WO2011080475A1

WO2011080475A1 - Cermet with improved heat conductivity and nuclear fuel comprising said cermet

Info

Publication number: WO2011080475A1
Application number: PCT/FR2010/052910
Authority: WO
Inventors: Pierre Matheron; Méryl BROTHIER; Jacques Lechelle; Tangui Derriennic
Original assignee: Commissariat A L'energie Atomique Et Aux Energies Alternatives
Priority date: 2009-12-31
Filing date: 2010-12-23
Publication date: 2011-07-07
Also published as: FR2954850B1; FR2954850A1

Abstract

The invention relates to a cermet having a specific microstructure comprising a steel matrix in which uranium dioxide (UO₂) particles are dispersed. Said cermet has a mechanical strength under irradiation and/or an improved heat conductivity. The invention also relates to a nuclear fuel comprising said cermet.

Description

IMPROVED THERMAL CONDUCTIVITY CERMET AND NUCLEAR COMBUSTIBLE COMPRISING THE CERMET.

DESCRIPTION

TECHNICAL AREA

The technical field of the present invention is that of nuclear fuels.

The present invention relates more particularly to a cermet comprising a steel matrix in which particles of uranium dioxide (UO ₂ ) are dispersed. For the purposes of the invention, a "cermet" designates a composite material comprising a metallic phase and a ceramic phase.

The invention also relates to the nuclear fuel comprising this cermet, intended in particular for supplying a nuclear reactor type Pressurized Water Reactor (PWR).

STATE OF THE ART

One of the ways to improve the operational safety of a Pressurized Water Reactor (PWR) is the use of a nuclear fuel that best evacuates the heat energy produced in the reactor core.

For this purpose, it has been proposed to use a cermet-type composite material in which particles of U0 ₂ , constituting the ceramic phase, are separated from each other by a metallic phase whose good property of thermal conductivity, inherent to metals, is put to use. However, the metal phase should not be too important a part of the cermet, because it also has the property of being neutron, which limits the yield of nuclear combustion.

In order to keep a sufficient rate of fissile material

(U0 ₂ ) Within the nuclear fuel, it would then be necessary to increase the U enrichment rate of the fissile material beyond the limits imposed by the Non Proliferation Treaty (NPT).

The NPT generally sets a U-limit

20% by weight of the uranium composing the υθ ₂ , a content lowered by the energy operator to 5%.

This content is respected for a cermet which has a metal content of between 10% and 30% of its volume.

There is therefore a need for a cermet with a low metal content which has also been improved with respect to the mechanical strength under irradiation and / or the thermal conductivity, in particular the conductivity in a radial direction of a nuclear fuel pellet composed in whole or in part of this cermet.

SUMMARY OF THE INVENTION

One of the aims of the invention is precisely to meet this need by providing a cermet having in particular a specific microstructure.

The cermet of the invention comprises a steel matrix in which particles of UO ₂ are dispersed which occupy 70% to 90% of the volume of the cermet and which have a mean size of between 50 μm and 400 μm and a mean sphericity coefficient, measured by the average of the ratio between the maximum dimension and the minimum dimension of each of said particles (Dmax / Dmin), between 1.1 and 4. The average sphericity coefficient of the particles of UO2 between 1.1 and 4 (preferably between 1.1 and 2.5) makes it possible to improve the thermal conductivity of the cermet of the invention, in particular the radial thermal conductivity.

According to a preferred embodiment of the invention, a mean sphericity coefficient of the UO2 particles of between 1.5 and 2.5 (preferably between 1.5 and 2) constitutes a very good compromise when it comes to to improve both radial and global thermal conductivities.

In the present description, the terms "axial" and "radial" refer to the axial or radial direction of a nuclear fuel pellet composed in whole or in part of the cermet of the invention. Such a pellet generally has dimensions of 8.2mm in diameter and 12mm to 13mm in height (REP pellet), or 15mm in height (MOx pellet).

The sphericity coefficient makes it possible to evaluate the flattening rate of an object such as, for example, an U02 granule or a U02 particle. It is measured by the ratio between its maximum dimension and its minimum dimension (Dmax / Dmin), each object being considered in first approximation as an oblate-shaped ellipsoid (flattened sphere).

In practice, the ratio Dmax / Dmin can be calculated from the axial section of a cermet pellet for which the dimension Dmax and Dmin for each particle of U02 is measured according to at least 6 fields of observation. The measurement is generally performed using an image analysis software, each image representing fifty particles. The average sphericity coefficient is then the average of the Dmax / Dmin ratios measured for each U02 particle. The mechanical strength under irradiation of the cermet of the invention is in turn improved by the fact that the metal phase constituting its matrix comprises a steel. Such an alloy has the advantage of having better mechanical compatibility with the constituent material of a nuclear fuel cladding generally made of steel. Thus, preferably, the cermet steel of the invention is identical or similar to the steel constituting the nuclear fuel cladding. This is for example a stainless steel, preferably selected from 316L steel, AIM, F17 or EM10.

This mechanical strength is further optimized by the fact that the U02 particles of the cermet have an average size of between 50 μm and 400 μm.

The average minimum size of 50m is an acceptable value with respect to the degraded volume ratio of the matrix under the action of the irradiation, by making it possible to increase the surface area ratio developed by the U02 particles / total volume. they occupy.

The average maximum size of 400 m is in turn related to the geometry of the rods containing the nuclear fuel pellets of a PWR reactor: in order to be able to produce a sufficiently compact stack of pellets, it is necessary to limit the edge effects of each pellet by retaining a ratio (pencil sheath diameter) / (U02 particle diameter) that is greater than 20.

Preferably, the average size of the U02 particles is between 200 μm and 250 μm.

The average particle size distribution of UO2 can be monomodal or multimodal, ie, this distribution is centered on one or more values, with a dispersion of the average size of the order of 10% around each value. A monomodal distribution can be obtained by performing a single sieving of the granules or UO ₂ particles so that their average size is centered on a value.

A multimodal distribution can be obtained by performing a number N of co-sieving granules or UO ₂ particles, so that the average size of each sieve is centered on a value distinct from that of any other sieve, and then bringing together the N fractions obtained so that the average size of the resulting mixture is centered on N values.

Generally, the number N of values on which the distribution of the mean size of the CPU granules is centered is preserved during sintering leading to the corresponding UC particles, especially when N = 1 (monomodal distribution).

In particular, a multimodal distribution makes it possible to increase the UO ₂ fuel content within the cermet, as well as its radial thermal conductivity, in particular that of its steel matrix.

According to an advantageous embodiment of the cermet of the invention, the distribution of the average size of the UC particles is bimodal. As illustrated below, this allows unexpectedly to increase both the radial and axial thermal conductivity.

Preferably, the average particle size distribution of UC 2 is from 50 μm to 65 μm (ie for example a mean size of between 100 μm and 150 μm, 140 μm and 190 μm, or 250 μm and 31 μm).

According to another advantageous embodiment, the minimum average distance between the particles of U0 ₂ (in particular in their radial direction) is between 0.27 m and 0.49 m. The invention also relates to the nuclear fuel comprising the cermet.

Preferably, this fuel is in the form of pellet. The cermet then composes the pellet such that, preferably, the principal dimension (Dmax) of the UO2 particles entering into the composition of the cermet is oriented in the radial direction of the pellet. The heat is thus evacuated more easily from the pellet.

Other objects, features and advantages of the invention will now be specified in the following description given by way of illustration and not limitation, with reference to Figures 1 to 7 attached.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 is a Scanning Electron Microscopy (SEM) shot of U02 pellets -

Figures 2 to 4 are snapshots of a section of a cermet according to the invention.

FIG. 5 is a graph showing the evolution of the thermal conductivity of a cermet as a function of the average sphericity coefficient and the average size distribution of its U02 particles.

Figure 6 is a graph showing the evolution of the average distance between U02 particles as a function of their average sphericity coefficient.

Figure 7 is a graph showing the evolution of the radial thermal conductivity of a cermet as a function of the average distance between its U02 particles -

DETAILED DESCRIPTION OF THE INVENTION

1. Making a cermet. The following example describes a method of manufacturing a cermet according to the invention. This process essentially comprises two series of steps: a first in which U02 granules are formed, followed by a second during which the cermet is obtained by sintering these granules with a steel powder.

1.1. Compaction.

A quantity of 200 g of a uranium dioxide powder (UO2) with a BET surface area equal to 3.3 m ² / g is placed in a cylindrical latex blind case. It is compressed by vibrations, then the case is closed by a plug of diameter adjusted and maintained by latex elastics.

The whole is immersed in a soluble oil bath contained in the cylinder of an isostatic press which is then closed hermetically.

In order to give it sufficient mechanical strength, the UO 2 powder is then agglomerated by isostatic compaction using a 150 MPa granulation pressure applied for 3 minutes. The rate of rise and fall in granulation pressure is 100 MPa / min. It can be between 50MPa / min and 500MPa / min.

After returning to atmospheric pressure, the case is removed from the press and a compacted U02 log is extracted.

Isostatic pressing makes it possible to compact large quantities of material at one time with a good homogeneity of density in the compact, which guarantees the achievement of a homogeneous density and therefore a behavior (in particular an average sphericity coefficient). homogeneous when formatting later.

As an alternative, other types of compaction may nevertheless be used, such as uniaxial compaction making it possible to make U02 compact disks. large diameter (20mm to 30mm) but thin (3mm to 4mm) to maintain density homogeneity in the compact. 1.2. Crushing.

Using an agate container and pestle, the U0 ₂ log (or the discs) are crushed by means of a pestle to obtain fragments of a medium size of order of 3mm to 5mm.

1.3. First sieving.

A sieve with a mesh size of 400 μm is placed above a second sieve of mesh size 315 mm, itself placed on a recovery container.

Small amounts (20g to 50g) of U0 ₂ fragments from crushing are placed on the sieve and forced through with a stainless steel spatula.

Fragments forced at 400 mm fall on the sieve at 315 mm, then the fragments of average size less than 315 mm fall into the container placed under the sieves.

Only the fraction 315-400ym, recovered in the sieve of 315ym, is preserved.

1.4. Spheroidization.

In order to achieve or tend towards a monomodal distribution of the average size of the screened fragments of U0 ₂ , the average sphericity coefficient of the latter is increased by placing them in a container whose bottom is covered with a self-adhesive abrasive disk N 400 (400 abrasive particles / cm ² ). They are agitated manually by means of a brush so as to give them a rotational movement allowing the abrasion of their sharp angles.

The operation continues until the average sphericity coefficient of the sieved UO ₂ fragments is less than or equal to 4 in order to obtain quasi-spherical U02 fragments or in the form of an ellipsoid.

Alternatively, the screened fragments of UO2 can also be spheronized using a powder mixer (Turbula® type for example). The abrasion then occurs by friction between the sieved UO2 fragments. The quantities treated with each batch are larger but the stirring time is longer.

An example of U02 pellets obtained after the spheronization step is illustrated in Figure 1. This step decreased the average size of the UO 2 pellets.

1.5. Second sieving.

A 315mM sieve is placed over a second 250m mesh sieve, itself placed on a recovery vessel.

The UO2 fragments obtained in the previous step are placed on the upper sieve and the whole is vibrated by means of an automatic sieve for 10 minutes with a small amplitude (0.4 mm) so as not to create fines. additional abrasion fragments between them.

At the end of this second sieving step, 30 g of UO 2 granules having an average sphericity coefficient of between 1.1 and 4 and a monomodal distribution of their average size of an extension of 65 μm are obtained. a range of 250ym to 315ym.

Most often, the average size of the UO2 granules is between 60 μm and 500 μm (preferably between 250 μm to 315 μm), with a distribution of this average size preferably ranging from 50 μm to 65 μm.

All previous operations (from the compaction to the second sieving) is repeated in order to obtain 100 g of granules of UC> 2. 1.6. Mixing UQ ₂ granules with steel powder.

100 g of UC granules and 30 g of 316L steel powder with a BET specific surface area of 0.2 m ² / g are mixed in a container by slow manual stirring in order not to generate fines. The UO ₂ granules then represent 70% by volume of the primary mixture obtained.

The BET surface area of the steel and UO ₂ powders is preferably measured using the three-point BET method known to those skilled in the art.

At the end of the mixing operation, it is avoided to manipulate the container to avoid the phenomena of weight segregation, namely that, in the container, the largest particles of U0 ₂ can, under the effect of vibrations, back on the surface of the finer steel particles. This phenomenon would lead to a heterogeneity of composition between the first shaped tablets and the last.

Alternatively, the primary mixture of the UO ₂ granules with the steel powder can be produced using a powder mixer (of the Turbula® type, for example), for example at 40 rpm for 1 hour. at 5 minutes.

Still alternatively, we can add a binder (for example Polyethylene Glycol (PEG) 3% molar)) which allows to glue the pellets of U0 ₂ and then add the steel powder that will stick on it.

1.7. Fitness.

The primary mixture of UO ₂ granules and steel powder is shaped to reduce its porosity and to flatten the UO ₂ granules.

For this, it undergoes a double-acting uniaxial pressing comprising a rise of the shaping pressure at a speed of 100 MPa / sec in order to reach a limiting shaping pressure of 100 MPa which is maintained for 5 minutes. at 10 s, then a reduction in the shaping pressure at a speed of 100 MPa / s, at the end of which a compact mixture is obtained in the form of a cylinder of diameter 10 mm and height 12 mm.

In order to limit the particles / matrix friction and the appearance of defects during demolding, zinc stearate or ammonium oxalate may be added to the primary mixture or sprayed onto the press matrix by means of an aerosol.

Other pressing is possible, such as uniaxial floating matrix pressing.

1.8. Sintering.

The compact mixture is placed in a nacelle, itself placed in a molybdenum metal furnace.

The furnace is placed under an oxidizing atmosphere (here composed of industrial argon, that is to say argon generally comprising 10 ppm to 30 ppm of O 2), which allows the UO 2 forming the granules to oxidize into the atmosphere. UO2 (uranium dioxide called "surstoichiometry").

The compact mixture is then sintered according to the following thermal cycle: temperature rise at a rate of 300 ° C./h (most often between 150 ° C./h and 300 ° C./h) up to a limit temperature of 1380 ° C. ° C (lower than the melting temperature of 316L steel, namely 1480 ° C) which is maintained for 3 hours.

During the temperature rise of this sintering step, the U02 + _x granules substantially densify to their final value at a temperature of 1000 ° C to form U02 + _x particles. Then, from 1000 ° C, the steel densifies in turn up to 1200 ° C in order to reach (approximately 80%) its final density: this sequential withdrawal (UO2 then steel) avoids the tensioning of the steel and thus the cracking of the material sintered, and allows to form a continuous metal matrix ensuring a good thermal diffusivity.

The sintering is completed by maintaining the limit temperature of 1380 ° C in order to reach the maximum degree of densification of the sintered material.

1.9. Treatment in a reducing atmosphere.

At the end of the sintering step, hydrogen is added to the reaction medium in order to obtain a reducing atmosphere consisting of a mixture Ar + 5% by volume of ¾.

The temperature limit of 1380 ° C is maintained under this atmosphere for one hour.

Such an atmosphere can reduce 1 'U0 ₂ + x 6Π

U0 ₂ , oo.

After cooling to room temperature according to a thermal gradient of 300 ° C./h (most often between 150 ° C./h and 300 ° C./h), a cermet composed of a 316L steel matrix is obtained in which U02 particles are dispersed, each of these components occupying respectively 30% and 70% of the volume of the cermet.

Generally, the average sphericity coefficient of U02 pellets is preserved during the manufacturing process of the invention. It then corresponds to the average sphericity coefficient of the U02 particles contained in the cermet.

Figures 2 and 4 are the snapshots of the same section of the cermet obtained. Figure 2 is an optical micrograph in which the U02 particles appear in dark gray. Figure 4 is a close SEM image of the same section in which the U02 particles appear this time in light gray and the steel matrix as a black-colored cross-section. The porosity within the U02 particles is also black. Figures 2 and 3 are the snapshots of the same section of the cermet obtained. Figure 2 is a close-up SEM image in which the UC particles appear in light gray and the steel matrix as a black-colored cross-section. The porosity within the UC particles is also black. Figure 3 is an optical micrograph in which UC particles appear in dark gray.

These figures make it possible to confirm that the cermet obtained by the manufacturing method of the invention has a steel matrix which is generally continuous and does not exhibit cracking.

2. Studies of the properties of the cermet.

Using thermodynamic simulations based on inventors' own calculation codes, the effects of various characteristics of the cermet of the invention were evaluated.

2.1. Influence of the average sphericity coefficient.

The radial and axial thermal conductivities were evaluated at a temperature of 1400K for cermets comprising U0 ₂ particles stacked in an ideal mode of cubic face-centered type for which the average sphericity coefficients and the distribution of the average height.

The results are grouped in Figure 5 which reproduces the following thermal conductivities:

- radial with a monomodal distribution of U0 ₂ particles in the form of spheres (the point Dmax / Dmin = 1) and ellipsoids (Dmax / Dmin variable and different from 1): curve (a),

- Radial according to a bimodal distribution of these spheres and ellipsoids, - axial according to a monomodal distribution of these spheres and ellipsoids: curve (b),

- Radial according to a bimodal distribution of these spheres and ellipsoids.

These results have shown that there is a maximum mean sphericity coefficient beyond which saturation occurs, as indicated by the curve (a) of the radial conductivity which tends to an asymptote when the Dmax / Dmin ratio increases.

Furthermore, it has been found that a value Dmax / Dmin of between 1.1 (significant gain on the radial conductivity with respect to the case Dmax / Dmin = 1) and 4 (gain on the radial conductivity close to the asymptotic value ) makes it possible to maintain an axial conductivity at least equal to 50% of its initial value (curve (b)). Even if the radial thermal conductivity has increased at the expense of the axial thermal conductivity, a Dmax / Dmin value of between 1.1 and 4 (preferably between 1.5 and 2.5) constitutes a good compromise when it is is to improve both the radial and global thermal conductivities.

In practice, the average sphericity coefficient (Dmax / Dmin) can be modulated by varying the granulation pressure (Pg) between 80 MPa and 150 MPa and the forming pressure (MP) between 400 MPa and 100 MPa, in a ratio Pm / Pg less than 9.

Thus, by way of illustration, FIGS. 3 and 4 show SEM images of cermets manufactured under the same conditions, except for the pressures Pm and Pg, in order to obtain particles of UC 2 having average sphericity coefficients. different, namely:

cermet of Figure 3: Pm = 100OMPa, Pg = 150MPa, Dmax / Dmin = 2.1;

cermet of Figure 4: Pm = 400MPa, Pg = 80MPa, Dmax / Dmin = 1.5. 2.2. Influence of the distribution of the average particle size of UQ ₂ .

The results of FIG. 5 also show that the axial conductivity degrades inversely with the evolution of the radial conductivity in a non-symmetrical manner.

However, unexpectedly, when the distribution of the average particle size of U0 ₂ is bimodal, the radial and axial thermal conductivities are both improved, in particular for a Dmax / Dmin value of between 1.5 and 2.5. , even equal to 2.

2.3. Influence of the distribution of the average particle size of UQ ₂ .

As illustrated in Figure 6 in the case of a monomodal or bimodal distribution of the average particle size of U0 ₂ , the average distance between these particles in a plane perpendicular to the thermal flux passing through a nuclear fuel pellet (called interparticular distance ) depends on the average sphericity coefficient (Dmax / Dmin).

Moreover, as illustrated by FIG. 7, it has been found that the radial thermal conductivity of a pellet composed of the cermet of the invention (comprising, for example, 28% of steel) is improved when the interparticular distance decreases or the coefficient of mean sphericity (Dmax / Dmin) increases, as the average particle size of U0 ₂ are distributed either monomodally or multimodally (especially bimodal).

For example, especially in the case of a monomodal distribution, a minimum interparticular distance of 0.27m.sup.i corresponds to an average sphericity coefficient of about 4 and a radial thermal conductivity of the order of 9.18W.m.sup.- ¹ K.sup.- ^1. . A maximum interparticle distance of 0.49ym corresponds to a mean sphericity coefficient of about 1.1 and a radial thermal conductivity of the order of 8.2W.m ^_1 K ^_1.

From an average sphericity coefficient of about 20, the minimum interparticle distance stabilizes at about 0.1 μm.

It will be apparent from the foregoing description that the cermet of the invention has a low metal content and exhibits mechanical strength under irradiation and / or thermal conductivity (particularly in a radial direction of a nuclear fuel pellet) which is improved.

At an equivalent porosity rate and for a steel content set for example at 20% by volume, the cermet of the invention can thus reach (at least) a radial thermal conductivity value greater than 80% (at 1000 K) compared to that of a material consisting of pure UO2.

Claims

1) Cermet comprising a steel matrix in which UO ₂ particles are dispersed which occupy 70% to 90% of the volume of the cermet and which have an average size of between 50 μm and 400 μm and an average sphericity coefficient, measured by the average of the ratio between the maximum dimension and the minimum dimension of each of said particles (Dmax / Dmin), between 1.1 and 4.

2) cermet according to claim 1, wherein the average sphericity coefficient is between 1.5 and 2.5.

3) cermet according to claim 1 or 2, wherein the average particle size of U0 ₂ is between 200ym and 250ym.

4) cermet according to any one of the preceding claims, wherein the distribution of the average particle size of U0 ₂ is bimodal.

5) Cermet according to any one of the preceding claims, wherein the distribution of the average size of the U0 ₂ particles is from 50m to 65m.

6) cermet according to any one of the preceding claims, wherein the minimum average distance between the particles of U0 ₂ is between 0.27ym and 0.49ym. 7) Nuclear fuel comprising the cermet as defined in any one of claims 1 to 6.

8) nuclear fuel according to claim 7, in the form of pellet. 9) Nuclear fuel according to claim 8, wherein the main dimension (Dmax) of the U02 particles used in the composition of the cermet is oriented in the radial direction of the pellet.