Note: Descriptions are shown in the official language in which they were submitted.
~9~7~6g
BACKGROUND OF THE INVENTION
The present invention relates to an improved method
for preparing a sinterable uranium dio~ide powder for use
in nuclear fuel preparation and, more particularly, to
the preparation of a sinterable uranium dioxide powder by
microwave radiation in a microwave induction furnace.
Uranium dioxide is the fuel most commonly used in
present day nuclear power reactors. Generally, uranium
dioxide powder is pressed and ~intered to form pellets
which are loaded into and sealed in slender, hollow metal
tubes called fuel rods. It is a plurality of such fuel
rods that establishes an accumulation of fissionable
material in sufficient concentration to support sustained
fission reactions within the core of a nuclear power re-
actor.
A num~er of techniques have been developed for pre-
pariny a sinterable uranium dioxide powder, generally the
starting compound in a nuclear fuel pellet preparation
process, the most common of which involves the decomposi-
tion and reduction of ammonium diuranate or the ADU method.ADU is produced by precipitation from a solution of uranyl
fluoride by the addition of ammonia and the ADU formed in
this manner has a very fine particle size which carries
through to uranium dioxide powder after thermal drying,
decomposition and reduction in an electrical resistance
furnace, radiant heat transfer dryer, kiln or a co~bina-
tion thereof.
Another common method for the manufacture of uranium
dioxide powder is the ammonium urànyl carbonate or AUC
method. The AUC is produced by precipitation from a solu-
tion of uranyl fluoride by the simultaneous addition of
NH3 and CO2; the RUC precipitated thereby is separatéd
t'q' ~h
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from the mother li~uor by filtering and washing, and the
uranium dioxide powder is formed by thermal decomposition
of the AUC and subsequent reduction of the resulting U3O8
to UO2 in a reducing atmosphere. The thermal decomposition
of the AUC and the reduction of the oxide into uranium
dioxide powder in hydro~en or other reducin~ ~as is nor-
mally carried out in an electrical resistance furnace or
in two such units, such as the so-called vortex-bed
furnaces.
Still another method for the manufacture o uranium
dioxide powder is the ura~yl nitrate hexahydrate or UNH
method. The UNH method proceeds by starting with uranyl
nitrate hexahydrate,UO2(NO3)2.6H2O, then heating and de-
composing the compound in an electrical resistance furnace
to form UO3, oxides of nitro~en and water vapor. The UO3
is then heated in an electrical resistance furnace in a
hydrogen reducing atmosphere to form uranium dioxide pow-
der and water vapor.
The prior art processes for preparing uranium dioxide
powder have in common the use of standard electrical resis-
tance or combustion-fired heating furnaces during the
decomposition and reduction steps, that i5, the decomposi-
tion to UO3 or U3O8 followed by reduction to uranium di-
oxide powder. Alternatively, uranium bearin~ compounds
are processed primarily utilizing radiant heat transfer
dryers and kilns. It is the object of the present inven-
tion to replace the conventionallv used electrical resis-
tance furnace, radiant heat transfer dryer and kiln with
microwave induction furnaces.
Heretofore, microwave induction has been used as a
heating mechanism almost entirely via the susceptance of
the water molecule to microwave radiation, that is, the
use of microwaves for the heating of materials has been
centred on the effects that microwaves have on water
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molecules. Microwaves cause rapid changes in the polariza-
tion of the water molecule, thereby generating heat. The
invention herein described discloses that uranyl nitrate
hexahydrate, ammonium diuranate and ammonium uranyl carbo-
nate also suscept to microwave radiation, generating heat.Accordingly, the electrical resistance furnace, radiant
heat transfer dryer and kiln, above discussed, can be
replaced by microwave induction furnaces during the prepa-
ration of uranium dioxide powdervia the ADU, AUC and UNH
powder preparation processes.
The present invention overcomes many of the short-
comings of the prior art heating devices by decreasing
material heatup time, allowing a greater range of process-
ing temperatures, shortening processing times, lowering
fluoride impurity levels, improving the ease in handling
gelatinous ADU or AUC filter cakes, conserving energy by
generating heat entirely within the target material, find-
ing greater utility in remote locations required for nuclear
fuel processing and providing a ceramically active, sinter-
able uranium dioxide powder product.
SUMMARY OF THE INVENTION
The present invention relates to an improved methodfor preparing a sinterable uranium dioxide powder for use
in nuclear fuel preparation utilizing the concept of micro-
wave radiation in a microwave induction furnace. Typically,a starting compound is selected from the group consisting
of uranyl nitrate hexahydrate, ammonium diuranate and
ammonium uranyl carbonate. The selected starting compound
is then heated in a microwave induction furnace for a
period of time sufficient for compound decomposition. The
decomposed compound is then heated in a microwave induction
furnace in a reducing atmosphere for a period of time suf-
ficient to reduce the decomposed compound to uranium dioxide
powder, after which, the uranium dioxide powder is cooled
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in a reducing atmosphere. After cooling, the powder is
available for use in a nuclear fuel preparation process.
The various features of novelty which characterize
the invention are pointed out with particularity in the
claims annexed to and forming a part of this disclosure.
E`or a better understanding of the invention, i~s operating
advantages and specific results obtained by its use,
reference should be made to the descriptive mattex in
which there is illustrated and described a typical embodi-
ment of the invention.
DETAILED DESCRIPTION OF T~IE PP~EFERRED EMBODIMENT
A sinterable uranium dioxide powder to be used in anuclear fuel preparation process is produced by first
selecting a commercially available starting material from
the compound group of uranyl nitrate hexahydrate, ammonium
diuranate and ammonium uranyl carbonate. The selected start-
ing material or compound is then heated in a microwave induc-
tion furnace for a period of time sufficient to decompose
the material, the composition of which may have a uranium
oxide stoichiometric range of from UO3 to U3O8. The pre-
ferred decomposition end product is U3O8 and the decompo-
sition may be conducted in either an oxidizing, air, 2'
or the like, mixed air-stream, or an inert atmosphere.
The decomposition step is conducted at a heating tempera-
ture in the range of about 400 to 600C when uranyl nit-
rate hexahydrate is selected as the starting compound.
The decomposition heating temperature is conducted in the
ranye of about 350 to 450C when either ammonium diuranate
or ammonium uranyl carbonate is selected as the starting
compound. The decomposed compound is then heated in a
microwave induction furnace in a reducing atmosphere con-
sisting essentially of a hydrogen-nitrogen gas mix~ure, or
the like, for a period of time sufficient to reduce the
decomposed compound to uranium dioxide powder; the
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reduction step is conducted at a heating temperature in
the range of about 450 to 550C notwithstanding -the start-
ing material selected from the aforementioned compound
group. The uranium dioxide powder i5 then cooled in a
reducing atmosphere to approximately room temperature.
After cooling the powder is read~ for use in a muclear
fuel preparation process.
Uranyl nitrate hexahydrate, ammonium diuranate and
ammonium uranyl carbonate were each subjected to microwave
radiation in a microwave induction furnace at approximately
2450 MHz, the frequency of the standard kitchen-type micro-
~ave oven, to determine the susceptance of each compound
to microwave radiation. It should be understood thak,
while a conventional microwave oven was selected for use
because of its ready availability, other microwave induc
tion fuxnaces operating at different frequencies would
also be operable. Additionally, one oven or a plurality
of ovens could be used for the decomposition and reduction
processes. Each uranium compound readily suscepted, heat-
ing rapidly. Other materials, however, such as niobia,alumina, silica, and graphite, when exposed to microwave
radiation, while suscepting, did not exhibit the rapid
heating found to be characteristic of the above compounds
of uranium.
Uranyl nitrate hexahydrate crystals suscepted to
microwave radiation in a microwave induction furnace in
an oxidizing atmosphere by first forming a liquid as the
hydrated water molecules were released, then decomposing
in a 400 to 600C temperature range, progressively drying,
releasing nitrous oxide gas and water ~apor and forming
uranium trioxide (UO3). The UO3 was then heated to a
temperature in the 450 to 500C range in a microwave
induction furnace in a reducing atmosphere wherein water
vapor was released and the UO3 was reduced to uranium di-
oxide powder which was then cooled in the reducing
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atmosphere to about room temperature.
Ammonium diuranate, available as a filter cake,suscepted to microwave radiation in a microwave induction
furnace in an oxidizing atmosphere by first releasing
water and drying in the microwave field and then by de-
composing in a 350 to 450~C temperature range, releasing
ammonia gas and water vapor and forming U3O8. The U3O8
was then heated to a temperature in the 450 to 550C
range in a microwave induction furnace in a reducing
atmosphere wherein water vapor was released and the U3O8
was reduced to uranium dioxide powder which was then
cooled in the reducing atmosphere to about room teperature.
Ammonium uranyl carbonate, subjected to the condi-
tions imposed upon ammonium diuranate, decomposed in much
the same manner as did the ammonium diuranate, releasing
gases of ammonia and water vapor with the additional
release of carbon dioxide gas and forming U3O8. The reduc-
tion of U3O8 to uranium dioxide powder, followed by cooling,
proceeded as did the reduction and cooling of ammonium di-
uranate.
Uranyl nitrate hexahydrate, ammonium diuranate andammonium uranyl carbonate decomposition and reduc-tion in
a microwave induction furnace or furnaces is accomplished
in processing times on the order of minutes rather than
the hours customarily associated with the use of conven-
tional electrical resistance furnacesO Additionally, the
processing of a glossy or gelatinous filter cake does not
hinder the microwave decomposition-reduction processes,
the presence of such cakes lengthen process times in
conventional furnaces and effect finished product ~uality.
Uranyl nitrate he~ahydrate, ammonium diuranate and ammoni-
um uranyl carbonate processes in a microwave field
produce a finished product o~ sinterable uranium dioxide
powder suitable for use in a nuclear fuel preparation
process.
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While in accordance with the provisions of the
statutes there is herein illustrated and described a
specific embodiment of the invention, those skilled in
the art will understand that changes may be made in the
form of the invention covered by the claims, and that
certain features of the invention may sometimes be used
to advantage without corresponding use of the other
features.
