Patent Number: 
Section: claims

1. A nuclear reactor core of the pressure tube kind in which a plurality of pressure tubes extend through a tank containing, in normal use, heavy water moderator, each of said plurality of pressure tubes comprising:a plurality of fuel elements held together in spaced relationship so as to permit the flow of coolant through spaces between adjacent fuel elements during normal operation, andeach of said fuel elements comprising a plurality of fuel pellets in a cladding comprising sapphire. 2. A nuclear reactor core according to claim 1, wherein the plurality of fuel elements form a bundle, said reactor core further comprising a plurality of support tabs secured to an outermost ring of the bundle of fuel elements to support the bundle in the pressure tube, the tabs comprising sapphire. 3. A nuclear reactor core according to claim 1, wherein each fuel element comprises fuel pellets in tubular cladding closed at each end with end caps. 4. A nuclear reactor core according to claim 3, wherein the tubular cladding and end caps are made of the same material. 5. A nuclear reactor core according to claim 4, wherein the end caps are joined to respective ends of the cladding by an aluminium oxy-nitride bond between respective juxtaposed surfaces of the cladding and end caps. 6. A nuclear reactor core according to claim 1, wherein the sapphire comprises single crystal sapphire. 7. The nuclear reactor core of claim 6 wherein the single crystal sapphire comprises an edge defined film fed growth formation. 8. The nuclear reactor core of claim 1 wherein each of said plurality of pressure tubes comprises a protective insulator sleeve comprising fused silica. 9. The nuclear reactor core according to claim 8, wherein the insulator sleeve has an interior liner or coating of sapphire. 10. A nuclear reactor core in which a plurality of pressure tubes extend through a tank containing, in normal use, heavy water moderator, each of said plurality of pressure tubes comprising: a plurality of fuel elements held together in spaced relationship so as to permit the flow of coolant through spaces between adjacent fuel elements during normal operation, and a protective insulator sleeve comprising fused silica, the fuel elements comprising fuel pellets in a cladding comprising sapphire. 11. A nuclear reactor core according to claim 10, wherein the insulator sleeve has an interior liner or coating comprising sapphire. 12. A nuclear reactor core according to claim 10, wherein the plurality of fuel elements form a bundle, said reactor core further comprising a plurality of support tabs secured to an outermost ring of the bundle of fuel elements to support the bundle in the pressure tube, the tabs comprising sapphire. 13. A nuclear reactor core according to claim 10, wherein the cladding is tubular and is closed at each end with end caps. 14. A nuclear reactor core according to claim 13, wherein the tubular cladding and end caps are made of the same material. 15. A nuclear reactor core according to claim 14, wherein the end caps are joined to respective ends of the cladding by an aluminium oxy-nitride bond between respective juxtaposed surfaces of the cladding and end caps. 16. A nuclear reactor core according to claim 15, wherein the sapphire comprises single crystal sapphire. 17. The nuclear reactor core of claim 16 wherein the single crystal sapphire comprises an edge defined film fed growth formation. 18. A fuel element for use in a nuclear reactor core in which a plurality of fuel elements are held together in spaced relationship so as to permit the flow of coolant through spaces between said plurality of fuel elements during normal operation, the fuel element comprising a plurality of fuel pellets in a cladding comprising sapphire. 19. A fuel element according to claim 18, wherein the cladding comprises tubular cladding closed at each end with end caps. 20. A fuel element according to claim 19, wherein the tubular cladding and end caps are made of the same said material. 21. A fuel element according to claim 20, wherein the end caps are joined to respective ends of the cladding by an aluminium oxy-nitride bond between respective juxtaposed surfaces of the cladding and end caps. 22. A fuel element according to claim 18, wherein the cladding material comprises single crystal sapphire. 23. The fuel element according to claim 22, wherein the single crystal sapphire of the cladding comprises an edge defined film fed growth formation.