Patent Number: 
Section: claims

1. A spherical fuel element for high temperature pebble bed nuclear reactors which is a fuel-sphere core surrounded by a fuel-free graphite matrix shell seamlessly bonded with a sphere core, wherein the sphere core comprises an A 3 graphite matrix containing coated fuel particles homogenously distributed in the sphere core and embedded into the A 3 graphite matrix, and wherein the main components of the graphite matrix of the shell are natural graphite, graphitized petroleum coke and optionally binder coke, and wherein the fuel-free shell of the fuel element sphere contains at least one of silicon carbide (SiC) or zirconium carbide (ZrC) or both, and the shell has an average nominal thickness of 0.5 mm to 5.5 mm. 2. Fuel element sphere according to claim 1 characterized in that the average nominal thickness of the shell is ≧2 mm. 3. Fuel element sphere according to claim 1 characterized in that the average nominal thickness of the shell is in the range of from 3 mm to 5 mm. 4. Fuel element sphere according to claim 3 characterized in that the shell contains silicon carbide and the silicon carbide proportion in the fuel-free shell is in the range of from 6 to 14% by weight. 5. The fuel element sphere according to claim 3 characterized in that the shell contains zirconium carbide and the zirconium carbide proportion in the fuel-free shell is in the range of from 10 to 30% by weight. 6. Method for production of fuel element spheres according to claim 1 characterized by molding the fuel element spheres and utilizing a molding powder for the shell containing at least one of silicon or zirconium oxides. 7. Method according to claim 6 wherein the at least one of silicon and zirconium oxide is suspended in a solution of methanol and phenol formaldehyde resin and this suspension is homogenously mixed with a graphite powder comprising natural graphite and graphitized petroleum coke by kneading at room temperature. 8. Method according to claim 7, wherein the conversion of said oxide into the corresponding carbide is effected by annealing the fuel element spheres in vacuum (P<10−2 hPa) and a maximum temperature of 2000° C. 9. Method according to claim 6, wherein the conversion of said oxide into the corresponding carbide is effected by annealing the fuel element spheres in vacuum (P<10−2 hPa) and a maximum temperature of 2000° C. 10. Fuel element sphere according to claim 2 characterized in that the shell contains silicon carbide and the silicon carbide proportion in the fuel-free shell is in the range of from 6 to 14% by weight. 11. The fuel element sphere according to claim 2 characterized in that the shell contains zirconium carbide and the zirconium carbide proportion in the fuel-free shell is in the range of from 10 to 30% by weight. 12. Fuel element sphere according to claim 1 characterized in that the shell contains silicon carbide and the silicon carbide proportion in the fuel-free shell is in the range of from 6 to 14% by weight. 13. The fuel element sphere according to claim 1 characterized in that the shell contains zirconium carbide and the zirconium carbide proportion in the fuel-free shell is in the range of from 10 to 30% by weight. 14. Fuel element sphere according to claim 1 characterized in that the shell contains silicon carbide and the silicon carbide proportion in the fuel-free shell is in the range of from 8 to 12% by weight. 15. Fuel element sphere according to claim 1 characterized in that the shell contains silicon carbide and the silicon carbide proportion in the fuel-free shell is in the range of from 9 to 10% by weight. 16. The fuel element sphere according to claim 1 characterized in that the shell contains zirconium carbide and the zirconium carbide proportion in the fuel-free shell is in the range of from 19 to 25% by weight. 17. The fuel element sphere according to claim 1 characterized in that the shell contains zirconium carbide and the zirconium carbide proportion in the fuel-free shell is in the range of from 20 to 23% by weight.