in the attached FIGURE, increase in the sulphur content, expressed as 
elemental sulphur, in the green pellets increases the grains size in the 
final pellets, under standardized sintering conditions. Indeed we have 
found that by the addition of sulphur in this fashion grains sizes in the 
range of 50 microns up to 1,000 microns are obtainable. 
The critical point in the process of this invention at which the sulphur 
content must be controlled is at the green pellet stage. A preferred range 
of sulphur content, expressed as elemental sulphur, at that point in the 
process is from about 20 ppm by weight, to about 1,000 ppm by weight. At 
this level of addition a grains size in the final pellet of up to 1,000 
microns can be achieved. Clearly a lower level of sulphur will only 
provide a grains size toward the lower end of this range: reference is 
again made to the attached FIGURE. 
However, the only point in the process at which it is feasible to control 
the sulphur content is at the beginning, in step (i) as detailed above. It 
is our experience that in proceeding through steps (i) to (iv), that is 
from the initial uranyl nitrate feed to an unsintered green pellet, about 
75% of the sulphur initially added is lost. It also appears that a scale 
factor is involved: in small scale laboratory work less sulphur seems to 
be lost than in larger scale industrial work. Thus it usually will be 
necessary to establish, by way of experiment, exactly what level of 
sulphur compound requires to be reacted initially in order to achieve a 
specified desired level of sulphur in the green pellets, and hence a 
specified grains size range in the sintered pellets. In our own operations 
we have found the losses to be of the order of 75% and hence if it is 
desired to have a sulphur level of 100 ppm in the green unsintered 
pellets, an addition of 400 ppm requires to be made initially. 
The form in which the sulphur is added to the uranyl nitrate in step (i) is 
not critical, and it can be chosen from a wide range of materials. However 
in its choice, it must also be remembered that some of the sulphur will 
persist through the sintering stage into the final pellets. Therefore 
substances containing sulphur which would interfere either with the 
chemistry leading to the uranium dioxide used in making the pellets, or in 
the pressing and sintering operation, or would cause problems when the 
pellets are used in a reactor, have to be avoided. A reagent which is 
easily available and meets all of these criteria is sulphuric acid, and 
hence this is the reagent we prefer to use. 
The following general comments apply to all of the subsequent examples. 
(a) URANYL NITRATE 
The nuclear grade uranyl nitrate used had the following chemical analysis: 
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Impurity Maximum Typical 
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Ag 1.0 0.1 
B 0.2 0.15 
Cd 0.2 &lt;0.2 
Cr 10 5 
Cu 50 1 
Fe 30 25 
Mn 5 &lt;1 
Mo 1 0.5 
Ni 15 5 
P 50 10 
Si 20 &lt;10 
Th 50 30 
V 30 &lt;10 
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These levels are in ppm on uranium present basis. 
(b) AMMONIUM NITRATE AND AMMONIA 
These were prepared from reagent grade materials. When recycle ammonium 
nitrate is used both the pH and concentration were adjusted, if need be, 
by conventional procedures. 
(C) URANIUM DIOXIDE ASSESSMENT 
The method used was to press the uranium dioxide powder to form green 
pellets, and then sinter these pellets in a hydrogen atmosphere at a 
temperature of up to at least 1600.degree. C. The pellets were then 
suitably sectioned, and the grains size assessed by means of an optical 
microscope as observed from the face of the section. 
(D) EXPERIMENTAL PROCEDURE 
The sulphur additions were carried out by adding a known amount of sulphur 
compound, generally as sulphuric acid, to uranyl nitrate, and then heating 
the mixture to a temperature of from about 300.degree. C. to about 
400.degree. C. in order to decompose the uranyl nitrate to uranium 
trioxide. 
The modified uranium trioxide thus produced was then added to a well 
agitated vessel containing ammonium nitrate solution at the desired 
temperature. 
The pH of the slurry, recorded during the run, generally dropped to a 
minimum value in the range 2.5 to 4.0. After the required reaction time, 
either aqueous (28%) or anhydrous ammonia was added to the slurry. After 
the ammonia addition, the slurry was usually repulped for 5 to 30 minutes 
prior to filtering, in order to verify that the pH was not still 
decreasing. 
Filtration was carried out at temperatures up to 70.degree. C., generally 
above 50.degree. C. After washing with hot water, the cake was dried at 
110.degree. C. 
Finally, the product was baked, reduced, pelletted, and sintered in 
conventional production equipment. 
Following this procedure a sequence of runs were made in which standardized 
processing and sintering conditions were used. The only variable changed 
was the amount of sulphur added. Inspection of the obtained sintered 
pellets gave the following results. The sulphur was added in step (i) as 
sulphuric acid. 
TABLE 1 
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Sulphur added, 
Sulphur content of 
Pellet UO.sub.2 particle 
ppm green pellets, ppm 
size, microns 
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40 10 11 
80 20 15 
160 40 38 
200 50 41 
240 60 80 
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Other experiments carried out under differing sintering conditions have 
shown that the presence of from 150 ppm to 300 ppm, of sulphur in the 
green pellets (measured as elemented sulphur) will provide grain sizes in 
the sintered pellets of from 500 to 700 microns. Under some sintering 
conditions grain sizes of up to at least 1,000 microns have been observed. 
The relevant variable in the sintering process appears to be the rate at 
which the green pellets are brought to sintering temperature. In the above 
Table the rate of rise was standardized at 200.degree. C. per min. The use 
of a higher rate of rise leads to larger final grain sizes when a sulphur 
source is present in the green pellets.