This invention relates to a method of preparing nuclear fuel including the step of depositing at least two adjacent series of layers ( 16, 18 ) around a kernel ( 12 ) of fissile material, each series comprising a layer of pyrolytic carbon ( 16 ) contiguous with a layer of silicon carbide ( 18 ) and each layer ( 16, 18 ) having a thickness of at most ( 10 ) micrometers, such that alternate layers of ( 16, 18 ) of pyrolytic carbon and silicon carbide are deposited around the kernel ( 12 ). The invention extends to a nuclear fuel element ( 10 ).
1. A method of preparing a spherical nuclear fuel particle, which method includes the step of depositing at least two adjacent series of spherically continuous layers around a kernel of uranium dioxide to form a spherical nuclear fuel particle, each series comprising a layer of pyrolytic carbon contiguous with a layer of silicon carbide and each layer having a thickness of at most 9 micrometers, with alternate layers of pyrolytic carbon and silicon carbide thus being deposited around the kernel.
2. The method as claimed in claim 1 , in which each layer has a thickness of between 3 micrometers and 9 micrometers.
3. The method as claimed in claim 2 , in which each layer of silicon carbide has a thickness of between 3 micrometers and 6 micrometers.
4. The method as claimed in claim 2 , in which each layer of pyrolytic carbon has a thickness of between 4 micrometers and 9 micrometers.
5. The method as claimed in claim 1 , in which the layers are deposited by chemical vapor deposition techniques.
6. The method as claimed in claim 5 , in which the deposition of layers is carried out at a temperature of at least 1,000 degrees Celsius in an argon environment.
7. The method as claimed in claim 5 , in which the deposition of layers is carried out at a pressure of between 1.3 kPa and 2.5 kPa.
8. The method as claimed in claim 7 , in which the deposition of layers is carried out at a pressure of 1.7 kPa.
9. The method as claimed in claim 1 , in which the silicon carbide deposited is of the beta polytype.
10. The method according to claim 1 , in which the kernel has a diameter of 500 micrometers.
11. A method of preparing a spherical nuclear fuel particle, which method includes the step of depositing at least two adjacent series of spherically continuous layers around a kernel of uranium dioxide to form a spherical nuclear fuel particle, each series comprising a layer of pyrolytic carbon contiguous with a layer of silicon carbide and each layer having a thickness of at most 9 micrometers, with alternate layers of pyrolytic carbon and silicon carbide thus being deposited around the kernel, with the deposition of the pyrolytic carbon and silicon carbide layers taking place as a continuous process by switching between the chemical precursors for deposition of the pyrolytic carbon and silicon carbide layers respectively such that transition zones comprising pyrolytic carbon mixed with silicon carbide are formed between each layer of pyrolytic carbon and a contiguous layer of silicon carbide.
12. The method as claimed in claim 11 , in which the transition zones between each layer of pyrolytic carbon and its contiguous layer of silicon carbide have a thickness of between 0.5 micrometers and 2 micrometers.
13. A method of preparing a spherical nuclear fuel particle, which method includes the step of depositing at least two adjacent series of spherically continuous layers around a kernel of uranium dioxide to form a spherical nuclear fuel particle, each series comprising a layer of pyrolytic carbon contiguous with a layer of silicon carbide and each layer having a thickness of at most 9 micrometers, with alternate layers of pyrolytic carbon and silicon carbide thus being deposited around the kernel, the method including a prior step, preceding the step of depositing at least two adjacent series of spherically continuous layers around the kernel of uranium dioxide, of forming a plurality of kernels of uranium dioxide by atomising a uranyl nitrate solution to form microparticles, followed by baking the microparticles at high temperature, to yield uranium dioxide microparticles.