METHOD FOR PREPARING HIGHLY RADIOACTIVE MATERIALS FOR TRANSMUTATION AND/OR COMBUSTION

PROCEDE POUR PREPARER DES MATIERES HAUTEMENT RADIOACTIVES EN VUE D'UNE TRANSMUTATION ET/OU D'UNE COMBUSTION

English Abstract

The invention concerns a method of preparing highly radioactive materials for
transmutation and/or burn-up by irradiation in a nuclear plant. The invention
proposes that the materials are first converted into liquid form by melting or
chemical dissolution and a porous carrier material which is essentially
insoluble in the liquefied materials is impregnated with the liquefied
materials and then heated in such a way that the materials are converted into
the finally required chemical form and density.

French Abstract

Procédé pour préparer des matières hautement radioactives en vue d'une transmutation et/ou d'une combustion par irradiation dans une centrale nucléaire. Selon l'invention, les matières sont liquéfiées par fusion ou par dissolution chimique, et un matériau support poreux pratiquement insoluble dans les matières liquides, est imprégné avec les matières liquéfiées puis soumis à un traitement thermique de sorte que les matières soient converties sous la forme chimique et avec la densité finalement souhaitées.

Claims

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CLAIMS
1. A method for preparing highly radioactive
materials for transmutation and/or combustion by irradiation
in a nuclear plant, characterized in that the materials are
liquefied by smelting or chemical dissolution, in that a
porous support material which is practically insoluble by
the liquified material is impregnated with the liquefied
,material and thereafter submitted to a heat treatment which
converts the radioactive materials into the chemical form
which is finally intended.
2. A method according to claim 1, characterized in
that the support material consists of powder or particles
and that it is brought into the desired shape after impregnation
and heat treatment by means of a mechanical pressure
or vibration.
3. A method according to claim 1, characterized in
that the support material is initially in pellet form.
4. A method according to claim 3, characterized in
that the pellets are additionally degassed prior to and
during the impregnation phase.
5. A method according to claim 3, characterized in
that the impregnation is limited in time in such a way that
the impregnation liquid does not arrive at the core of the
pellets.
6. A method according to claim 3, characterized in
that the pellets are immersed only partly into the impregnation
liquid.
7. A method according to claim 3, characterized in

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that the pellets are coated on a portion of their surface
prior to impregnation with a layer which is impermeable to
the impregnation liquid.
8. A method according to one of claims 1 to 7,
characterized in that a final heat treatment is performed
(sintering).

Description

' CA 02249347 1998-09-16
2498 US
METHOD FOR PREPARING HIGHLY RADIOACTIVE MATERIALS
- FOR TRANSMUTATION AND/OR COMBUSTION
The invention refers to a method of preparing highly
radioactive materials for transmutation and/or combustion by
irradiation in a nuclear plant.
The transmutation and combustion of radioactive
materials is applied especially in the frame of the disposal
of nuclear materials. As a result of the transmutation and
combustion radioactive materials are obtained having a
drastically shortened half-lifetime thereby loosing their
radiological danger within tolerable time periods , or
materials having a lower radiologically dangerous radiation
intensity.
It would now be possible by applying processes analog
to those for the production of nuclear fuel, to transform
the materials to be submitted to transmutation or combustion
into powders, to mix them with inactive powders, to compress
them into pills (pellets, targets) and to enclose them in
tubular containers which are then put into a nuclear plant
in view of transmutation and combustion. However, this
method entails the creation of highly radiotoxic dust which
deposits on the walls of hot cells or glove boxes and con-
stitutes an additional radiation charge for the persons
working in this area. Moreover there exists a risk that the
powders are incompletely mixed and that during the success-
ive irradiation in the nuclear plant hot points appear and
may oblige to stop prematurely the transmutation and combus-
tion.
Object of the invention is therefore a method which
does not produce at all or only insignificantly toxic dust
and which insures a very uniform distribution of the highly
radioactive materials such that hot points are avoided

' CA 02249347 1998-09-16
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during irradiation.
This ob;ect is achieved according to the invention by
a method as defined in claim 1. It is essentially character-
ized in that a convenient porous support material is impreg-
nated by the highly radioactive materials which have been
converted into liquid form. In the case of a powdery or
particle like support material the latter is preferably
pressed into pellets. If on the contrary the support
material is already in a pellet shape prior to impregnation
it is preferable degas it prior to impregnation in order to
facilitate a uniform distribution of the radioactive
material in the support material.
In some applications it is important to improve the
mechanical stability of the pellets in order to insure their
integrity during the entire impregnation phase. This can be
achieved by keeping the central core of the pellets free of
highly radioactive materials, ensured by a convenient limi-
tation of the impregnation time. By this means the mechan-
ical stability of the pellets is conserved.
Furthermore it can be useful to exclude a portion of
the pellet surface from impregnation in order to avoid the
creation of thermal fission energy in the core of the pel-
lets during irradiation, thus improving the mechanical
stability. To this end either the pellets are immersed only
partially into the impregnation liquid or the pellets are
partly coated with an layer impermeable for the liquid prior
to impregnation.
The invention will now be described in more detail by
means of some preferred embodiments.
The porous support material may be used in powder,
granulate particles or microspheres form of for example
uranium oxide, plutonium oxide, thorium oxide, yttrium
oxide, cerium oxide or mixtures therefrom, for example
spinel and YAG (yttrium-aluminum-garnet). However, this list
is not at all exhaustive and depends upon the type of

CA 02249347 1998-09-16
material which should be submitted to transmutation or
combustion.
Even carbides, and nitrides of the mentioned elements
or other elements can be used. The materials to be submitted
to transmutation and combustion may be different isotopes of
plutonium, americium, neptunium, curium and other actinides
as well as fission products, for example technetium. These
materials are transformed into liquids either by high tem-
perature fusion or by chemical dissolution in an appropriate
solvent.
The support material must be sufficiently porous in
order to allow impregnation by the liquid material. Moreover
the support material should not be dissolved to a substan-
tial extent by the liquid material during impregnation. The
mechanical constitution of the support material depends on
the required degree of impregnation. One can employ powders
and granulates produced by precipitation or transformation
processes, microspheres obt~ine~ in a so-called droplets-to-
particle conversion method (SOL-GEL) or pellets obtained
from powders, granulates or microspheres.
If the support material is during impregnation in
power, granulate or microspheres form, then this material is
given the desired shape after impregnation by applying
mechanical pressure or vibration.
According one possible realisation pellets are
pressed and burned from a powdery support material. Such
pellets present a porosity of about 40% and can be impreg-
nated without difficulties. In the simplest case they are
immersed into a smelt or a solution of a highly radioactive
material. To this end the pellets are placed in a defined
position on a grip whereupon the latter is slowly lowered
into the liquid. The impregnation rate depends on the dimen-
sions of the pores, the viscosity and surface tension of the
liquid, the moistening rate of the pellet material and the
impregnation material.

CA 02249347 1998-09-16
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If a very uniform distribution of the liquid in the
pellet is required then the presence of gas bullets
imprisoned in the pellet can be detrimental since non-im-
pregnated zones will remain. This problem can solved by
submitting the impregnation tank to a vacuum prior to the
admission of the impregnation liquid whereby the gas bullets
are eliminated.
In some applications it is desired to only affect the
outer layer of the pellets by the impregnation. This can be
achieved by an appropriate choice of the impregnation time
thereby ensuring that the impregnation liquid only pe-
netrates to a predetermined depth into the pellets. It is
further possible to deposit on a portion of the pellet
surface a protective coating which is impermeable for the
liquid in order to avoid penetration through this surface
portion and to maintain without impregnation a core region
of the pellet which is not attainable by the liquid. The
advantage of this is that the fission energy created during
irradiation of the highly radioactive material in a nuclear
plant causes the production of heat mainly at the pellet
surface. This heat can be easily evacuated while the solid
core of the pellet prevents the latter from collapsing.
After the impregnation the highly radioactive
material which has been introduced into the pellet in liquid
form can be converted by a heat treatment into a desired
chemical form (e.g. oxide, nitride or carbide) and sintered.
In another embodiment the support material can
consist of microspheres, as mentioned, obtained by the SOL-
GEL method. Due to the high porosity (about 80~) such micro-
spheres are particularly adapted to a high rate impreg-
nation. Thus a cylindrical column can be filled with micro-
spheres and a nitrate solution of the highly radioactive
material is added thereto. The dimensions of the column are
chosen in such a way that critical conditions cannot occur.
After the impregnation the rem~;~;ng nitrate solution of the

CA 02249347 1998-09-16
radioactive material is pumped back to a supply tank. The
part of this material remaining between the spheres in then
converted into an oxide, nitride or a carbide. This conver-
sion is realised under an appropriate atmosphere in a con-
ventional furnace or by means of microwaves. Thereafter themicrospheres are compacted into the shape required for the
irradiation in the nuclear plant.
The impregnation of microspheres and their compacta-
tion later-on into pellets can be performed in a completely
automatic manner such that the personnel is not submitted to
an additional radiation charge. In addition this method does
not deal with dust of highly toxic materials.
Instead of the impregnation of microspheres in a
column the microspheres can equally be put into a basked and
immersed into the liquid, or the microspheres can be moved
or stirred while the liquid material is sprayed thereon.
The essential advantages of the method according to
the invention are as follows:
1 - The process steps during which highly radioactive
materials are processed, are limited to the liquification
and impregnation steps.
2 - The method is perfectly adapted to a remotely con-
trolled and largely automatic process.
3 - Toxic dust or radioactive waste are nearly not exist-
ent.
4 - The radiation charge of the personnel is substan-
tially reduced.
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