Patent Number: 
Section: claims

1. Method of manufacturing nuclear elements from ceramic balls comprising:a) placing ceramic nuclear fuel balls in a container made from ultra-porous material in order to form a stack of nuclear fuel balls,b) performing densification of the stack of nuclear fuel balls by chemical vapour infiltration of at least one chemical species, wherein performing densification comprises passing the at least one chemical species in vapor form by three-dimensional diffusion through the bulk of the ultra-porous material, and wherein the chemical vapour infiltration is three-dimensional, such that the entire surface of the stack of nuclear balls is evenly exposed to the chemical species and such that a simultaneous infiltration takes place in the whole stack of nuclear balls, andc) removing the container;wherein the nuclear fuel element thus produced has a composite structure comprising a stack of nuclear fuel balls, such that the interstitial space between the nuclear fuel balls is at least partially filled with a matrix formed by said at least one chemical species, so that the nuclear fuel balls are held together by the matrix as a single solid unit and the compactness of the nuclear fuel element is of at least 90%. 2. Method according to claim 1, in which step c) takes place during step b). 3. Method according to claim 1, in which step b) comprises the infiltration of an organometallic precursor of the MethylTrichloroSilane (MTS) type leading to the obtaining of a silicon carbide matrix. 4. Method according to claim 1, in which the removal of the container during step c) takes place by polishing or machining. 5. Method according to claim 1, in which step b) comprises a first densification substep of 15 hours before the removal of the container and a second densification substep of 15 hours after the removal of the container and takes place at a temperature of approximately 1000° C. 6. Nuclear fuel element obtained by:a) placing ceramic nuclear fuel balls, comprising fissile material, in a container made from ultra-porous material in order to form a stack of nuclear fuel balls,b) performing densification of the stack of nuclear fuel balls by chemical vapour infiltration of at least one chemical species, wherein performing densification comprises passing the at least one chemical species in vapor form by three-dimensional diffusion through the bulk of the ultra-porous material, such that the entire surface of the stack of nuclear balls is evenly exposed to the chemical species and such that a simultaneous infiltration takes place in the whole stack of nuclear balls, andc) removing the container;wherein the nuclear fuel element has a composite, such that the interstitial space between the nuclear fuel balls is at least partially filled with a matrix formed by said at least one chemical species, so that the nuclear fuel balls are held together by the matrix as a single solid unit,wherein the unit of power per unit of volume is proportional to the concentration per unit volume of fissile material of the nuclear fuel element, the unit of power per unit of volume being proportional to the ratio between the quantity of fissile material contained in the fuel element and the volume of the fuel balls, andwherein the nuclear fuel element has a compactness of at least 90%. 7. Nuclear fuel element according to claim 6, in which the balls all have substantially the same diameter. 8. Nuclear fuel element according to claim 6, in which the step b) comprises the infiltration of an organometallic precursor of the MethylTrichloroSilane (MTS) type leading to the obtaining of a silicon carbide matrix. 9. Container made from ultra-porous material for implementing a method of manufacturing nuclear elements from ceramic balls comprising the steps of:a) placing ceramic nuclear fuel balls in the container in order to form a stack of the ceramic balls,b) densification of the stack of the ceramic balls by chemical vapour infiltration of at least one chemical species, comprising passing the at least one chemical species in vapor form by three-dimensional diffusion through the bulk of the ultra-porous material, such that the entire surface of the stack of the ceramic balls is evenly exposed to the chemical species, andc) removal of the container, said container comprising a coating that is anti-adherent vis-à-vis a densification chemical species. 10. Container according to claim 9, in which the ultra-porous material has a porosity of around 98%. 11. Container according to claim 9, in which the porous material is a carbon foam. 12. Container according to claim 9, cylindrical in shape with a circular cross section comprising a bottom at one end. 13. Container according to claim 11, with an outside diameter of between 9 mm and 11 mm, a height of between 12 mm and 14 mm, a wall thickness of between 0.4 mm and 1 mm, and an internal depth of around 1 mm. 14. Container according to claim 9, produced by machining. 15. The container of claim 9, wherein the densification chemical species is powdery boron nitride.