Patent Application
20110170653
This invention generally pertains to the field of nuclear fuel and, in particular, it relates to a nuclear fuel cladding for helium-cooled “high temperature” nuclear reactors as well as a process for manufacturing the same.
Among the nuclear reactors of the future, the Fast Neutron Reactor (FNR), which uses helium as the coolant gas (the so-called “He-GFR” reactor), may be mentioned. This reactor is a so-called “high-temperature” reactor because during operation the temperature of its core generally lies in the range between 800° C. and 1200° C.
As described in patent application EP 1 913 600, a nuclear fuel cladding employed in such a reactor may be provided as a plate, a cylinder, a sphere or a network of cavities.
When subjected to the above-mentioned temperature conditions, this cladding requires the use of high-melting point refractory materials (in order to ensure sufficient thermomechanical stability to maintain the fuel within the cladding) and should have a high thermal conductivity under irradiation (in order to optimally transfer the generated thermal energy towards the coolant gas during operation of the nuclear reactor).
Ceramics, although they meet these criteria, are generally too brittle to withstand the operating conditions of a nuclear fuel cladding.
Indeed, fission reactions within the nuclear fuel generate solid and gaseous fission products which cause cladding swelling. When subjected to such loads, the ceramics that form the cladding may break-up and cause a loss of fuel confinement.
In order to avoid such a loss, it would therefore be beneficial to use a ceramic matrix composite (CMC) material of the SiCf/SiC type, in order to achieve improved mechanical properties. Such a material is generally made of a two-dimensional or three-dimensional arrangement of silicon carbide fibers (referred to as SiCf), which contributes to the reinforcement of the SiC ceramic matrix in which it is incorporated.
However, for a given temperature, the thermal conductivity of CMCs of the SiCf/SiC type may strongly decrease after it has been subjected to irradiation.
During the operation of a “He-GFR” nuclear reactor subjected to high temperatures, such CMCs therefore prove to be inappropriate for the removal of thermal energy from the nuclear fuel cladding towards the coolant gas.
It is accordingly one of the objects of this invention to provide a nuclear fuel cladding totally or partially made of a composite material which, when irradiated and at temperatures of between 800° C. and 1200° C., is capable of mechanically maintaining the fuel within the cladding while enabling optimal transfer of the thermal energy generated towards the coolant gas.
The subject matter of the present invention thus relates to a nuclear fuel cladding totally or partially made of a ceramic matrix composite material comprising silicon carbide SiC, fibers as a reinforcement for the matrix, and an interphase layer provided between the matrix and the fibers, the matrix comprising at least one carbide selected from titanium carbide TiC, zirconium carbide ZrC, or ternary titanium silicon carbide Ti3SiC2.
As shown below, when irradiated and at temperatures between 800° C. and 1200° C. (preferably, between 800° C. and 1000° C., or even equal to 800° C.), the nuclear fuel cladding according to the present invention has a thermal conductivity which allows the transfer of heat towards the coolant to be improved, whilst at the same time preserving the thermomechanical (high melting point) and mechanical (reduced brittleness) properties, which are inherent to CMCs and which enable optimum confinement of the fuel contained within the cladding.
According to a preferred embodiment, the ceramic matrix composite material further comprises silicon carbide SiC. Thus, for example, the amount of silicon carbide SiC, represents less than 50% (typically, from 1% to 50%) by volume of the matrix, preferably less than 25% (typically from 1% to 25%), and still more preferably, less than 10% (typically, from 1% to 10%). The addition of varying quantities of SiC enables the matrix properties (such as its thermal conductivity) to be optimally adapted to the prevailing conditions. The incorporation of SiC within the matrix also allows its thermomechanical compatibility with SiC fibers to be improved: for example, the matching between the thermal expansion coefficients allows the effects of a differential expansion between the matrix and the fibers, which could lead to cracking of the nuclear fuel cladding, to be reduced.
In a preferred embodiment, the silicon carbide SiC in the matrix represents between 5% and 15% by volume of the matrix (in particular when the matrix contains TiC). Unexpectedly, as illustrated below, such a matrix composition makes it possible to achieve optimal thermal conductivity.
Optionally, the matrix has a columnar microstructure.
As for the fibers, they may be totally or partially ordered. Thus, they generally originate from a fibrous pre-form, which is most often made of fibers, which instead of being arranged randomly, are ordered. Thus, in particular, the fibers may have a form such as that of a two-dimensional weave (braids, for instance), a pseudo two-dimensional weave (such as a braided fabric which is sewn afterwards), a three-dimensional weave, a knit fabric, or felts.
Preferably, the fibers are in the form of braids or felts, whenever the nuclear fuel cladding has the form of a tube or a plate, respectively.
Concerning their composition, the fibers are made of SiC, such that they are particularly well suited to the context of the present invention because SiC has an excellent neutron and thermal stability.
Furthermore, an interphase layer is provided between the fibers and the matrix.
This layer may be totally or partially made of a compound comprising several superimposed layers, such a compound preferably being pyrolytic carbon.
The superimposed nature of such layers may be:
The interphase layer may have a mean thickness in the range between 10 nm and 500 nm, preferably between 10 nm and 50 nm, and still more preferably between 10 nm and 30 nm, whereby a reduction in this thickness most often results in an improvement in the mechanical properties.
The porosity of the composite material, which forms all or part of the nuclear fuel cladding according to this invention, is preferably 10% (or even 5%) by volume or less in order to promote high thermal conductivity.
A further object of this invention is to provide a process for manufacturing a nuclear fuel cladding according to the present invention. This process comprises the preparation of the composite material according to the following consecutive steps:
a) making a fiber pre-form from the fibers,
b) depositing said interphase layer by chemical vapor infiltration onto said pre-form,
c) depositing said matrix by chemical vapor infiltration onto said pre-form coated with said interphase layer.
The fiber pre-form generally has a geometry which is close to that of the nuclear fuel cladding to be manufactured. After the manufacturing process of the present invention has been performed, this cladding therefore is most often in its final form or requires at most a few regrinding operations.
Preferably, the chemical vapor infiltration of step c) is performed using a mixture of precursors comprising i) at least one compound selected from a titanium-, zirconium- or silicon-based compound, ii) a hydrocarbon and iii) hydrogen.
More preferably, these precursors are then such that:
Preferably, at least one of the chemical vapor infiltrations (namely, the infiltration performed to make the interphase layer according to step b) or that performed to deposit the matrix according to step c)) is of the pulsed type.
Other objects, features and advantages of the present invention will become more apparent from the following description, which is given by way of non-limiting example.
The following examples illustrate that part of the manufacturing process of the invention in which the ceramic matrix composite (CMC) material, intended to enter into the composition of the nuclear fuel cladding, is prepared.
As discussed above, the use of a fiber pre-form having a shape and dimensions which are close or identical to those of the nuclear fuel cladding enables, at the end of the manufacturing process, according to the present invention, such a cladding to be obtained in the form of a blank or even in its final form.
These example embodiments are followed by characterizing, before and after irradiation, the thermal properties of ceramic materials (without fibers or the interphase layer), which are representative of the prepared CMCs.
1—Manufacture of Ceramic Matrix Composite (CMC) Materials to Enter into the Composition of the Nuclear Fuel Cladding of the Invention.
The following manufacturing operations are performed using a process known to those skilled in the art, namely the chemical vapor infiltration process, known as CVI.
Using CVI, carbide may be formed from precursors and then deposited onto a fiber pre-form. Such precursors are generally available in gaseous form.
A particular type of CVI is pulsed-CVI, such as described, for example, in EP 0 385 869 or “T. M. Besmann, Ceram. Trans., Vol 58, pages 1-12, 1995”.
In pulsed CVI, the precursors are transported by a pulse sequence within the reactor vessel (for example an oven). For each pulse, the pressure of the precursors within the oven changes over time according to the following three phases:
1.1—Manufacturing a CMC of the SiCf/TiC Type
Using the CVI process, a fiber pre-form made of ordered fibers of silicon carbide SiC is coated with an interphase layer having a mean thickness of a few tens to a few hundreds of nanometers, composed of a lamellar compound, such as pyrolytic carbon (PyC).
The fiber pre-form is then placed in an oven at 1050° C. with heated walls and subjected to a primary vacuum.
Thereafter, using pulsed CVI, the CMC matrix is made by depositing, under a working pressure of 5 kPa, titanium carbide TiC through a vapor phase reaction starting with the gaseous precursors TiCl4, CH4 and H2 initially contained in a mixture vessel under a pressure of 40 kPa.
To obtain as homogeneous a carbide deposit as possible, in terms of composition and microstructure, it is preferred to limit the deposition rate by working at a low temperature (typically in the range between 900° C. and 1200° C.) and under a low working pressure (typically in the range between 1 kPa and 10 kPa).
It should be noted that parameters other than the temperature and pressure may also influence the carbide deposit's homogeneity. In particular, these are the nature of the hydrocarbon, the proportion of carbon and the dilution factor.
Thus, for example, for a TiC deposition:
The flow rate of the vector gases, CH4, and especially H2, and the control of the boiling temperature of TiCl4 allow the TiCl4 flow rate, and therefore the dilution factor α and carbon proportion mC/Ti, to be controlled.
The pressure within the oven also depends on this flow-rate as well as on the opening time of the shut-off valve.
In the present case, the deposition parameters are as follows:
A CMC of the SiCf/TiC type has thus been obtained, in which the matrix is made of stoechiometric TiC and has a columnar microstructure and a mean thickness of 40 μm.
1.2—Manufacturing a CMC of the SiCf/ZrC Type
Operating conditions similar to those described in the above example may be employed for preparing a CMC in which the matrix is composed of zirconium carbide ZrC. The only specific parameters are then as follows:
1.3—Manufacture of a CMC of the SiCf/TiC—SiC Type.
In this case, CMCs are prepared in which the matrix has a mixed composition, such that it is composed of both titanium carbide TiC, and silicon carbide SiC.
Pulsed CVI is particularly well suited to the manufacturing of mixed matrices because of the possibility to easily change the proportion of TiC to SiC by changing, for example, the number of pulses as a function of the precursors of each of these carbides. This feature has been used to prepare three mixed SiCf/TiC—SiC CMCs in which the matrix had the following TiC/SiC compositions in volume percentage: 90/10, 75/25, 50/50.
Several pulsed CVI deposition modes may be considered.
In a first embodiment, the number of pulses in each of the TiC and SiC deposition sequences is reduced such that the deposited layer is discontinuous.
The TiC deposition conditions are those mentioned in the previous example.
For SiC deposition, the conditions are also similar to those of TiC deposition, except for the following parameters:
It should be noted that the deposition rate of the SiC layer is proportional to the temperature and pressure values shown.
In the present case, the following parameters effectively used for depositing SiC are as follows:
The pulse sequence structure is as follows: 2 pulses for TiC deposition, followed by one pulse for SiC deposition.
The obtained mixed matrix is made of stoechiometric SiC and TiC and has a mean thickness of 40 μm.
In a second embodiment, nano-sequenced depositions are made, namely depositions for which layers of different characteristics, having a mean thickness of 10 to 100 nm, are deposited consecutively. For that purpose, the precursor pulses specifically intended for either TiC or SiC are produced consecutively (for instance, 40 pulses for SiC and 80 pulses for TiC, or 20 pulses for SiC and 40 pulses for TiC).
In a third embodiment, the SiC and Tic precursors are introduced together. Generally, the precursors and operating conditions are then selected from the following:
2—Thermal Properties of Ceramic Matrix Composite (CMC) Materials Comprising TiC.
Ceramic materials (without fibers and an interphase layer), with the same composition as the matrix of the four TiC-based CMCs made previously, are manufactured by sintering under pressure.
The four ceramic materials have the following TiC/SiC compositions in volume percentage: 100/0, 90/10, 75/25, 50/50.
These ceramic materials allow the relative thermal conductivities of the four previously made TiC-based CMCs to be determined because, even though the absolute value of their thermal conductivities differs from that of the corresponding CMCs, their relative values are comparable. In other terms, the behaviors of the thermal conductivity of these ceramic materials with respect to one another are similar and are indicative of the behaviors of the four previously prepared CMCs.
In practice, the thermal diffusivity of ceramic materials is measured at different temperatures.
Given the density and the mass heat capacity (noted Cp) of these ceramic materials, the thermal conductivity is then computed according to the formula k=α·ρ·Cp, in which:
The Cp(T) formulas used for TiC and SiC are as follows:
When the ceramic material has a mixed composition (for example 75% TiC+25% SiC), its mass heat capacity is the weighted mean of the mass heat capacities of each of the carbides.
After computation, the change in thermal conductivity (trend curves) as a function of temperature is obtained, as the shown for example in
It may be concluded from
The following thermal conductivity measurements have confirmed such a behavior of ceramic materials after irradiation.
These measurements were performed according to the same procedures on five irradiated ceramic materials, namely the four previous ceramic materials and one ceramic material made of 100% SiC (that is, ceramic materials having the following volume TiC/SiC compositions: 100/0, 90/10, 75/25, 50/50, 0/100).
The irradiation consisted in simulating a neutron flux by implanting Kr ions having an energy of 74 MeV so as to reach an irradiation dose of 1 dpa (displacement per atom), in order to create two damage areas, namely one for nuclear interactions (which simulates neutron damage) and one for electron interactions. The thermal conductivity was measured at 800° C. in the nuclear interaction area.
The results are summarized in
Complementary results have also shown that, after irradiation by Au ions (4 MeV, 8 dpa), the thermal conductivity at 800° C. of a ceramic material made of TIC is greater than that of a ceramic material made of SiC.
It therefore appears that, to manufacture a nuclear fuel cladding, the use of a ceramic matrix composite material comprising SiC fibers, an interphase layer and a matrix comprising at least one carbide selected from titanium carbide TIC, zirconium carbide ZrC, or ternary silicon titanium carbide Ti3SiC2, allows the thermal conductivity of said cladding to be improved under irradiation at temperatures generally ranging between 800° C. and 1200° C.
During the operation of a “He-GFR” reactor, the nuclear fuel cladding of the present invention may thus mechanically maintain the nuclear fuel and ensure heat transfer towards the coolant gas, more efficiently than in the case of a cladding made of a CMC of the SiCf/SiC type.
| Number | Date | Country | Kind |
|---|---|---|---|
| 08/05127 | Sep 2008 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR2009/001105 | 9/18/2009 | WO | 00 | 3/17/2011 |
