Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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AN IMPROVED METHOD AND BINDER FOR THE
MANUFACTURE QF NUCLEAR ~UEL PELLETS, AND THE PRODUCT
FIELD OF THE INVENTION
This in~ention relates generally to the ceramic
art and the formation of sintered bodies from
particulate oxide materials. It is particularly
concerned with a process for producing consolidated
units of particulate ceramic materials, including
the compressing of such particles into coherent and
handleable compacts for subsequent sintering to
integrated bodies. The invention is specifically
directed to the manufacture of nuclear fuel
pellets or units from particulate materials containing
uranium dioxide.
BACKGROUND OF THE INVENTION
~ issionable nuclear fuel comprises a variety of
co~position~ and forms of fissionable materials, including
ceramic co~pounds of uranium, plutonium and thorium.
Fuel compounds ~or commercial power generating reactors
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typically comprise oxides of uranium, plutonium and
thorium, and mixtures thereof. The generally most
suitable and commonly used fuel for such commercial
nuclear reactors is uranium dioxide. Such commercial
fuel materials can be combined with minor amounts of
other ingredients, including neutron flux controlling
additives such as gadolinium.
Commercially produced uranium dioxide is a fine,
fairly porous powder, a form which is not suitable as such
for use as fuel in commercial reactors. A number of means
have been developed and used to convert powdered uranium
dioxide into a form suitable for use as a fuel in power
generating nuclear reactors. One commonly used technique
has been to sinter appropriately sized bodies of the
powdered uranium dioxide material at high temperatures
to develop strong diffusion bonds between the individual
power particles.
However, the sintering technique requires a
preliminary compressing of the loose powder into a shaped,
and self-retaining compacted body of particles of
sufficient strength and integrity to survive handling and
the sintering procedure. The operation of compressing
fine particles into a body or coherent compact with
acceptable low reject levels, and with the strength and
uniformity for enduring subsequent handling and firing
and grinding to size has been a subject of considerable
concern and investigation in the nuclear fuel industry.
Conventional organic or plastic binders commonly
used in powder fabrication have been considered to be
unsuitable in nuclear fuel processing operations.
Entrainment of any binder residues such as carbon within
the sintered nuclear fuel product is unacceptable in
reactor service. Moreover, the presence of any organic
binder among the particles inhibits the formation during
sintering of strong diffusion bonds between the particles,
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and adversely affects the density of the sintered product.
T~e complete removal of binders, or their decomposition
products, prior to sintering is especially difficult, and
usually requires a costly additional operation in the fuel
manufacture.
Accordingly, a common method has been to die press
uranium dioxide powder into approximately sized "green"
(unfired) compacts without the assistance of any binder.
This approach however has resulted in very costly high
rates of rejects and scrap material recycling because of
the weakness of such green, binder-free compacts of
powder.
U.S. Patent No. 4,061,700, issued December 6, 1977,
to Gallivan, and assigned to the same assignee as this
1; application, discloses a distinctive group of fugitive
binders that improved the production of sintered pellets
of particulate nuclear fuel materials for nuclea~ reactors.
The fugitive binders of this patent function without
contaminating the resulting fuel product, and they permit
~ the formation of effective bonds between sintered par-
ticles during firing without deleteriously affecting the
desired porosity of the fused pellet.
The disclosure of the said U.S. Patent No. 4,061,700,
and of U.S. Patent Nos. 3,303,273; 3,923,933; and
3,927,154, also assigned to the same assignee as the
instant application, and relating to significant aspects
in the subject field of producing nuclear fuel pellets
from particulate fissionable ceramic material for
reactor service.
The prior art techniques or means such as disclosed
in ~.S. Patent No. 4,061,~00, have been found wanting in
some conditions and circumstances. For instance it has
been observed that the fugitive binders of the aforesaid
patent do not provide consistent results as to pellet
strength and integrity irrespective of the blendingconditions and particle characteristics of the uraniu~
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dioxide powder. Specifically the severity of agitation in
blending, relative humidity and temperature, and duration of
storage, as well as the uranium dioxide powder properties
such as size, surface area and moisture content are all
factors that apparently can detract from the uniformity of
the physical attributes provided by such fugitive binders.
These shortcomings are more evident when higher rates of
die pressing are applied.
More effective and practical fugitive binder systems
have been provided in this art for imparting improved plas-
ticity to such particulate ceramic materials for their con-
solidation into coherent compacts with a minimum of rejects
over a wide range of production rates, including high speed
pressing with continuous rotary presses. Examples of such
improved fugitive binder systems comprise the amine-type
binder of U.S. Patents 4,427,579, issued January 24, 1984
to Gaines, Jr. et al and 4,389,341, issued June 21, 1983 to
Gaines, Jr. et al.
However, it has been found that the improved plastic
properties provided by such binder systems may not be lasting
in that they exhibit a tendency to diminish over prolonged
periods of time following their blending with nuclear fuel
material. Thus, it is not feasible to store or retain over
extended periods molding compositions comprising admixtures
of particulate ceramic materials and such amine binders.
This shortcoming imposes an impediment to production schedul-
ing and any shipping which entail prolonged periods.
Experience with the amine carbamate-type or amine
carbonate-type of fugitive binder admixed with uranium dioxide-
containing nuclear fuel material indicates that exposure tomoisture is a likely factur in diminishing the initially
effective plasticity provided by such binders. Moreover,
elevated temperatures have also been found to reduce the
plastic properties of these admixtures. Thus, over long
periods of time, molding combinations of particulate ceramic
materials containing uranium dioxide and such amines tend
to lose their stability by becoming more brittle and less
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amenable to rapid compression molding, with the result
of a high rate of rejects during compresion molding.
';I~MMARY OF THE INVENTION
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This invention comprises a method of producing
coherent compacts from particulate ceramic material wherein
the ceramic material is rendered and retained more plastic,
or less brittle, for compression molding over extended
periods of time. Thus the invention entails a process
comprising a combination of specified ingredients employed
together, including the essential combination providing
the carbonate or carbamate containing fugitive binder for
the compression molding step.
OBJECTS OF THE INVENTION
It is a primary object of this invention to provide
a method and fugitive binder compositions for rendering
particulate ceramic material more amenable to compression
molding over extended periods of time, and the product
thereof.
It is another object of this invention to provide
means for overcoming the brittle nature of ceramic
particles and to impart enduring plasticity to such a
materials whereby it can be compression molded under
essentially all compacting conditions and at high rates
with a lower and feasible level of rejects for extended
periods.
A further object of this invention is to extend
the time for compression molding of particulate ceramic
materials comprising uranium dioxide powder and uranyl
oxalate-carbonate composition fugitive binders into
coherent compacts, and to preserve the plastic properties
and molding amenability thereof.
A still further object of this invention is to
provide a method of producing nuclear fuel pellets
comprising uranium dioxide from particulate ceramic
material wherein a combination of the particulate ceramic
and binder, and its properties are stable and can be
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compression molded to a coherent compact at fast rates
with minimal rejects due to fractures over long periods
of time following the combining of said ingredients.
Another object of this invention is to provide a
compression moldable particulate material comprising
uranium dioxide which has lasting plasticity and is
amenable to compression molding over prolonged periods
of time.
DETAII.ED DESCRIPTION OF THE INVENTION
This invention comprises a method for producing
a fissionable nuclear fuel product in pellet form from
particulate ceramic material utilizing a fugitive binder
system which is subsequently removed during the sintering
operation. The method relates to compression molding of
particulate ceramic material containing uranium dioxide
powder combined with a unique fugitive binder composition
containing carbonate or carbamate of the invention to form
coherent compacts of apt dimensions, and thereafter sinter-
ing the compacts to produce integrated bodies of fissionable
nuclear fuel suitable for use in nuclear reactors.
The particulate fissionable nuclear fuel materials
for use in this invention comprise various materials used
as nuclear fuels for nuclear reactors, including ceramic
compounds such as oxides of uranium, plutonium and thorium.
Preferred fuel compounds consists of uranium oxide, plutonium
oxide, thorium oxide, and mixtures thereof.
The particulate nuclear fuel material in the
practice of the invention can also include various additives
such as high neutron absorbing materials comprising gadolinium
to moderate neutron flux densities.
Fugitive binder systems suitable for use in the
practice of this invention includes the amine-containing
carbonate or carbamate compounds disclosed and claimed in
the above-identified U.S. Patents 4,427,579 and 4,389,341.
These include amine compounds selected from the group
consisting of amine carbonates and amine carbamates such
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as carbonates or carbamates of ethylene diamine; mono-
methylene; 3,3 diaminodipropylamine; 1,3 diaminopropane;
1,6 diaminohexane; n butyl amine; diethylene triamine;
and 1,7 diaminoheptane.
In this invention the above amine-containing
carbonate or carbamate compounds and ammonium oxalate are
added to the particulate ceramic fuel material containing
uranium dioxide. The combined binder ingredients and pro-
duct thereof are then blended substantially uniformly
through the particulate material.
According to this invention, the improved fugitive
binder system providing lasting plasticity to the particu-
late ceramic nuclear fuel material comprising uranium
dioxide which endures adverse conditions over prolonged
periods, is prepared as follows.
Ammonium oxalate, which is of low solubility in
water, is added to uranium dioxide powder together with an
aqueous solution of an amine compound. ~ydrolysis of the
amine increases the pH of the solution which facilitates
dissolution of the ammonium oxalate with rapid evolution of
an ammonia gas at elevated temperatures. Thus, following
a typically brief period upon combining the ammonium oxalate
with the amine in aqueous solution of for example about 10
to 30 minutes or longer, and a temperature of at least about
65C, for said binder forming ingredients to interact, they
form a reaction product providing a fugitive binder. The
combined and interacted ingredients including the formed
binder composition are dried to remove excessive moisture
by any suitable means such as the application of heated
nitrogen gas moderately increased in temperature up to
about 150C.
This unique binder system which imparts a high
degree of plasticity, has been found to be stable and
highly resistant to reaction with water and in turn
deterioration. Moreover it has also been found to be
stable at relatively elevated temperatures of up to about
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~5C, over extended periods. At higher temperatures of
about 200C it aptly decomposes for its effective elimina-
tion in the subse~uent sintering step. Accordingly, the
unique binder system of this invention is not degraded
when blends thereof with particulate nuclear fuel material
are stored or otherwise retained for long periods of time,
even under adverse conditions of high humidity and high
temperature.
Proportions of such binder forming ingredients for
the compositions of this invention preferably comprise the
use of an amine carbonate or carbamate compound identified
above in amounts of from about 0.5% to about 7% by weight
based upon the weight of the nuclear fuel material. The
ammonium oxalate is included in amounts of from about 0.25~
to about 4~ by weight of the nuclear fuel material. Quanti-
ties of such binder ingredients in excess of the above upper
amounts generally do not provide a proportionally commensurate
benefit in bonding capacity, and may introduce unwanted
effects that compromise any advantages or the costs of includ-
ing greater amounts of these ingredients.
As noted hereinbefore, the amine carbonate or carba-
mate compound is dissolved in water to facilitate the inter-
action of the less soluble ammonium oxalate with the amine.
Amine water solutions of any practical concentration for
achieving the interaction and addition to the particulate
ceramic nuclear fuel can be employed. For instance excesses
of water which must be removed later in the operation are
not expedient. However, amine compound water solutions of,
for example, about 40% to about 50% solids are generally
appropriate. Excessive water is removed following the
reaction period. Suitable means for water removal comprise
nitrogen gas at a temperature of up to about 150C through
the particulate mixture of ingredients and their eaction
product for a typical period of about 20 to about 60 minutes,
or longer if appropriate.
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A binder system prepared of the foregoing
ammonium oxalate and amine compound in solution in
accordance with this invention is blended uniformly
with the particulate ceramic nuclear fuel material
containing ceramic dioxide, and the resultant mixture
can thereafter be compressed into a coherent compact
of suitable dimensions pursuant to the procedures and
means of the art. The method of this invention imparts
a high of plasticity and enables the prompt use, or
long delayed use, such as resulting from extended
storage or shipment, of the blend of fugitive binder
and particulate fuel material. Moreover, the enduring
plasticity attrib~table to the invention is adequate
for the effective use of the blends in high speed,
lS continuous production rotary press devices and operations
for long periods after preparing the blend to provide
unfired coherent compacts.
The "green" (unfired) coherent compacts thus
formed are then sintered in accordance with the practices
and procedures of the art to expel binder material and
integrate the ceramic particles into a uniform and
continuous body. The sintered product, typically in
the form of a pellet, is thereafter ground to specified
dimensions for its designated service.
81ending of the added binder can be effected with
any appropriate "dry" mixing apparatus including low
shear blenders such as fluidized bed, slab and ribbon
blenders, and high shear or intensive blenders such as
vibratory mills, ball mills and centrifugal mills.
A preferred blending apparatus comprises
vibratory mills of the type described in pages 8-29 to
8-30 of Perry and Chilton's 5th edition of Chemical
Engineering Handbook, McGraw-Hill Book Co
An example of a preferred procedure for the practice
of this invention, and of the fugitive binder product
~hereof, is as follows:
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A charge of enriched uranium dioxide powder,
granulated to a substantially uniform particle size, is
deposited in a vibratory mill (Sweco Inc. Vibro-Energy
mill) for blending with the binder forming ingredients.
Solid ammonium oxalate in powder form is added to the
uranium dioxide in an amount of about 0.8 percent based
on the weight of the uranium dioxide. A water solution
of about 48% by weight ethylenediamine carbamate is also
added to the uranium dioxide in an amount providing about
1.63~ by weight of water and about 1.5~ by weight of
solid ethylenediamine carbamate based on the weight of
the uranium dioxide. The ratio of ammonium oxalate to
the amine carbamate is about one part to 1.9 parts by
weight. The combined ingredients are blended in the
vibratory mill for a suitable period of about 25 minutes
at a temperature of at least 65C to provide for their
interaction to produce an effective fugitive binder
comprising uranyl oxalate-carbonate.
The combined and interacted ingredients are dried
and ammonia gas formed during the interaction is driven
from the combined mass of ingredients by applying a
heating gaseous medium at a temperature of not greater
than about 150C.
The product of the interaction of these ingredients
as set forth above has been identified as uranyl oxalate-
carbonate, having the formula:
[U2 (CO3) (C2o4) ~ 2H2] 2H2O
A chemical analysis of two binder preparations
according to the above procedure and ingredients is as
30 follows:
Batch } Batch 2
H2O , ppm 4958.0 5433.0
C2O4= (oxalate)0.416~ 0.411%
C2 (carbonate)0.270~ 0.288
U (u+6) 1.46% 1.42%
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Molar ratios were determined to be:
U::C2O4 ::CO3 H2O
Batch 1 Batch 2
1::0.77::1::3.4 1::0.78::1.1::3.9
SInfra-red spectra measurements confirm the above
formulation.
