FR2611695A1 - Process for the manufacture of nuclear fuel - Google Patents

Abstract

Process for the manufacture of nuclear fuel from mixed oxides or of advanced type such as carbide, nitride or carbonitride of uranium and/or plutonium. Microspheres of at least one oxide of fissile or fertile metal are calcined in an oven 10 at a temperature between 400 and 800 DEG C. These microspheres are then conveyed to an impregnation column 36 where they are impregnated with an aqueous solution originating from a storage vessel 38 and containing a component of the final product to be obtained in order that this component may be absorbed into the microspheres. The latter are then conveyed into an oven 66 where they are subjected to a heat treatment at a temperature of between 500 and 1000 DEG C. The granules obtained can then be converted by pressing and sintering into nuclear fuel pellets. Application to the production of nuclear fuel for fast neutron reactors.

Description

PROCESS FOR THE MANUFACTURE OF NUCLEAR FUEL
DESCRIPTION
The present invention relates to a method for the
manufacture of usable nuclear fuel in particular
for the production of mixed oxide nuclear fuel (U,
Pu) O or also nuclear fuel called "advanced type", 2
that is to say carbides, nitrides or carbonitrides
uranium and plutonium.

The technique currently used to manufacture the
nuclear fuel (U, Pu) O is to prepare first
2
separately from UO uranium oxide and plutonium oxide
2
PuO in powder form and to mix the two oxides in
2
suitable proportions. The mixture is then transformed into
pellets and the latter are sintered in an oven at a temperature of the order of 1500 to 17000C. For the manufacture of
nuclear fuel of the carbide, nitride or carbonitride type of uranium and plutonium, powders are first produced
carbide, nitride or carbonitride of fissile metals as
it is mixed with powders of uranium and / or plutonium oxides, for example UO or (U, Pu) O or with powders of
2 2 same chemical composition.

 However, these methods have certain drawbacks. First of all, the handling of very fine powders containing radionuclides emitting gamma photons and / or neutrons requires draconian precautions to avoid any risk of diffusion of contamination by very fine powder. On the other hand, the mechanical mixing of the powders does not always make it possible to obtain perfectly homogeneous mixtures and can therefore result in an imperfect distribution of the plutonium in the oxide matrix. This results in an incomplete solubility of the fuel in nitric acid during its reprocessing. Finally, the manufacture of nuclear fuel of the carbide, nitride or carbonitride type by the process of the mechanical mixing of powders presents difficulties in the central content oxygen of the final product and handling of these powders which are pyrophoric.

 The object of the present invention is to remedy these drawbacks by means of a process for the manufacture of nuclear fuel which makes it possible to obtain pellets for fuel elements having very good homogeneity and which allows precise adjustment of the composition, in particular as regards concerns the relationship between the quantity of fertile elements and fissile elements.

According to the main characteristic of the process which is the subject of the invention, it comprises the following stages consisting in: (a) - calcining, at a temperature between 400 and 800 OC,
and preferably between 500 and 6000C,
microspheres of at least one oxide of a fissile metal or
fertile, (b) - impregnate these microspheres with an aqueous solution
containing at least one of the following: uranium,
plutonium, carbon and nitrogen, and (c) - subject the microspheres thus impregnated to a treatment
thermal at a temperature between 500 and 10000C and
preferably between 800 and 9000C.

 Preferably, the microspheres are obtained, prior to step (a), by the sol-gel process.

 The microspheres are generally microspheres of uranium oxide and / or plutonium, possibly of thorium.

As for the calcination of step (a), it is carried out, depending on the case, in air or in a reducing atmosphere.

 Preferably, the microspheres are maintained in a state of fluidization during step (b).

 As for step (c), it is carried out, depending on the case, under vacuum or under a reducing atmosphere. Optionally, this atmosphere may contain a constituent element of the final product to be obtained, for example nitrogen in the case of nitrides.

The invention will appear better on reading the description which follows, given by way of purely illustrative and in no way limitative example, with reference to the appended drawings in which:
FIG. 1 is a block diagram showing the main elements of a device which can be used for implementing the method which is the subject of the invention, and
- Figure 2 is a schematic view in more detailed vertical section of certain elements of this device.

Referring to the drawings, it can be seen that this device firstly comprises an oven 10 inside which the starting materials undergo calcination. This oven first comprises an enclosure 12 surrounded by a heating resistor 14,
the whole being protected by a mass of insulator 16. The microspheres, which, preferably, were previously prepared by the sol-gel process, are stored in a tank 18, the lower part of which is connected to one end of the tank 12 by a pipe 22 fitted with a valve 24. In this example, the reservoir 18 comprises a vertical cylindrical part 19 extended at its lower part by a conical part 21 whose diameter decreases as one moves away from part 19. If necessary, so that the reservoir 18 has a subcritical geometry, one can provide inside of it a core or an ogive 20 having a shape similar to that of the reservoir 18 so that your powder is stored only in
The space between this core 20 and the internal walls of the tank. The microspheres fall from it into the enclosure 12 through the pipe 22 and fall on a conveyor 26 which
Crate them at the other end of the oven. The running speed of the conveyor is determined as a function of the time necessary to calcine the microspheres. If the calcination is to take place under a controlled atmosphere, the enclosure 12 is swept by a suitable gas current flowing against the microspheres. The gas inlet and outlet pipes are symbolically represented in the drawing by the arrows in phantom 23 and 25 respectively.

At its end opposite to that where the microspheres are introduced, the oven is connected by a pipe 27 fitted with a valve 28 to a tank 30 where the microspheres are stored before the impregnation step. The reservoir 30 is connected, by a
line 32 fitted with a valve 34, to an impregnation column 36 with a fluidized bed in which the operation for impregnating the microspheres takes place. The aqueous solution used for this is stored in a tank 38 connected to the column 36 by a pipe 40 fitted with a valve 42. In order to eliminate the water and acid vapors which are produced in the impregnation column 36, a condenser 44 is provided, connected, by a turbine 46, to the upper part of the column 36. A reservoir 48, placed below the condenser 44, is connected to the latter by a pipe 50 which makes it possible to recover the condensates.

 The column 36 has a shape similar to that of the reservoir 18 with an upper cylindrical part extended by a lower part of substantially conical shape. A pipe 52 fitted with a valve 54 connects the lower part of the column 36 to the upper part of a storage tank 56.

The latter is used to store the microspheres which have been impregnated in column 36 before subjecting them to the heat treatment which constitutes step (c) of the process which is the subject of the invention. A pipe 58 serves for the introduction of the gas used to keep the microspheres in a fluidization state when they are inside the column 36. This pipe 58, which opens into the pipe 52 at the level of the valve 54, is equipped with a heat exchanger 60 in order to preheat the gas to a temperature of the order of 1500C. A heating device arranged around the column 36 makes it possible to maintain in the latter
The temperature at 1500C.

The lower part of the tank 56 is connected by a pipe 62 fitted with a valve 64 to an oven 66 in which the heat treatment which constitutes step (c) of the process which is the subject of the invention is carried out. This oven 66 can consist of
the same way as the oven 10 with an enclosure 68 for the passage of the microspheres, this tank being surrounded by a heating resistor 70 and protected by a mass of thermal insulator 72.

The oven 66 can also be swept by a gas, the inlet and outlet pipes being symbolized by the arrows in phantom 71 and 73 respectively.

Figure 2 shows in more detail the constitution of certain elements of the device object of
the invention. It can be seen in this figure that the elements intended to contain the microspheres, that is to say the storage tanks 30 and 56 as well as the impregnation column 36, can, if necessary, be surrounded by a neutron protection or biological 33, the latter being shown diagrammatically in dashed lines in FIG. 2. If necessary, the reservoir 38 intended to contain the impregnation solution for the microspheres can also be protected in the same way.

 Still for security reasons, it is preferable that the column 36 and the reservoir 56 be of subcritical geometry, that is to say constituted so as to distribute the mass of the microspheres over the largest possible space. For this, it is possible, for example, to provide a core 37 inside the column 36 and at the lower part thereof, the part 37 possibly having, for example, the appearance of a double cone: the microspheres are located between this double cone and the interior walls of the column 36. As for the reservoir 56, it may be a double-walled reservoir, that is to say an exterior wall 57 and an interior wall 59 defining between them a thin space 61. The microspheres are stored in this space 61, which makes it possible to have a mass of thin microspheres.

It can also be seen in FIG. 2 that the pipe 40 connecting the reservoir 38 to the column 36 opens out at the upper part thereof by a nozzle 43 which makes it possible to project the liquid in the state of droplets onto the microspheres. As for the condenser 44, it can be in the form of a column fitted with trays 45 and provided with a refrigerator 47. Finally,
the heat exchanger 60 can be in the form of an enclosure 63 inside which the gas circulates along a
coil 65, the enclosure 63 being heated by means of
resistances 67.

 We will now describe the progress of a nuclear fuel manufacturing operation using this device.

 The microspheres, which have been previously prepared, preferably by the sol-gel process, are introduced and stored in the reservoir 18.

At the start of the operation, the oven 10 is started up until it reaches the desired temperature for
the calcination operation, the conveyor 26 is started and the valve 24 is opened so that the microspheres fall thereon.

If necessary, the enclosure 12 is swept by an appropriate gas.

During this operation, the valve 28 is open and the valve 34 closed so that the microspheres are discharged into the reservoir 30.

 When the desired quantity of microspheres has been calcined in the oven 10, we proceed to the next operation which is the impregnation of the microspheres with the solution 39 which is contained in the tank 38. For this, the valve 34 is opened so that the microspheres contained in the reservoir 30 fall into the column 36. The valve 42, which was previously closed, is then opened, so that the solution 39 enters the column 36 through the nozzle 43.

 During the impregnation operation, the valve 54 is closed. It should be noted however that, the mass of microspheres contained in the column 36 having to be maintained in a state of fluidization by means of the gas introduced by the line 58, the valve 54 must be designed so as to be able to prevent the passage of the microspheres from column 36 to tank 56 while allowing gas to pass from bottom to top along line 52.

The valve 54 is then opened so that the microspheres
impregnated material falls into tank 56. If they must be
stored before performing Step (c), the valve 64 is closed. At the appropriate time, the latter is opened so that the microspheres fall into the oven 66 where the heat treatment which constitutes step (c) of the process which is the subject of the invention is carried out.

We will now describe some examples of nuclear fuel preparation using the device
illustrated in Figures 1 and 2.

EXAMPLE 1
In this example, nuclear fuel pellets of mixed oxide of (U, Pu) O2 were prepared. For this, we started with a charge of 2000 g of microspheres of oxide U308 obtained by the sol-gel process. These microspheres were first calcined in air at 6000C in the oven 10 for 30 minutes, then impregnated in column 36 with 2 liters of a 1M nitric solution containing 50.88 g / l of plutonium and 50.88 g / l of uranium. After impregnation, the microspheres were subjected in the oven 66 to a heat treatment lasting 30 minutes at 8500C in a reducing atmosphere which contained 95% of argon and 5% of hydrogen. Granules of (U, Pu) O were thus obtained which could be transformed by pressing and sintering into
2 cylindrical pellets with a density of 10.5 g / cm. In this example,
The pellets finally obtained had a diameter of 10 mm and a height of 15 mm.

EXAMPLE 2
In this example, we sought to adjust the plutonium concentration of a charge of mixed oxide microspheres (U, Pu) O
2
A charge of 20429 microspheres of (U, Pu) O having the composition was prepared by the sol-gel method.
2 stoichiometric (U Pu) O. These microspheres have been
0.75 0.25 2 calcined at 7000C for 30 minutes under a reducing atmosphere containing 95Z of argon and 5X of hydrogen. This solution was then impregnated with 7 liters of a 0.42M UO2 (NO3) solution. The microspheres thus impregnated were then
subjected to a heat treatment lasting 30 minutes at 9000C under a reducing atmosphere containing 95Z of argon and 5X of hydrogen. We finally obtained granules whose
stoichiometric composition was (ut 82 Pu 18) 2. These 0.18 0, 2 granules were then pressed and sintered to obtain cylindrical pellets with a diameter of 10 mm and a height of 15 mm.

 The process which is the subject of the invention can also be used for the manufacture of so-called "advanced type" nuclear fuel, that is to say carbides, nitrides or carbonitrides of uranium and plutonium. For example, carbide granules can be obtained by introducing a certain amount of carbon into the impregnation solution 39, then by subjecting the impregnated microspheres to a carbothermic reduction at high temperature under vacuum or in an inert atmosphere. As for the nitride or carbonitride granules, they can be obtained by subjecting the microspheres to thermal reduction under a nitrogen atmosphere.

EXAMPLE 3
In this example, pellets of advanced type nuclear fuel, i.e. uranium carbonitride and plutonium (U, Pu) CN, were prepared.
0.8 0.2 0.2 0.8
Microspheres of (U Pu) O with an average diameter of 0.1 mm were prepared by the sol-gel method. These microspheres have
0.8 0.2 2.2 was impregnated with 10 liters of an aqueous CHO solution
12 22 It having a concentration of 82.39 g / l. After complete drying, the impregnated microspheres, which constituted a mixture of (U
Pu) O and CHO, were transferred to a crucible
0.22 2.2 12 22 11 of tungsten and heated under a nitrogen atmosphere to 14500C. The temperature rise was carried out at the speed of 1500C per hour with a stop of 2 hours at 7000C and a maintenance of 6 hours at the maximum temperature of 14500C. The product thus obtained was put into the form of pellets by cold pressing.

These pellets were then in turn transformed into high density fuel pellets by heating to a
temperature between 1700 and 19000C. We thus obtained
23009 of fuel based on uranium carbonitride and
plutonium, The product obtained having the chemical formula
approximate (UO 8 PU0 # 2) CN
0.2 0.2 0.2 0.8
Thus, the process which is the subject of the invention presents
particularly interesting advantages since it allows
obtain different types of nuclear fuel (oxides
mixed or advanced fuel type) with good
homogeneity and that the process makes it possible to adjust the composition
of SOL-GEL initial products. In addition, all operations taking place inside containers connected
to each other by pipes fitted with valves, this decreases
Manipulations and therefore the risk of spreading contamination.

 Finally, it is understood that the invention is not limited to the sole embodiments which have just been described, but that variants can be envisaged without thereby departing from the scope of the invention. Thus, in the device of Figures 1 or 2, we can replace an element by an equivalent element to perform the same operations. One can, for example, replace ovens 10 and 66 with ovens of another type or replace them with a simple heated crucible if the quantity of powder handled allows it. Likewise, the cores 20 of the reservoir 18 and 37 of the column 36 can be eliminated if the quantities of powder handled make it possible to constantly maintain a subcritical geometry. It is also possible, without departing from the scope of the invention, to replace them with parts of different shape. As for the reservoir 56, if a particular embodiment has been described in which the subcritical geometry was obtained by the use of a double envelope, it would not go beyond the scope of the invention using another structure provided that the geometry remains subcritical.

Claims (10)

 1. Method for manufacturing nuclear fuel of mixed oxides or of advanced type, characterized in that it  includes the following steps of  (a) - calcining, at a temperature between 400 and 800 CC,  and preferably between 500 and 6000C,  microspheres of at least one oxide of a fissile metal or  fertile, (b) - impregnate these microspheres with an aqueous solution  containing at least one of the following: uranium,  plutonium, carbon and nitrogen, and  (c) - subjecting the microspheres thus impregnated to a treatment  thermal at a temperature between 500 and 10000C and  preferably between 800 and 9000C.  2. Method according to claim 1, characterized in that the microspheres are prepared, prior to step (a), by the sol-gel process.  3. Method according to claim 1, characterized in that step (a) is carried out in air.  4. Method according to claim 1, characterized in that step (a) is carried out under a reducing atmosphere.  5. Method according to claim 1, characterized in that said fissile or fertile metal is chosen from: uranium, plutonium, and thorium.  6. Method according to claim 1, characterized in that during step (b), the microspheres are maintained in the fluidization state.  7. Method according to claim 1, characterized in that step (c) is carried out under vacuum.  8. Method according to claim 1, characterized in that Step (c) is carried out under a reducing atmosphere.  9. Method according to claim 1, characterized in that step (c) is carried out in an atmosphere containing a component of the product to be obtained.  10. Method according to claim 9, characterized in that said constituent element of the product to be obtained is nitrogen.