BACKGROUND OF THE INVENTION
The present invention relates to a process for fabricating nuclear
reactor fuel pellets having large grain sizes from highly active UO&sub2; powder.
In particular, the present invention concerns a method for controlling the sintered
density of UO&sub2; pellets to a predetermined range.
When UO&sub2; pellets are used as the fuel in nuclear reactors, it
is important that the fuel density be high within a reasonable limit so that a
more compact reactor core can be designed, and that the thermal conductivity of
the pellets is sufficiently high.
However, when the sintered density of the pellets is too high, swelling
of the pellets during irradiation becomes too great, thereby damaging a tube in
the reactor. Accordingly, UO&sub2; pellets commonly used in light-water reactors
are usually designed so that the sintered density is in the range of from 94 to
97 % TD (theoretical density).
One recent technical innovation is to prolong the useful life of the
reactor fuel. This is called the plan for "high burnup", and it is now being studied
seriously. In order to execute this plan, it is imperative to restrain the fission
gas (FP gas) in the pellets as much as possible.
It is well known that producing large crystal grain sizes is effective
in confining FP gas in the pellets. However, the conventional technology only produced
grain sizes of at most about 10 to 20 µm.
In light of the above, the applicants have proposed a process for
fabricating UO&sub2; pellets with large-grain size crystals in JP-A-62-297215,
JP-A-63-45127, EP Patent Application Nos. 87119391.8, 87100721.3 and 891007720.5.
These processes have a common effect of producing in that they make crystalline
grains of large size by controlling the conditions of ammonium diuranate (ADU)
With the processes described in the applications and patents, it is
possible to control the crystalline grain size, however at the time these previous
patents were filed, the applicants did not consider to control the sintered density
of the pellets. In other words, when pellets are fabricated pellets having grain
sizes larger than 20 µm by the process described above, the sintered density of
the resulting pellets are as high as 98 to 99% TD.
In order to reduce the sintered density of the sintered body, if
required, it has been common to add a pore-former agent to the raw material powder,
which cause the formation of pores when the agent sublimates during sintering.
The applicants believe that the method is applicable to the fabrication process
for pellets composed of large-grain size crystals. Although a pore-former agent
of this kind is effective in reducing the sintered density, the agent is likely
to have adverse effects on UO&sub2; grain growth. In other words, the formation
of crystalline grains with large grain size is disrupted. Accordingly, it is desired
to develop a method for controlling both the sintered density of the pellets and
the crystalline grain sizes.
SUMMARY OF INVENTION
Accordingly, it is an object of the present invention to provide a
process for fabricating UO&sub2; pellets having the sintered density of pellets
within the range of from 94 to 97% TD, when pellets having large crystalline grain
sizes in excess of 20 µm are fabricated from highly active UO&sub2; powder, and
thereby restrain the fission gasses generated during the irradiation in the pellets
thereof to enhance the safety of irradiation.
According to an aspect of the present invention, there is provided
a process for fabricating UO&sub2; pellets comprising
- (1) uniformly adding at least one pore-former agent in the range of 0.3 to
1.4% by weight, to uranium dioxide powders to form a starting material, the pore-former
agent decomposing and sublimating below 600 °C and having average grain size in
the range of 5 to 500 µm,
- (2) compacting the uranium dioxide powders, including the pore-former agent,
to form green pellets, and
- (3) sintering the green pellets to form sintered UO&sub2; pellets having large
According to the present invention, Accordingly, both the sintered
density of the pellets and the crystalline grain sizes can be easily controlled
to the desired range, which was not previously possible.
A process for fabricating UO&sub2; pellets, according to the present
invention, will be specifically described below.
First, a pore-former agent is added to highly active UO&sub2; powder.
From experimental results, the inventors have determined that the following conditions
must be met in order to maintain a sintered density in the range between 94 to
- (1) It is necessary that the particle size of the pore-former agent be between
5 and 500 µm, and preferably between 10 and 100 µm. When the particle size is less
than 5 µm, the pores left behind after the sublimation of the agent during sintering
disrupted the growth of the crystals, and thus, the crystalline grain sizes in
the pellets is small. On the other hand, when the particle size exceeds 500 µm,
large pores are formed on the surface of pellets. These large pores must be avoided
because they accelerate the absorption of the water into the pellets.
- (2) It is necessary to use a pore-former agent that decomposes and sublimates
below 600 °C, more preferably at 500 °C. When the decomposition temperature is
higher than 600 °C, the pore-former agent is confined to the inside of the pellets
during the sintering process and cracks or holes appear in the pellets. Ammonium
acetate, ammonium carbonate, ammonium bicarbonate, ammonium oxalate, ammonium
alginate, stearic acid, and the like, are pore-former agents that meet the above
requirements. These compounds are used either alone or in a mixture.
- (3) The appropriate amount of these pore-former agent is in the range of 0.3
to 1.4% by weight of the UO&sub2; powders. The sintered density of the pellet will
not be in the range of 94 to 97% TD, when the added amount falls outside this range.
Subsequently, UO&sub2; powder, to which a pore-former agent has been added, is
fabricated into pellets by any of the methods described below.
- (a) UO&sub2; powder, to which a pore-former agent has been added, is filled
into a mold and pressed to make a pressed body.
- (b) A lubricant is added uniformly to UO&sub2; powder to which a pore-former
agent has been added. Thus prepared, the powder is filled into a mold and subjected
- (c) UO&sub2; powder, to which a pore-former agent has been added, is filled
into a mold coated with a lubricant. Subsequently, the compacting step is carried
out on the filled mold.
- (d) UO&sub2; powder that includes a pore-former agent is roughly molded into
a lump which is pulverized to obtain granules. The granules are compacted by any
of the processes described in (a) to (c) above. Stearic acid, zinc stearate, lithium
stearate, stearic amide, ethylene-bis-stearic amide, methylene-bis-stearic amide,
polyethylene glycol, and the like, are suitable compounds for the lubricant. These
compounds are used either alone or in mixtures. When a lubricant of this kind
is added to the raw material powder, or when a lubricant is applied to the mold,
the press-compacting is easily carried out.
When the granules are made by the method described in (d), the size
of the granules should be less than 2000 µm, and more preferably less than 1000
µm. When the size exceeds 2000 µm, it is difficult to fill a designated amount
of the granule into a mold and perform the compacting step.
The following examples are cited to prove the usefulness of the invention.
ADU (ammonium diuranate), produced by the reaction between 300gU/l
of UO&sub2;F&sub2; aqueous solution and aqueous ammonia, was calcined and reduced
to form UO&sub2; powder. After 0 to 1.5% by weight of pore-former agents, whose
particle size had been adjusted to several different particle sizes, was uniformly
added to the UO&sub2; powder, the pressed bodies were formed under the pressure
of 3 t/cm² by the use of various compacting processes as described above. Ammonium
oxalate and zinc stearate were selected as the pore-former agent and the lubricant,
respectively. The pressed body was heated to 1750 °C for four hours in hydrogen
atmosphere to produce sintered pellets. Table 1 shows the relationship between
pellets fabrication conditions and physical properties of the pellets.
As is clearly seen from Table 1, the pellets fabricated by the processes
according to the present invention have a sintered density between 94 and 97% TD.
Furthermore, significant reduction of the crystalline grain size, and reduced probability
of having large pores, both due to the presence of a pore-former agent, is apparent.
Physical Properties of Pellets
Particle Size (µm)
Density (% TD)
Grain Size (µm)
less than 2000
less than 2000
less than 2000
less than 5
Significant Reduction of Grain Size
Large open porosity
One or more compounds were selected as pore-former agents from the
group consisting of ammonium acetate (C 1), ammonium carbonate (C 2), ammonium
bicarbonate (C 3), ammonium oxalate (C 4), ammonium alginate (C 5) and stearic
acid (C 6). 1.0% by weight of the selected compound or mixture thereof was added
to the same UO&sub2; powder as in Example 1. The resulting UO&sub2; powder was
subjected to rough compacting, pulverization and granulation to form granules.
Subsequently, one or more compounds were selected as a lubricant from the group
consisting of stearic acid (D 1), zinc stearate (D 2), lithium stearate (D 3),
stearic amide (D 4), ethylene-bis-stearic amide (D 5), methylene-bis-stearic amide
(D 6) and polyethylene glycol (D 7), and 0.2% by weight of the selected compound
or compounds was added to the granules to make the mixture. Furthermore, the mixture
was subjected to the same compacting and sintering as in Example 1 to form pellets.
Table 2 shows the relationships between the fabrication conditions of pellets and
the physical properties of the pellets.
As is clearly seen from Table 2, the pellets having the excellent
physical properties are fabricated by using the above-described pore-former agents
and lubricants in the process according to the present invention.
Grain Size of Pellets (µm)
C 1 + C 4
C 1 + C 2 + C 4
C 1 + C 2 + C 3 + C 4
C 1 + C 2 + C 3 + C 4 + C 5
D 1 + D 2
D 1 + D 2 + D 3
D 1 + D 2 + D 3 + D 4
D 1 + D 2 + D 3 + D 4 + D 5
D 1 + D 2 + D 3 + D 4 + D 5 + D 6
D1 + D2 + D3 + D4 + D5 + D6 + D7