The invention relates to the fabrication of uranium dioxide pellets
for use as nuclear reactor fuel where the pellets are formed by pressing and sintering.
Background of the Invention
Commercially operated nuclear reactors for electrical power are generally
fueled by enriched uranium dioxide pellets sealed inside a metal cladding tube.
The pellets are pressed from a highly sinterable ceramic-grade uranium dioxide
powder to form unfired, or "green" pellets, which are then fired in a furnace in
a controlled ambient gas until they take on the desired sintered characteristics.
The characteristics of the ceramic powder, as well as a description of the pressing
process and of various additives used in such a process are described, for example,
in U.S. patent 4.430.276 issued Feb. 7, 1984 to Radford et al. This patent discloses
that the addition of certain metal oxides to the sintering powder, can result in
desirable changes, such as by increasing the grain size in the microstructure of
the finished pellet.
The step of pressing the powder to form a green pellet requires the
addition of a die lubricant to the press tooling or to the ceramic powder itself,
or both, in order to reduce friction during pressing and to ensure an undamaged
release of the green pellet from the tooling. The addition of lubricants to ceramic
powder prior to pressing is a technique recognized in the ceramic art in general
and in the uranium dioxide pelletizing art in particular. Metallic soaps are considered
especially useful for this purpose. Various metallic soap compounds have been disclosed
previously as being useful for this purpose. Among the candidates, zinc stearate
is widely used in a powder form as an additive blended with the ceramic powder.
When the green pellets which have been pressed with a zinc stearate
lubricant are fired, the stearate organic components are volatilized and removed
from the furnace as fugitives. The zinc metallic component is also vaporized and
removed from the pellets. However, within the furnace gas the zinc forms oxides
which deposit on the walls of the furnace and in the flue. The resulting depositions
are troublesome contaminants.
Summary of the Invention
In accordance with the novel method of the present invention, a dry
lubricant consisting of a stearate of aluminum is added to the uranium dioxide
sintering powder and dry blended with it prior to the pressing of green pellets.
The aluminum hydroxide distearate is highly effective as a die lubricant. Furthermore,
while the distearate radical is removed as a fugitive in the course of the sintering
process, the aluminum hydroxide component is converted to aluminum oxide, or alumina
particles, which function as a grain size control agent by promoting larger grain
sizes which are favorable for effectively containing fission products within the
pellets during a fuel burnup cycle.
Brief Description of the Drawings
FIG. 1 is an elevated perspective of a sintered pellet made in accordance
with the present invention.
FIG. 2 is a flow chart describing the steps of the novel process.
In the process of the invention, aluminum hydroxide distearate powder,
i.e. Al(OH) (C&sub1;&sub8;H&sub3;&sub5;O&sub2;)&sub2;, is mixed together in dry
powder form with a dry uranium oxide ceramic powder. The ceramic powder may have
minor quantities of other additives, such as burnable poisons, plutonium, etc.
in addition to the uranium dioxide itself, but for convenience will be referred
to herein simply as "ceramic powder." The amount of aluminum hydroxide distearate
added is 0.1 to 0.4 weight percent of the ceramic powder base.
The function of the aluminum hydroxide distearate is two-fold. First,
it becomes a direct replacement for zinc stearate as the die lubricant used in
the pellet pressing. The aluminum hydroxide distearate is added by the same procedure
and blending process as zinc stearate. Lubricating qualities of aluminum hydroxide
distearate and zinc stearate are substantially equivalent. Second, the aluminum
metal of the compound is not fugitive, but becomes bonded throughout the fuel oxide
matrix as a fuel stabilizing agent, a function previously performed by added alumina
powder. Alumina at these concentrations also increases grain size in the sintered
pellet an additional 2-5 micrometer beyond the grain size obtained when no alumina
is present. Controlled grain size is necessary to regulate the fission gas release
while the fuel is in service. Aluminum stearate and alumina additives provide
equivalent grain size results when compared directly in terms of the molar concentration
of aluminum atoms.
An important additional advantage of using aluminum hydroxide distearate
to replace zinc stearate and alumina is the benefit of increasing the pellet end
capping threshold (ECT). This threshold refers to the maximum pressing pressure
which is feasible for obtaining an acceptable pellet yield. Direct comparison tests
have shown that aluminum hydroxide distearate will permit pellet pressing to higher
green density parameters without the spurious formation of so-called "end cap flaws"
in the form of microfissures in the end face when the green pellet is removed from
the die. Instead, it results in the production of stronger green pellets and thus
increases product yield.
In one example of the process in accordance with the invention, a
nuclear fuel pellet 10 shown in FIG. 1 is manufactured by a process as described
in general terms by the flow chart of FIG. 2. In the process, a steel drum containing
200 kilograms of highly sinterable uranium dioxide ceramic powder which was precompacted
and granulated in particles less than 1.6 mm with moisture content less than 2000
ppm H&sub2;O/UO&sub2;, of which about 12% is U&sub3;O&sub8;, an oxidized form of
uranium dioxide which results from processing of scrap pellets for reuse, has added
to it 400 grams of aluminum hydroxide distearate, amounting to 0.2 weight percent.
The aluminum hydroxide distearate is "technical grade" dry powder material which
typically contains about 19% total ash, about 4.5% free fatty acids, and about
0.5% moisture. It has a particle size such that 98% will pass through a 74 micrometer
screen, 95% will pass through a 44 micrometer screen, and all particles will pass
through a 210 micrometer screen. The powders are blended for 5-15 minutes by a
drum tumbling machine until uniformly mixed.
The blended powder mix is next loaded into a mechanical pelletizing
machine which presses it into green pellets with a force of between 27,58 x 10&sup7;
Pa (40,000 psi) and 51,71 x 10&sup7; Pa (75,000 psi), depending upon the particular
properties of the powder mix.
The green pellets are loaded on trays and fed into a firing furnace
in a reducing atmosphere with water vapor controlled for a dew point from 21-35
degrees Celsius and fired at 1700-1780 degrees Celsius for 2-3 hours, after which
they are cooled at room temperature. At this point the sintering process is complete
and the pellets may be processed further in other respects.
In another example of a process in accordance with the present invention,
all the steps and quantities are the same as in EXAMPLE I above, with the exception
of the added die lubricant. In this case the lubricant is aluminum hydroxide tristearate.
It is added in the amount of 0.28 weight percent. A greater relative quantity
of the aluminum hydroxide tristearate is required than was of the aluminum distearate
because the amount of the aluminum element in the compound is critical in stabilizing
the grain size of the sintered product. Therefore, the amount of the stearate of
aluminum added, regardless of the nature of the stearate radical, should be based
on the aluminum element being in a concentration of 0.025 to 0.335 molar percent
(30-250 microgram Al/gram U) of the blended powder mix.
The aluminum tristearate exhibits excellent lubrication properties
for this application. However, the nature of the manufacturing process for the
aluminum tristearate is at present such that the product is presently not normally
available without a substantial proportion of excessively large particles. The
large particles result in excessively large pore sizes in the sintered pellets,
which lead to defects. It is nevertheless contemplated that aluminum hydroxide
tristearate would be useful for this purpose if obtainable in an appropriate powder
form. Its lubricating properties are acceptable, whereas those of aluminum hydroxide
monostearate are not sufficient. Higher proportions of aluminum hydroxide monostearate
provide excessive amounts of aluminum dopants in proportion to stearate radicals
The process of the present invention is to be distinguished from
processes in which metals and/or lubricants are added in a fluid state, such as
a slurry. Wet processes, when used for fissionable materials, require serious consideration
of possible criticality conditions for the mix. In addition, wet processing makes
it difficult to control the concentrations of the constituents, due to settling
and other phenomena which degrade homogeneity of a suspension. The dry mixing of
the compounds, on the other hand, is relatively free of such considerations and
readily provides close control of the constituent proportions.