Compositions – Radioactive compositions – Nuclear reactor fuel
Reexamination Certificate
2001-05-25
2003-12-02
Bos, Steven (Department: 1754)
Compositions
Radioactive compositions
Nuclear reactor fuel
C423S261000, C376S901000
Reexamination Certificate
active
06656391
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a process for preparing a powder of uranium dioxide with physico-chemical properties suitable for the preparation of a uranium and plutonium mixed oxide (MOX) fuel used in light water reactors.
More precisely, it relates to obtaining a powder which is castable, fine and with restricted particle size, intended to be mixed with another powder (a primary mixture rich in Pu) and which has the following properties:
good spontaneous flow properties,
a homogeneous particle size located in the range between 20 and 100 &mgr;m,
a high apparent density to allow optimum filling of different equipment used for manufacturing fuel (grinder, mixer, containers, transfers, pressure foot feed, etc.),
elementary particles sufficiently solid to withstand the different mechanical stresses undergone during manufacture (convection mixing, filling and emptying containers, transfer by pneumatic track, etc.),
good compaction capability, and
excellent reactivity to natural sintering.
STATE OF PRIOR ART
At present, a powder of uranium dioxide with these properties is obtained by preparing it by a wet conversion process of uranyl nitrate in uranium dioxide.
This wet conversion process consists of a precipitation of ammonium diuranate (ADU) followed or not by grinding to the appropriate particle size, as described in U.S. Pat. No. 3,394,997 [1], and U.S. Pat. No. 3,998,925 [2]. Document FR-A-2 088 170 [3] describes a conversion process of uranyl nitrate in sinterable uranium dioxide by drying-pulverisation of a solution of uranyl nitrate and formic acid, followed by calcination.
The powders obtained by these wet conversion processes can be used directly for the industrial manufacturing of MOX fuel without it being necessary to carry out a supplementary stage of mechanical granulation of the powder.
In fact, the powders have excellent flow properties, which give the final UO
2
—PuO
2
mixture sufficient fluidity for withstanding the high output rates.
On the other hand, the preparation process of uranium dioxide using dry conversion of uranium hexafluoride UF
6
in UO
2
, such as described in FR-A-2 060 242 [4] and FR-A-2 064 351 [5], does not make it possible at present to obtain a powder able to be used directly for manufacturing MOX fuels. Dry conversion of UF
6
is carried out industrially in a compact oven and plays on two reactions consecutively:
the first is the hydrolysis reaction which transforms gaseous UF
6
into solid uranium oxyfluoride UO
2
F
2
,
the second relates to the reducing pyrohydrolysis of UO
2
F
2
, which leads to the formation of UO
2
under pulverulent form.
The UO
2
powder produced according to the process described in [4] and [5] is difficult to use for preparing MOX fuels since it is usually cohesive and has a low apparent density, which makes direct utilisation very tricky for industrial application because of its very poor compaction behaviour.
When this process according to references [4] and [5] is used to prepare fuels with a base of UO
2
for reactors, granulation of the powder is next carried out by a mechanical process comprising a pre-compaction of the powder, followed by a crushing, then a sieving in order to obtain a mechanical granulate with good flow properties, which in certain circumstances is submitted to a spheroidizing operation to increase the castability of the powder further. This mechanical granulation leads to high particle sizes which can reach and even exceed 500 &mgr;m.
Another powder granulation technique for UO
2
has been described in FR-A-1 438 020 [6]. In this case, a paste is prepared by mixing a powder of UO
2
or UO
2
—PuO
2
, with a solution of a binder in an organic solvent with a very low hydrogen content, such as trichloroethylene, then drying this paste by pulverisation. Thus granulates of high dimension reaching 250 &mgr;m are obtained.
For manufacturing MOX fuel, where an intimate mixture of the two constituents (UO
2
powder and PuO
2
powder) is needed, such high particle sizes cannot be allowed for UO
2
. It is indispensable to have UO
2
particle sizes lower than 100 &mgr;m in order to obtain the specifications required for this fuel.
The aim of the present invention is a treatment process for a uranium dioxide powder obtained by dry method, to convert it into a powder suitable for direct use in the manufacture of MOX fuel.
DESCRIPTION OF THE INVENTION
According to the invention, the process for preparing a powder of sinterable uranium dioxide UO
2
comprises the following stages:
1) to prepare an aqueous suspension of a powder of UO
2
obtained by dry process from uranium hexafluoride, said suspension comprising 50 to 80% by weight of UO
2
and at least one additive chosen among deflocculation agents, organic binders, hydrogen peroxide H
2
O
2
and a powder of U
3
O
8
, in quantity such that the viscosity of the suspension does not exceed 250 mPa.sec, and
2) to atomise this suspension and dry it in a hot gas, at a temperature between 150° and 300° C., to obtain a powder of depleted UO
2
with an average particle size of 20 to 100 &mgr;m.
First of all, in this process, an aqueous suspension of uranium dioxide is prepared, comprising a very high dry matter content, but with a viscosity as low as possible, not exceeding 250 mPa.sec, so that it is suitable for the following operation of atomisation-drying of the suspension into calibrated granulates.
The fact that water is used to prepare the suspension is very interesting since it makes it possible to limit the quantities of organic products likely to introduce impurities into the final product to very low levels.
According to the invention, the method for preparing the suspension can be simple, quick, reproducible, and can lead to very fluid suspensions able to be carried by pumping to the injection nozzle of the atomiser without difficulty. Very high levels of dry material can be reached in order to obtain dense, full and completely spherical particles of powder. Furthermore, this process can be transposed to a production unit of industrial capacity.
In order to prepare this suspension, at least two additives are generally used, constituted respectively of:
1) at least one deflocculation agent, and
2) at least one additive chosen among organic binders, hydrogen peroxide and/or a powder of U
3
O
8
, these latter additives all playing the role of binder during the operation of atomisation-drying.
In the document EP-A-0 092 475 [7], the use of hydrogen peroxide is described for improving the resistance to compression of raw pellets of UO
2
or UO
2
UO
2
—by forming a layer of hydrated oxide on the initial powder. This layer is formed by pulverising a solution of H
2
O
2
on the metal oxide powder, the quantity of H
2
O
2
solution representing 2 to 15% of the weight of this powder. In this case, the quantity of H
2
O
2
used is significant.
The deflocculation agent is intended to fluidify the suspension. It can be constituted by an organic product which can be easily eliminated, for example ammonium polymethacrylate such as the product marketed by the company Polyplastic S.A. under the name DARVAN C which is a 25% aqueous solution of ammonium polymethacrylate.
The quantity by weight of the deflocculation agent used (ammonium polymethacrylate) generally represents 0.03 to 0.16% by weight of dry matter in the suspension.
The organic binders are added to the suspension to encourage agglomeration of the powder during drying in an atomiser. Organic binders which can be eliminated easily are chosen. As examples of such binders, polyvinyl alcohol and polyethylene glycol can be mentioned.
Oxygenated water can play the same role as an organic binder, as can a powder of U
3
O
8
, but in the two cases, it can be advantageous to add a small proportion of an organic binder as well.
Thus, according to a first embodiment of the invention, the solution comprises a deflocculation agent and an organic binder, the quantity by weight of the organic
Bauer Mireille
Bonnerot Jean-Marc
Brunaud Laurent
Bos Steven
Commissariat a l'Energie Atomique
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