GB2527539A - Composite fuel - Google Patents

Abstract

The present invention relates to a heterogeneous composite fuel for nuclear reactors, the composite fuel comprising: at least a first fissile component comprising a fissile metal, such as uranium, plutonium or thorium; and a second, reinforcement component comprising particulate silicon carbide (SiC); wherein the fuel has, by comparison to the fissile metal alone, a larger elastic modulus, a smaller coefficient of thermal expansion and no less thermal conductivity. The silicon carbide particles are preferably between 0.5 and 10 microns in diameter, more preferably between 1.5 and 2.5 microns, and make up between 40 and 50 percent of the fuel by volume, more preferably between 44 and 46 percent. Pellets of the fuel may be manufactured by mixing the powdered fissile metal and silicon carbide to ensure a uniform distribution thereof, compacting the mixture in a steel die by hot pressing under an inert atmosphere, and finally sintering each pellet to complete densification and eliminate porosity.

Description

COMPOSITE FUEL

The present invention concerns fuels for nuclear reactors. In particular it concerns fissile metal fuels.

Fissile metals are attractive as nuclear fuels, for a number of reasons including good thermal conductivity. Some early nuclear reactors such as the Magnox reactors in the UK were designed to take advantage of the physical properties of fissile metals.

However it was later discovered that when fissile metal is irradiated, it may over time become geometrically unstable, and thermal expansion and irradiation damage may lead to increasing risk of mechanical failure. As a consequence, metal fuel requires to be removed from nuclear reactors at relatively low fission conversion rates.

Because of these problems, fissile metals are currently considered as obsolete and are no longer used in commercial reactors (except for the final operating Magnox reactor at Wylfa which uses uranium metal and is expected to cease operation soon). So despite certain advantages, fissile metals have been abandoned as nuclear fuels because of operational difficulties.

The nuclear industry has moved instead to compound fuels, for example uranium dioxide. Such materials have better geometric stability which makes them attractive operationally, but they generally have poor thermal conductivity and low mechanical strength. NB: In this specification and in the claims, the word "compound" is used to mean a chemical compound comprising at least two elements (and not a mixture).

The present invention comprises heterogeneous compositions designed to retain the advantages of fissile metal fuels (such as high thermal conductivity) while increasing thermal, mechanical and irradiation stability so that a larger proportion of fissile metal fuel in nuclear reactors may be used productively in fission.

The behaviour of a metal nuclear fuel during its time in a nuclear reactor may be modified by incorporating a reinforcement material. The reinforcement material may be selected on the basis of its physical and chemical properties, and the resulting properties of the composite.

The addition of reinforcement materials to compound fuels has been considered in the past, but does not bring significant improvements unless the reinforcement fraction is very large. However, such a level of reinforcement may act to the detriment of other key properties (such as fission cross section, density etc).

Although metals and their compounds may both be used as fuels, they differ widely from a material science view. Accordingly, modifications of chemical, mechanical and/or thermo-physical properties of one class are not applicable to the other.

The reinforcement material and composite fuel should have good elastic properties and be resistant to plastic deformation. Specifically desirable is a high elastic modulus so as to reduce deformation due to stresses.

The reinforcement material and composite fuel should be thermally stable, to resist deformation and changes during temperature fluctuations. Specifically desirable are a high melting point and low coefficient of thermal expansion. A reinforcement material is preferred with no allotropic transformations below the melting point of the fissile material.

The reinforcement material and composite fuel should have high thermal conductivity, so that heat generated is efficiently conducted to the outer surface of the fuel element.

The reinforcement material should be chemically compatible with the fissile component, so that there is minimal chemical interaction between them at their interface.

The reinforcement material should have low neutron radiative capture cross section and be resistant to radiation damage.

Various reinforcement materials may be considered for reinforcement of fissile metals: Graphite is known as a reinforcement material in general metal composites (mainly as fibres). Graphite is characterised by high strength (-4 GPa) and extremely high thermal conductivity (up to 160 WmFC1). The overall coefficient of thermal expansion of graphite is low (7.8 x 106 K-1), but it shows anisotropy in expansion. Graphite is characterised by a low modulus of elasticity (which varies depending on its grade) and a very low neutron absorption cross section. Chemically, graphite can react with fissile metals, for example with uranium metal to form uranium carbides, so the fraction of carbon reinforcement has to be restricted in order to limit chemical interactions.

Ceramic nuclear fuels including uranium carbide (UC) and uranium nitride (UN) are also reinforcement candidates. Such fuels combine geometrical stability during irradiation with high fissile content. UC and UN are characterised by their relatively high thermal conductivity and low coefficients of thermal expansion. Disadvantages include degradation during irradiation, and chemical interaction with the fissile material.

The present invention uses silicon carbide as a reinforcement material.

Silicon carbide is characterised by a very high elastic modulus (-400 GPa) and high strength. It has high thermal conductivity (114 WmK), and a very low thermal expansion coefficient (4 x 1 o K1). Because of good thermal stability, it may be employed at high temperature. It sublimes at about 3,000 K (for reference, uranium melts at about 1,400 K, thorium about 2,020 Kand plutonium about 910 K). Silicon carbide is chemically unreactive, so chemical interaction with fissile components is minimal. Its neutron absorption cross section is very small.

Silicon carbide is known as a coating or layering material within fuel assemblies (see for example Gamier et al W02013/009534). In contrast the present invention uses silicon carbide within a composite fuel.

Silicon carbide has been employed as a coating in TRISO (tn-structural isotropic) nuclear fuel (see for example U53291921 Chin et al). US3723581 (Boettcher) similarly proposes a coating of silicon carbide. In contrast the present invention uses silicon carbide within a composite fuel.

W0201 2/1 29677 (Sherwood, Torxx) and also US2O1 2/01 40867 (Venneri) propose a composite fuel of uranium dioxide and silicon carbide in a TRISO environment. In contrast the present invention uses silicon carbide and a fissile metal as a composite fuel.

Silicon carbide has been proposed as a cladding material for nuclear fuel elements (see for example W02012174548 Feinroth et al). In contrast the present invention uses silicon carbide within a composite fuel.

There have been studies of composite fuels using uranium compounds and silicon carbide: W02014/028731 (Subhash, University of Florida) proposes a fuel comprising uranium dioxide and silicon carbide. Although the title of W02014/028731 includes the words "high thermal conductivity", the text states the thermal conductivity of uranium dioxide as about 2.8 Wm1K1 (page 1 line 21). It then claims (page liline 4) an "approximately three fold enhancement", from which a resulting value of about 9 Wm1K1 may be deduced. This is well below the value for uranium metal and well below the values of the composite fuels of the present invention.

GB87891 1 (general Electric Company) proposes (similarly to Subhash) a mixture of uranium dioxide with either silicon carbide or beryllium oxide reinforcement.

US3214499 (Burnham) proposes a composite fuel comprising uranium carbide and silicon carbide inter a/ia.

US3329744 (Kaufmann) also proposes a composite fuel containing silicon carbide, but only for fissile compound materials. Unlike the present invention, Kaufmann reflects the industry position of abandonment of metal fuels, giving a list of many other fissile materials for its composite (column 5 lines 39-42), but omitting fissile metals.

The present invention is a fuel for nuclear reactors, where the fuel is a heterogeneous composite material, comprising at least a first fissile component comprising fissile metal and a second reinforcement component comprising particulate silicon carbide, and where the fuel has by comparison to the fissile metal alone: a larger elastic modulus, a smaller coefficient of thermal expansion and no less thermal conductivity.

The fissile metal may comprise substantially a single chemical element, such as uranium, thorium or plutonium.

The fuel may be engineered for use in a nuclear reactor designed for use of another fuel, by substantially matching at least one physical property of the fuel to the same property of the other fuel. The compound fuel may comprise a compound of uranium, for example uranium dioxide. Macroscopic neutron fission cross section may be such a physical property.

The diameters of particles of the reinforcement component may lie substantially in the range 0.5 to 10.0 microns inclusive, 1.0 to 5.0 microns inclusive, and/or 1.5 to 2.5 microns inclusive.

The reinforcement component may comprise between ten and sixty percent by volume of the fuel, forty and fifty percent by volume of the fuel and/or forty four and forty six percent by volume of the fuel.

The fuel may be formed into fuel pellets.

Consider desired properties of a composite fuel: Elastic modulus is a measure of the ability of a material to resist elastic deformation under stress. The elastic modulus of a composite material is given from the elastic moduli of the component materials by the well known Shtrikman model (Shtrikman et at., "A variational approach to the theory of the elastic behaviour of multiphase materials," Journal of Mechanics and Physics of Solids, vol. 11, pp. 127-140, 1963.) Thermal conductivity of nuclear fuels is an important measure, both for safety and efficiency. The thermal conductivity of a composite material is given from the thermal conductivities of the component materials by the well known Maxwell model (Maxwell, A Treatise on Electricity and Magnetism, Oxford University Press, 1904).

The coefficient of thermal expansion describes the strain induced due to temperature change. The coefficient of thermal expansion of a composite material is given from the coefficients of the component materials by the well known Turner model (Turner, "Thermal Expansion Stresses in Rein foced Plastics," Journal of Research of National Bureau of Standards, vol. 37, pp. 239-250, 1946.) Figure 1 shows variation of the elastic modulus of a composite fuel with varying proportions of reinforcement material.

Figure 2 shows variation of the coefficient of thermal expansion of a composite fuel with varying proportions of reinforcement material.

Figure 3 shows variation of the thermal conductivity of a composite fuel with varying proportions of reinforcement material.

Figure 4 shows the coefficient of thermal expansion of three materials at different temperatures.

Figure 5 shows the variation of the overall yield strength of reinforced uranium versus the size of reinforcement particles of silicon carbide.

Figure 6 shows variation of the overall thermal conductivity of reinforced uranium versus the size of reinforcement particles of silicon carbide In the figures the series labels are SiC (silicon carbide), C (graphite), UC (uranium carbide), UN (uranium nitride) and U02 (uranium dioxide).

To allow the selection of a suitable reinforcement material, it is important to review the properties of composite fuels with varying proportions of a range of candidate reinforcement materials: Figure 1 shows the elastic modulus of uranium metal altered by a range of reinforcement materials in varying proportions. The x-axis shows the fraction of the reinforcement material. The y-axis shows the elastic modulus (in units of GPa). The chart shows that among these candidate materials only silicon carbide increases (improves) the elastic modulus of the composite material.

Figure 2 shows the coefficient of thermal expansion of uranium metal altered by a range of reinforcement materials in varying proportions. The x-axis shows the fraction of the reinforcement material. The y-axis shows the coefficient of thermal expansion (in units of 1O K-1). The chart shows that all the candidate materials reduce (improve) the thermal expansion of the composite material, and that silicon carbide has the most beneficial effect.

Figure 3 shows the thermal conductivity of uranium metal altered by a range of reinforcement materials in varying proportions. The x-axis shows the fraction of the reinforcement material. The y-axis shows the thermal conductivity (in units of Wm1K1).

The chart shows that both graphite and silicon carbide increase (improve) the thermal conductivity of the composite material. The other candidates show little or detrimental effect.

Figure 4 shows the coefficient of thermal expansion of three materials at different temperatures. The x-axis is the temperature (in degrees Kelvin) and the y-axis is the coefficient of thermal expansion (in units of 106 K1) Plot U represents uranium metal, plot "U + SiC" represents uranium metal reinforced with silicon carbide (45%) and for reference plot U02 represents uranium dioxide.

The chart of Figure 4 shows that reinforcing uranium with silicon carbide significantly reduces the overall thermal expansion across the operating temperature range. About 50% reduction of expansion is obtained by 45% volume reinforcement by silicon carbide. The expansion of this composite fuel is comparable to that of uranium dioxide.

The data illustrated in the Figures shows that excellent improvements in elastic modulus (and more importantly in the coefficient of thermal expansion) while retaining high conductivity are obtained using silicon carbide.

Thus the present invention uses silicon carbide as the reinforcement material.

Radiation damage resistance is improved by the silicon carbide reinforcement providing traps for point defects.

Many current reactors use fuels based on uranium dioxide as the fissile material. It is therefore convenient to engineer the new composite fuel of the present invention for use in such existing reactors. In this case, the properties of the new fuel may be matched to existing fuels.

A preferred property to match may be the macroscopic neutron fission cross section (MNFCS"). The MNFCS of the composite fuel should be similar to the MNFCS of uranium dioxide fuel.

The MNFCS of the composite can be easily calculated, because the silicon carbide reinforcement has negligible MNFCS. Consequently the MNFCS of the composite is the product of the MNFCS of the fissile metal and the fraction of the fissile metal in the composite fuel. As the proportion of silicon carbide increases, the MNFCS of the composite correspondingly decreases.

Thus for a uranium metal composite, the volume fraction of uranium metal in the new engineered composite is given by the MNFCS of uranium dioxide (0.11 cmj divided by the MNFCS of uranium metal (0.20 cm1) which gives proportions of 55% uranium metal and 45% silicon carbide reinforcement. At this value, the required enrichment of uranium 235 for the new composite nuclear fuel is the same as that required by current reactor designs fuelled with uranium dioxide.

A resulting embodiment of the present invention is a heterogeneous composite fuel comprising 55% by volume uranium metal and 45% particulate silicon carbide.

Other matching criteria may be used as appropriate to engineer materials for other embodiments, and different volume fractions may result.

Other fissile materials such as thorium and plutonium may also be reinforced by silicon carbide. Because each material has different physical properties, the above calculation yields different ratios of fissile material to reinforcement.

In certain embodiments other factors may be more important such as the irradiation stability. Such factors can be substantially matched and then the MNFCS can be optim ised by adjusting the isotopic composition of the fissile fuel.

To maxim ise ease of manufacturing, minimise cost and to benefit from improved plastic deformation resistance and improved thermal stability, a heterogeneous fuel comprising particulate reinforcement is preferred.

The size of the reinforcement particles plays a role in determining the yield strength and thermal conductivity of the composite fuel. The variation of these two properties versus the reinforcement particle size is now considered. It is assumed here that particulate reinforcements can be approximated as spheres.

Figure 5 shows the variation of the overall yield strength of reinforced uranium versus the size of reinforcement particles of (45%) silicon carbide. The x-axis is the average radius of the silicon carbide particles (in nanometres), and the y-axis is the yield strength in megapascals. This shows that improvements in the yield strength are larger for smaller particles.

Figure 6 shows variation of the overall thermal conductivity of reinforced uranium versus the size of reinforcement particles of (45%) silicon carbide. The x-axis is the average radius of the silicon carbide particles (in nanometres), and the y-axis shows the thermal conductivity (in units of Wm1K1). It shows that larger particles contribute to improved thermal conductivity.

Both high yield strength and high thermal conductivity are desirable. However, because these depend differently on the particle size, a compromise is required. Thus a reinforcement particle radius of about 2 microns is chosen as the preferred value for embodiments where these criteria are paramount; but of course other sizes may be used in embodiments where one or other of these parameters assumes greater relative importance.

Silicon carbide of the required grade is available commercially from chemical supply companies.

The mean free path of neutrons in silicon carbide within a nuclear reactor may be estimated by well known techniques and is about 26 millimetres. This is orders of magnitude larger than the particle size, so silicon carbide particles experience negligible neutron interactions, and so have minimal effect on the reactor neutron population.

There now follows an example four stage manufacturing process for fuel pellets made from a heterogeneous composite comprising uranium metal and silicon carbide.

As is well known to those skilled in the art, the materials and parameters described here may be varied appropriately to produce composite fuel pellets adapted to different specific embodiments.

Caution! Those skilled in the art will appreciate that some of the materials and intermediates described below are flammable, corrosive, explosive, toxic and/or radio-active, and may spontaneously ignite. As is well known, appropriate caution, protective procedures and specific handling techniques must be applied.

Stage 1 Powder Preparation: Small pieces of uranium metal are heated to 3,000 t in a hydrogen rich environment to form uranium hydride at elevated pressure (to increase reaction rate). Hydride powder is then decomposed at high temperature (5,000 C) to produce uranium metal powder.

Stage 2 Mixing: In order to ensure uniform distribution of component materials, the desired proportions of metal powder and silicon carbide powder are placed in a closed container and mixed well (for example mechanically, by ultrasound or by blowing inert gas).

Stage 3 Powder Compaction: The composite powder is shaped into pellet form in a steel die by hot pressing at 660 C and 150 MPa hydraulic pressure. This is performed in an inert medium (for example argon and/or helium) to prevent oxidation.

Stage 4 Sintering: To complete densification and eliminate porosity, each pellet is sintered between 800 -1,100 C for four hours. Sintered pellets are then quenched to room temperature to obtain random orientation of grains.

The analyses applied above to uranium may also be applied to other fissile metals including without limitation thorium and plutonium, so that composite fuels comprising silicon carbide with those other fissile metals may be similarly designed and produced.

While the present invention has been described in generic terms, those skilled in the art will recognise that the present invention is not limited to the cases described, but can be practised with modification and alteration within the scope of the appended claims. The Description and Figures are thus to be regarded as illustrative instead of limiting.

Claims (2)

1. CLAIMS1 A fuel for nuclear reactors, where: the fuel is a heterogeneous composite material comprising at least: a first fissile component comprising fissile metal and a second reinforcement component comprising particulate silicon carbide, and where the fuel has by comparison to the fissile metal alone: a larger elastic modulus, a smaller coefficient of thermal expansion and no less thermal conductivity 2 A fuel as in claim 1 where the fissile metal substantially comprises a single chemical element 3 A fuel as in claim 2 where the fissile metal substantially comprises uranium 4 A fuel as in claim 2 where the fissile metal substantially comprises thorium A fuel as in claim 2 where the fissile metal substantially comprises plutonium 6 A fuel as in any previous claim engineered for use in a nuclear reactor designed for use of another fuel by substantially matching at least one physical property of the fuel to the same property of the other fuel 7 A fuel as in claim 6 where the other fuel comprises a compound of uranium 8 A fuel as in claim 7 where the other fuel comprises uranium dioxide 9 A fuel as in claims 6 to 8 where the substantially matched property is the macroscopic neutron fission cross section A fuel as in any previous claim where the diameters of particles of the reinforcement component lie substantially in the range 0.5 to 10.0 microns inclusive 11 A fuel as in claim 10 where the diameters of particles of the reinforcement component lie substantially in the range 1.0 to 5.0 microns inclusive 12 A fuel as in claim 11 where the diameters of particles of the reinforcement component lie substantially in the range 1.5 to
2. 2.5 microns inclusive 13 A fuel as in any previous claim where the reinforcement component comprises between ten and sixty percent by volume of the fuel 14 A fuel as in claim 13 where the reinforcement component comprises between forty and fifty percent by volume of the fuel A fuel as in claim 14 where the reinforcement component comprises between forty four and forty six percent by volume of the fuel 16 A fuel as in any previous claim formed into fuel pellets Amendments to the claims have been filed as follows Claims 1 A replacement fuel for a nuclear reactor that was designed for uranium dioxide; where: the said fuel is engineered so that its macroscopic neutron fission cross section substantially matches that of uranium dioxide; the said fuel is a heterogeneous composite material comprising at least fissile metal and particulate silicon carbide 2 A fuel as in claim 1 where the fissile metal substantially comprises a single chemical element 3 A fuel as in claim 2 where the fissile metal substantially comprises uranium 4 A fuel as in claim 2 where the fissile metal substantially comprises thorium LI") 5 A fuel as in claim 2 where the fissile metal substantially comprises plutonium 6 A fuel as in any previous claim where the diameters of particles of silicon carbide 0 lie substantially in the range 0.5 to 10.0 microns inclusive 7 A fuel as in claim 6 where the diameters of particles of silicon carbide lie substantially in the range 1.0 to 5.0 microns inclusive 8 A fuel as in claim 7 where the diameters of particles of silicon carbide lie substantially in the range 1.5 to 2.5 microns inclusive 9 A fuel as in any previous claim where silicon carbide comprises between ten and sixty percent by volume of the fuel A fuel as in claim 9 where silicon carbide comprises between forty and fifty percent by volume of the fuel 11 A fuel as in claim 10 where silicon carbide comprises between forty four and forty six percent by volume of the fuel 12 A fuel as in any previous claim formed into fuel pellets