WO2005086173A2

WO2005086173A2 - Nuclear fuel provided with a coating

Info

Publication number: WO2005086173A2
Application number: PCT/IB2005/050733
Authority: WO
Inventors: Leszek Andrzej Kuczynski
Original assignee: Pebble Bed Modular Reactor (Proprietary) Limited
Priority date: 2004-03-01
Filing date: 2005-03-01
Publication date: 2005-09-15
Also published as: AR047914A1; WO2005086173A3

Abstract

This invention relates to a method of preparing a nuclear fuel including the step of depositing a coating including silicon nitride bonded silicon carbide on a nuclear fuel surface. The invention extends to a nuclear fuel element (10) and to a nuclear fuel particle (30).

Description

NUCLEAR FUEL

THIS INVENTION relates to nuclear fuel. More particularly, the invention relates to a method of preparing a nuclear fuel, to a nuclear fuel particle and to a nuclear fuel element. In a nuclear reactor of the high temperature gas-cooled type, use is made of fuel comprising a plurality of spherical fuel elements. The fuel elements comprise particles or kernels of a fissile material in a matrix. The fuel spheres are known as pebbles and the reactor of this type is generally known as a pebble bed reactor.

According to one aspect of the invention, there is provided a method of preparing a nuclear fuel, which method includes the step of depositing a coating including silicon nitride bonded silicon carbide on a nuclear fuel surface.

The coating may be deposited on the nuclear fuel surface by at least one technique selected from the group of deposition techniques consisting of chemical vapour deposition, sublimation growth, dipping and chemical bath deposition.

More particularly, depositing the coating may include the steps of, at a temperature of between about 1 300 degrees Celsius and about 1 950 degrees Celsius, depositing a mixture of silicon and silicon carbide on a nuclear fuel surface, and nitriding the nuclear fuel surface.

According to another aspect of the invention, there is provided a method of preparing a nuclear fuel, which method includes the step of depositing a coating including silicon nitride bonded silicon carbide around a nuclear fuel particle, which includes a kernel of fissile material. The coating may be deposited around the nuclear fuel particle by at least one technique selected from the group of deposition techniques consisting of chemical vapour deposition, sublimation growth, dipping and chemical bath deposition.

According to still another aspect of the invention, there is provided a method of preparing a nuclear fuel, which method includes the steps of at a temperature of between about 1 300 degrees Celsius and about 1

950 degrees Celsius, depositing a mixture of silicon and silicon carbide around a nuclear fuel particle having a kernel of fissile material to yield a coated nuclear fuel particle; and treating at least part of the deposited mixture of silicon and silicon carbide to reduce the porosity thereof.

Treating the mixture of silicon and silicon carbide to reduce its porosity may include nitriding the coated nuclear fuel particle. According to yet another aspect of the invention, there is provided a method of preparing a nuclear fuel, which method includes the steps of at a temperature of between about 1 300 degrees Celsius and about 1 950 degrees Celsius, depositing a mixture of silicon and silicon carbide around a nuclear fuel particle having a kernel of fissile material to yield a coated nuclear fuel particle; and nitriding the coated nuclear fuel particle.

By "nitriding" is to be understood treating the coated nuclear fuel particle in an atmosphere capable of supplying nitrogen to a surface of the fuel particle such that nitrogen diffuses into the surface and combines with nitride-forming elements therein. Typically, the mixture of silicon and silicon carbide is deposited by at least one technique selected from the group of deposition techniques consisting of chemical vapour deposition, sublimation growth, dipping and chemical bath deposition.

The abovementioned temperature range is to be understood to represent an ambient temperature, and the temperature of the surface of the nuclear fuel particle can typically be expected to be between about 20 degrees to about 200 degrees Celsius below a wall temperature of the coater apparatus. It is to be appreciated that the temperature of the surface of the nuclear fuel particle varies with a random probability distribution and that, generally, the greater the difference between the ambient and surface temperatures, the higher will be the deposition rate. More particularly, the mixture of silicon and silicon carbide may be deposited at a temperature of between about 1 370 degrees Celsius and 1 800 degrees Celsius.

The method may include the step of increasing the density of the nitrided mixture of silicon and silicon carbide.

The method may include the step of depositing a further amount of silicon carbide on the nitrided mixture of silicon and silicon carbide. Preferably, the further amount of silicon carbide deposited is stoichiometric silicon carbide. The Applicant believes that the deposition of the further amount of silicon carbide will reduce the porosity of, and hence densify, the nitrided mixture of silicon and silicon carbide by permeation of interstices defined in the nitrided mixture of silicon and silicon carbide. The method may include increasing the temperature during the nitriding step to a temperature of between about 1600 degrees Celsius and about 1850 degrees Celsius. The method may include further increasing the temperature during the step of depositing the further amount of silicon carbide to a temperature of between about 1700 degrees Celsius and about 1900 degrees Celsius.

The step of depositing the mixture of silicon and silicon carbide may be carried out at a pressure of between about 1.1 kPa and about 8 kPa. The method may include increasing the pressure during the step of nitriding of the nuclear fuel particle to a pressure of between about 1.6 kPa and about 10 kPa. The method may include further increasing the pressure during the deposition of the further amount of silicon carbide to a pressure of between about 1.9 kPa and about 10.1 kPa.

The method may include the prior step of forming a plurality of kernels of fissile material by atomising a uranyl nitrate solution to form microparticles, followed by baking the microparticles at high temperature to provide a kernel of fissile material, more particularly, uranium dioxide.

Typically the particles are about 0.5 mm in diameter.

The method may further include depositing a carbon material or compound, selected from the group consisting of porous carbon, pyrolytic carbon and silicon carbide, around each kernel of fissile material.

Preferably, the method includes the steps of, in sequence, depositing porous carbon on the surface of the kernel of fissile material, depositing pyrolytic carbon around the kernel, depositing a mixture comprising silicon and silicon carbide around the kernel, nitriding the silicon and silicon carbide deposited around the kernel to give a nitrided mixture of silicon and silicon carbide, and depositing a further amount of silicon carbide around the kernel. In a preferred embodiment, a further amount of pyrolytic carbon is thereafter deposited around the kernel. The method may include depositing silicon carbide between deposition of the pyrolytic carbon and the mixture of silicon and silicon carbide. Depositing the mixture of silicon and silicon carbide on the surface of the nuclear fuel particle may include transporting a precursor chemical for deposition, selected from the group consisting of silicon vapour, methylchlorosilane, dimethylchlorosilane and trimethylchlorosilane, to the surface of the nuclear fuel particle. Naturally, however, any other suitable chemical precursor may be used.

Nitriding of the nuclear fuel particle may be carried out for between about 20 minutes and about 15 hours. The method may include the step of treating the nuclear fuel particle in a pure argon environment at a pressure of about 300 MPa. Treating the nuclear fuel particle in the pure argon environment may include increasing the temperature of the argon environment to a temperature of about 1 800 degrees Celsius. The temperature may be increased by means of microwave heating. Treating the nuclear fuel particle in the argon environment may further include allowing the nuclear fuel particle to cool to room temperature over a period of about 4 hours. The Applicant believes that this will increase the gas tightness of the coating thereby improving the retention of fission products within the coating.

According to a further aspect of the invention, there is provided a method of preparing nuclear fuel, which method includes the step of depositing a coating including silicon nitride bonded silicon carbide on a surface of a nuclear fuel element including a core comprising a plurality of fuel particles, each having a kernel of fissile material, dispersed in a matrix.

Depositing the silicon nitride bonded silicon carbide may include depositing a mixture comprising silicon and silicon carbide on the surface of the nuclear fuel element core and thereafter nitriding the coated nuclear fuel element core.

Deposition of the silicon and silicon carbide mixture may take place at a temperature of between about 1 300 degrees Celsius and about 1 950 degrees Celsius by chemical vapour deposition techniques.

According to a still further aspect of the invention, there is provided a method of preparing a nuclear fuel, which method includes the steps of at a temperature of between about 1 300 degrees Celsius and about 1 950 degrees Celsius, depositing a mixture of silicon and silicon carbide on a surface of a nuclear fuel element including a core comprising a plurality of fuel particles, each having a kernel of fissile material, dispersed in a matrix; and nitriding the coated nuclear fuel element core.

By "nitriding" is to be understood treating the coated nuclear fuel element core in an atmosphere capable of supplying nitrogen to a surface of the coated fuel element core such that nitrogen diffuses into the surface and combines with nitride-forming elements therein.

The mixture of silicon and silicon carbide is typically deposited by chemical vapour deposition techniques. Preferably the deposition of the silicon and silicon carbide mixture takes place at a temperature of above about 1 370 degrees Celsius. More particularly, the silicon and silicon carbide mixture is deposited at a temperature of between about 1 370 degrees Celsius and 1 800 degrees Celsius.

The method may include the additional step of depositing a further amount of silicon carbide on the nitrided mixture of silicon and silicon carbide. Preferably, the further amount of silicon carbide deposited is stoichiometric silicon carbide. The Applicant believes that the deposition of the further amount of silicon carbide will reduce porosity of, and hence densify, the nitrided silicon and silicon carbide mixture by permeation of interstices defined in the nitrided silicon and silicon carbide mixture. Depositing the silicon and silicon carbide mixture on the surface of the nuclear fuel element core may include transporting a precursor chemical for deposition, selected from the group consisting of silicon vapour, methylchlorosilane, dimethylchlorosilane and trimethylchlorosilane to the surface of the nuclear fuel element core. Naturally, however, any other suitable chemical precursor may be used.

Typically, the nuclear fuel element cores will be suspended in the chemical precursor. More particularly, the chemical precursor is transported to the nuclear fuel element core in the gas phase at a predetermined rate of flow according to its temperature and pressure such that a nuclear fuel element core of mass between about 200 g to about 220 g and optimally about 210 g, is suspended in the chemical precursor.

The nitrided silicon and silicon carbide mixture may be deposited to a thickness of between about 8 micrometres and about 90 micrometres, preferably about 40 micrometres.

The method may include increasing the temperature during the nitriding step to a temperature of between about 1600 degrees Celsius and about 1850 degrees Celsius. The method may include further increasing the temperature during the step of depositing the further amount of silicon carbide to a temperature of between about 1650 degrees Celsius and about 1950 degrees Celsius. The step of depositing the mixture of silicon and silicon carbide may be carried out at a pressure of between about 1.1 kPa and about 8 kPa. The method may include increasing the pressure during the step of nitriding of the nuclear fuel element to a pressure of between about 1.4 kPa and about 9 kPa. The method may include further increasing the pressure during the deposition of the further amount of silicon carbide to a pressure of between about 1.7 kPa and about 9.5 kPa, preferably about 9.5 kPa.

Nitriding of the coated nuclear fuel element core may be carried out for between about 20 minutes and about 15 hours.

The method may include the step of treating the coated nuclear fuel element core in a pure argon environment at a pressure of about 300 MPa. Treating the coated nuclear fuel element core in the pure argon environment may include increasing the temperature of the argon environment to a temperature of about 1 800 degrees Celsius. The temperature may be increased by means of microwave heating. Treating the coated nuclear fuel element core in the argon environment may further include allowing the coated nuclear fuel element core to cool to room temperature over a period of about 4 hours.

The method may include the prior step of forming the nuclear fuel element core by mixing a plurality of fuel particles, each having a kernel of fissile material, with a matrix material and pressing the mixture into spheres. The matrix material may include graphite powder and phenolic resin.

According to yet another aspect of the invention, there is provided a coated nuclear fuel particle which includes a nuclear fuel particle, having a kernel of a fissile material, and a coating including silicon nitride bonded silicon carbide deposited around the nuclear fuel particle.

The silicon nitride bonded silicon carbide deposited may have a thickness of between about 1 micrometre and about 35 micrometres. The or each nuclear fuel particle may have at least one carbon material or compound, selected from the group consisting of porous carbon, pyrolytic carbon and silicon carbide, deposited around the kernel. More particularly, the coated nuclear fuel particle may have deposited around the kernel, in sequence, porous carbon, pyrolytic carbon, and silicon nitride bonded silicon carbide. In a preferred embodiment, pyrolytic carbon is thereafter deposited around the kernel.

The porous carbon may be deposited to a thickness of between about 8 micrometres and about 140 micrometres, preferably about 95 micrometres. The pyrolytic carbon may be deposited to a thickness of between about 6 micrometres and about 90 micrometres, preferably about 40 micrometres in each instance.

According to a further aspect of the invention, there is provided a nuclear fuel element which includes a core comprising a plurality of coated nuclear fuel particles, as hereinbefore described, dispersed in a matrix.

According to still another aspect of the invention, there is provided a nuclear fuel element which includes a core comprising a plurality of fuel particles, incorporating a fissile material, dispersed in a matrix, and a coating including silicon nitride bonded silicon carbide deposited around the core.

Each fuel particle may have a kernel of fissile material surrounded by at least one coating layer of a carbon material or compound. More particularly, each fuel particle may have at least one carbon material or compound selected from the group consisting of porous carbon, pyrolytic carbon, silicon carbide and silicon nitride bonded silicon carbide deposited thereon.

The fuel particles may be dispersed in a graphite matrix. More particularly, the matrix may comprise graphite powder and phenolic resin. The nuclear fuel element may be generally spherical in shape. The fuel element may have a diameter of between about 10 millimetres and about 90 millimetres, preferably between about 55 millimetres and about 60 millimetres.

The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings and the following Examples. In the drawings, Figure 1 shows a part-sectional perspective view of a coated nuclear fuel particle in accordance with the invention; and Figure 2 shows a cross-sectional perspective view of a nuclear fuel element in accordance with the invention.

Example 1

A plurality of nuclear fuel particles was formed by atomisation of uranyl nitrate to form microspheres. The microspheres were then gelled and baked at high temperature, ie. calcinated, to yield uranium dioxide particles, each to provide a kernel of fissile material for a nuclear fuel particle.

A batch of uranium dioxide particles was suspended in a deposition chamber of a chemical vapour deposition reactor, the deposition chamber having an argon environment. The deposition chamber was heated to a temperature of about 1 000 degrees Celsius.

Porous carbon was deposited on the surface of the uranium dioxide particles to a thickness of 95 micrometres. Next, pyrolytic carbon was deposited on the surface of the porous carbon coating to a thickness of 40 micrometres.

A methylchlorosilane source was then connected to an inlet end of the deposition chamber and the temperature was raised to 1 375 degrees Celsius. A mixture comprising silicon and silicon carbide was deposited to about 30 micrometres thickness, by the decomposition of the methylchlorosilane, on the pyrolytic carbon. The deposition of the silicon and silicon carbide mixture was carried out at a pressure of 1.5 kPa.

The fuel particles were then nitrided by fumacing in a nitrogen atmosphere at a temperature of 1820 degrees Celsius and a pressure of 1.9 kPa to yield a mixture of silicon carbide and silicon nitride crystals. The fuel particles were treated in the nitrogen atmosphere for 4 to 5 hours to permit nitrogen to permeate the full depth of the silicon and silicon carbide mixture deposited to yield silicon nitride bonded silicon carbide.

Stoichiometric silicon carbide was deposited on the silicon nitride bonded silicon carbide at a temperature of between 1760 degrees Celsius and 1780 degrees Celsius and a pressure of 1.6 kPa. Finally, a further amount of pyrolytic carbon was deposited on the coated fuel particles to a thickness of 40 micrometres.

The batch of coated nuclear fuel particles was then treated in an argon environment at a pressure of 300 MPa, in which they were heated rapidly by use of a microwave heater to a surface temperature of 1 000 degrees Celsius. This was followed by stable cooling to room temperature for at least four hours. Reference is made to Figure 1 of the drawings, in which reference numeral 10 refers generally to a coated nuclear fuel particle of the invention, prepared in accordance with the above Example. The coated fuel particle 10 includes a kernel 12 comprised of a uranium dioxide particle. A coating, generally indicated by reference numeral 14, is deposited on the kernel 12, the coating 14 including porous carbon 16 deposited to a thickness of about 95 micrometres, pyrolytic carbon 18 deposited to a thickness of about 40 micrometres, silicon nitride bonded silicon carbide 20 deposited to a thickness of about 30 micrometres, and further pyrolytic carbon 22 of 40 micrometres thickness. Figure 1 is for illustrative purposes only and it is to be appreciated that, as a result of the coating/deposition process being carried out whilst at all times maintaining a high temperature, i.e. not permitting the temperature to drop to below 1000 degrees Celsius, boundaries between different compounds/materials deposited are not clearly defined in practice and a composite coating is formed.

Discussion

The mixture of silicon and silicon carbide deposited has a maximum porosity of not more than 20% and comprises almost 100% beta- polytype silicon carbide owing to the high deposition temperature employed. Where aluminium and other similar impurities are permissible in the coating layer, the deposition temperature may be at most 1 800 degrees Celsius. For aluminium free coatings, the deposition temperature may be increased to at most 1 950 degrees Celsius.

Nitriding the mixture of silicon and silicon carbide deposited decreases the porosity thereof to between 0.5 % and 10 %. The additional amount of stoichiometric silicon carbide deposited further reduces the porosity of, and hence densities, the nitrided silicon and silicon carbide mixture by permeation of interstices defined in the nitrided silicon and silicon carbide mixture.

The thermal gradient imposed during the stages of deposition of silicon and silicon carbide mixture, nitriding and deposition of stoichiometric silicon carbide facilitates infiltration of open pores/interstices defined in the silicon and silicon carbide mixture by the nitrogen and the stoichiometric silicon carbide respectively. Similarly, the increasing gas pressure encourages infiltration of interstices. The increasing pressures employed during coating stages also serve to improve strength properties of the silicon nitride bonded silicon carbide. The step of depositing a further amount of silicon carbide as stoichiometric silicon carbide may be omitted in optimal conditions of deposition of silicon and silicon carbide mixture and nitriding steps. In each case, the deposition temperature reflects an ambient temperature, and the temperature of the surface of the nuclear fuel particle can typically be expected to be between about 20 degrees to about 200 degrees Celsius below a wall temperature of the coater apparatus. It is important that a single silicon carbide crystallographic type, viz. Beta-type silicon carbide, is deposited. Where the fraction of alpha-type silicon carbide is greater than 5 %, this leads to degradation of coating properties and reduced fission product- and, more particularly, silver isotope- retention in an operating temperature of above 1 700 degrees Celsius.

Treatment of the coated fuel particles by rapid heating followed by cooling in an argon environment additionally improves gas tightness and increases the diffusion boundary of each coated fuel particle. The Applicant believes that the method of the invention will yield coated nuclear fuel particles having improved fission product retention properties. More particularly, the Applicant believes that diffusion of silver isotope products through the coated fuel particle's protective layers will be reduced by the introduction of dense silicon nitride bonded silicon carbide. It is believed that the introduction of the silicon nitride bonded silicon carbide will not result in degradation of particle parameters such as strength, gas tightness, and the like. Further, it is believed that the improved fission product retention of the coated fuel particles of the invention will permit of increased nuclear reactor operational temperatures.

Example 2

A plurality of nuclear fuel particles was formed by atomisation of uranyl nitrate to form microspheres. The microspheres were then gelled and baked at high temperature, i.e. calcinated, to yield uranium dioxide particles, each to provide a kernel of fissile material for a nuclear fuel particle.

Carbon material and carbon compounds, such as, for example carbon materials and compounds selected from the group consisting of porous carbon, pyrolytic carbon, silicon carbide and silicon nitride bonded silicon carbide, were deposited on the surface of the uranium dioxide particles to yield generally spherical fuel particles of diameter about 1 millimetre. Some 15 000 of these coated fuel particles were mixed with 200 g of a matrix material comprising 80 % by mass graphite powder and 20 % by mass phenolic resin. The resulting mixture was pressed into spheres which were machined to a diameter of about 50 millimetres to yield nuclear fuel element cores including a plurality of fuel particles, incorporating a fissile material, dispersed in a matrix and having a mass of about 210 grams each.

A batch of such nuclear fuel element cores was suspended in a deposition chamber of a chemical vapour deposition reactor, the deposition chamber having an argon environment. To ensure free stable flotation of the fuel element cores within the coater volume, chemical precursors for deposition were transported through the deposition chamber at a rate of flow according to their temperature and pressure that was sufficient to maintain the fuel element cores in a suspended condition. Graphite was deposited on the core to thickness of about 5 millimetres. A methylchlorosilane source was then connected to an inlet end of the deposition chamber and the temperature was raised to 1 375 degrees Celsius. A mixture comprising silicon and silicon carbide was deposited to a thickness of about 240 micrometres, by the decomposition of the methylchlorosilane, on the surfaces of the fuel element cores. The deposition was carried out a pressure of 1.7 kPa. The coated fuel element cores were then nitrided by furnacing in a nitrogen atmosphere at a temperature of 1800 degrees Celsius and a pressure of 1.8 kPa to yield a mixture of silicon carbide and silicon nitride crystals. The coated fuel element cores were treated in the nitrogen atmosphere for 4.5 hours to permit nitrogen to permeate the full depth of the silicon and silicon carbide to yield silicon nitride bonded silicon carbide.

Stoichiometric silicon carbide was thereafter deposited on the silicon nitride bonded silicon carbide at a temperature of 1820 degrees Celsius and a pressure of 1.83 kPa.

The batch of fuel elements was finally treated in an argon environment at a pressure of 300 MPa, in which they were heated rapidly by use of a microwave heater to a surface temperature of 1 800 degrees Celsius.

This was followed by stable cooling to room temperature for at least four hours.

Reference is made to Figure 2 of the drawings, in which reference numeral 30 refers generally to a coated nuclear fuel element of the invention, prepared in accordance with Example 2 above. The coated fuel element 30 includes a core 31 of spherical form and having a diameter of about 50 millimetres. The core 31 includes of the order of 15 000 coated fuel particles 32, each having a kernel of fissile material (not shown), dispersed in a matrix 34. The matrix 34 comprises graphite powder and phenolic resin. About 5 millimetres of graphite 36 is deposited on the core 31 followed by silicon nitride bonded silicon carbide 38 to a thickness of about 240 micrometres.

Discussion

It is believed that the silicon nitride bonded silicon carbide coating of the fuel elements will serve to reduce diffusion of uranium- containing fission products into the coolant or working fluid of a nuclear reactor employing the fuel elements as its nuclear fuel. Further, due to the mechanical and working fluid (typically helium) wear resistance properties of the silicon nitride bonded silicon carbide protective outer layer, the production of irradiated dust as a result of fuel element wear will be reduced, which in turn will permit of the elimination or reduction of fuel sphere erosion factors from nuclear reactor safety calculations. Simulations show that the number of damaged fuel elements will be reduced ten-million fold and that the probability of fuel element disintegration or cracking will be reduced one hundred times, as a result of the high contact pressure resistance of the silicon nitride bonded silicon carbide protective outer layer.

The Applicant believes that the method of the invention will yield nuclear fuel elements having improved fission product retention properties. Further, it is believed that the implementation of a silicon nitride bonded silicon carbide layer as an outer protective coating on the spherical nuclear fuel elements will result in increased fuel element internal temperatures which in turn will influence the heat transfer to the working fluid of the nuclear reactor during operation thereof. It is believed that the resistance to oxidation, in the event of air ingress into the nuclear reactor, of the fuel elements will be improved in that a silicon oxide layer will form on the fuel element surface which will inhibit further oxidation and limit oxidation of the matrix material over extended periods of air exposure.

Claims

CLAIMS:

1. A method of preparing a nuclear fuel, which method includes the step of depositing a coating including silicon nitride bonded silicon carbide on a nuclear fuel surface.

2. A method as claimed in Claim 1 , which includes the steps of, at a temperature of between 1 300 degrees Celsius and 1 950 degrees Celsius, depositing a mixture of silicon and silicon carbide on a nuclear fuel surface, and nitriding the nuclear fuel surface.

3. A method as claimed in Claim 2, in which the mixture of silicon and silicon carbide is deposited by at least one technique selected from the group of deposition techniques consisting of chemical vapour deposition, sublimation growth, dipping and chemical bath deposition.

4. A method as claimed in Claim 2 or Claim 3, in which the nuclear fuel surface is that of a nuclear fuel particle, which includes a kernel of fissile material.

5. A method as claimed in Claim 2 or Claim 3, in which the nuclear fuel surface is that of a nuclear fuel element, including a core comprising a plurality of fuel particles, each having a kernel of fissile material, dispersed in a matrix.

6. A method as claimed in any one of Claims 2 to 5, inclusive, in which the mixture of silicon and silicon carbide is deposited at a temperature of between 1 370 degrees Celsius and 1 800 degrees Celsius.

7. A method as claimed in Claim 6, which includes increasing the temperature during the nitriding step to a temperature of between 1600 degrees Celsius and 1850 degrees Celsius.

8. A method as claimed in any one of Claims 2 to 7, inclusive, in which the step of depositing the mixture of silicon and silicon carbide is carried out at a pressure of between 1.1 kPa and 8 kPa.

9. A method as claimed in Claim 8, which includes increasing the pressure during the step of nitriding of the nuclear fuel surface to a pressure of between 1.4 kPa and 10 kPa.

10. A method as claimed in any one of Claims 2 to 9, inclusive, which includes the step of increasing the density of the nitrided mixture of silicon and silicon carbide.

11. A method as claimed in Claim 10, which includes depositing a further amount of silicon carbide on the nitrided mixture of silicon and silicon carbide.

12. A method as claimed in Claim 11 , in which the further amount of silicon carbide deposited is stoichiometric silicon carbide.

13. A method as claimed in Claim 4, which includes the prior step of forming a plurality of kernels of fissile material by atomising a uranyl nitrate solution to form microparticles, followed by baking the microparticles at high temperature each to provide a kernel of uranium dioxide.

14. A method as claimed in Claim 13, which includes depositing at least one carbon material or compound, selected from the group consisting of porous carbon, pyrolytic carbon and silicon carbide, around each kernel of fissile material.

15. A method as claimed in Claim 14, which includes the steps of, in sequence, depositing porous carbon on the surface of the kernel of fissile material, depositing pyrolytic carbon around the kernel, depositing a mixture comprising silicon and silicon carbide around the kernel, nitriding the silicon and silicon carbide deposited around the kernel to give a nitrided mixture of silicon and silicon carbide, and depositing a further amount of silicon carbide around the kernel.

16. A method as claimed in Claim 15, in which a further amount of pyrolytic carbon is thereafter deposited around the kernel.

17. A method as claimed in Claim 5, which includes the prior step of forming the core by mixing a plurality of fuel particles, each having a kernel of fissile material, with a matrix material and pressing the mixture into spheres.

18. A method as claimed in Claim 17, in which the matrix material includes graphite powder and phenolic resin.

19. A coated nuclear fuel particle which includes a nuclear fuel particle, having a kernel of a fissile material; and a coating including silicon nitride bonded silicon carbide deposited around the nuclear fuel particle.

20. A nuclear fuel particle as claimed in Claim 19, in which the silicon nitride bonded silicon carbide deposited has a thickness of between 1 micrometre and 35 micrometres.

21. A nuclear fuel particle as claimed in Claim 19 or Claim 20, which has at least one carbon material or compound, selected from the group consisting of porous carbon, pyrolytic carbon and silicon carbide, deposited around the kernel.

22. A nuclear fuel particle as claimed in Claim 21 , which has deposited around the kernel, in sequence, porous carbon, pyrolytic carbon and silicon nitride bonded silicon carbide.

23. A nuclear fuel particle as claimed in Claim 22, in which pyrolytic carbon is thereafter deposited around the kernel.

24. A nuclear fuel element which includes a core comprising a plurality of coated nuclear fuel particles, as claimed in any one of Claims 19 to 23, inclusive, dispersed in a matrix.

25. A nuclear fuel element which includes a core comprising a plurality of fuel particles, each including a kernel of fissile material, dispersed in a matrix; and a coating including silicon nitride bonded silicon carbide deposited around the core.

26. A nuclear fuel element as claimed in Claim 25, in which each fuel particle has a kernel of fissile material surrounded by at least one coating incorporating a carbon material or compound, selected from the group consisting of porous carbon, pyrolytic carbon, silicon carbide and silicon nitride bonded silicon carbide, deposited thereon.

27. A nuclear fuel element as claimed in Claim 25 or Claim 26, in which the matrix includes graphite.

28. A nuclear fuel element as claimed in Claim 27, in which the matrix comprises graphite powder and phenolic resin.