Patent Number: 048184772
Section: claims

1. A nuclear reactor core comprising a plurality of generally cylindrical fuel rods, each of said fuel rods comprising a cladding containing a plurality of nuclear fuel pellets stacked end-to-end within said cladding, wherein substantially each fuel pellet of substantially all of the fuel rods is coated with a layer of a refractory material interposed between the fuel pellet and the cladding of sufficient thickness to effectively prevent direct contact between the pellet and the cladding, said layer comprising a burnable poison having a non-naturally occurring tailored isotopic composition adjusted to reduce, in a controlled fashion, initial excess reactivity in said reactor core. 2. The reactor core of claim 1 wherein said burnable poison comprises a boride. 3. The reactor core of claim 2 wherein said boride comprises zirconium diboride. 4. The reactor core of claim 3 wherein said boride contains boron which is depleted in isotope boron-10 with respect to natural boron. 5. The reactor core of claim 2 wherein said layer is on the order of 10-100 microns thick. 6. A nuclear fuel rod comprising a cladding tube having ends and end plugs arranged to close said ends, a plurality of fuel pellets stacked end-to-end within said cladding tube between said end plugs, substantially all of said fuel pellets being coated with a barrier layer comprising a refractory material of sufficient thickness to effectively prevent interaction between said pellets and said cladding tube, said refractory material comprising a burnable poison having a tailored, non-naturally occurring isotopic composition determined on the basis of said thickness. 7. The fuel rod of claim 6 wherein said isotopic composition comprises a boride containing boron-10 and boron-11 in controlled ratios. 8. The fuel rod of claim 7 wherein said boride comprises zirconium diboride. 9. The fuel rod of claim 7 wherein said boride is depleted in boron-10 with respect to natural boron. 10. The fuel rod of claim 6 wherein said layer is on the order of about 10-100 microns thick. 11. A method of controlling excess reactivity and preventing interaction between fuel pellets and fuel rod cladding in a nuclear reactor core having an initial excess reactivity comprising the steps of: providing a predetermined amount of a burnable poison having a naturally occurring isotopic composition;  tailoring said isotopic composition from said naturally occurring isotopic composition to effectively counteract said initial excess reactivity; and  applying a layer of refractory material to substantially all of said fuel pellets, wherein said layer comprises said predetermined amount of burnable poison and is disposed between the fuel pellets and the cladding, said layer being of sufficient thickness to effectively prevent interaction between said pellets and said cladding.  operating said reactor core to burn at least some of said burnable poison; and  supplying additional neutron absorbing material to said core to control excess reactivity.  determining an amount of burnable poison required to control said initial excess reactivity;  determining, based on a predetermined thickness of fuel pellet coating effective for preventing said interaction and said amount of burnable poison required, an isotopic composition of said burnable poison;  coating substantially all of the fuel pellets in said core with a layer of a refractory material of said predetermined thickness, said refractory material comprising said amount of burnable poison having said determined isotopic composition.  providing a predetermined amount of a burnable poison having an isotopic composition;  controlling said isotopic composition to effectively counteract said initial excess reactivity;  applying a layer of refractory material to substantially all of said fuel pellets, wherein said layer comprises said predetermined amount of burnable poison and is disposed between the fuel pellets and the cladding, said layer being of sufficient thickness to effectively prevent interaction between said pellets and said cladding; and  wherein said reactor core is divided into a plurality of regions distinguished by an enrichment value of nuclear fuel in said fuel pellets, and wherein said step of controlling further comprises adjusting the isotopic composition of said layer in accordance with the enrichment in each of said regions. 12. The method of claim 11 further comprising the steps of: 13. The method of claim 12 wherein said reactor is cooled by a flowing coolant and the step of supplying additional neutron absorbing material comprises supplying a chemical shim to said coolant after said burnable poison in said layer has been at least partially depleted. 14. The method of claim 11 wherein said burnable poison comprises a boride having a boron isotopic composition, and said step of controlling comprises either enriching or depleting an isotope boron-10 in said boride with respect to natural boron in at least some of said rods. 15. A method of controlling excess reactivity of a nuclear reactor core having an overall initial enrichment which creates an initial excess reactivity while preventing interaction between fuel pellets and fuel rod cladding in said core, comprising the steps of: 16. The method of claim 15 wherein said reactor core is uniformly enriched and said layer has a uniform isotopic composition. 17. The method of claim 15 wherein said reactor comprises at least two regions, each of said regions being characterized by a different initial enrichment, wherein said step of determining said isotopic composition further comprises determining said isotopic composition of the coatings for each of said regions based on said different initial enrichments. 18. The method of claim 15 wherein said fuel rods have a characteristic void space and wherein said fuel rods are initially pressurized, said method further comprising the step of adjusting said initial pressurization based on an amount of gas pressure released into said void space by said burnable poison during reactor operation. 19. The method of claim 15 wherein said predetermined thickness is between about 10 microns and 100 microns. 20. A nuclear reactor core comprising a plurality of generally cylindrical fuel rods, each of said fuel rods comprising a cladding containing a plurality of nuclear fuel pellets stacked end-to-end within said cladding, wherein substantially each fuel pellet of substantially all of the fuel rods is coated with a layer of a refractory material interposed between the fuel pellet and the cladding of sufficient thickness to effectively prevent direct contact between the pellet and the cladding, said layer comprising a burnable, boride poison having an isotopic composition adjusted to reduce excess reactivity in said reactor core; wherein a first portion of said plurality of fuel rods contain fuel pellets coated with zirconium diboride having natural boron and a second portion of said plurality of fuel rods contain fuel pellets coated with zirconium diboride depleted in isotope boron-10. 21. A nuclear reactor core comprising a plurality of generally cylindrical fuel rods, each of said fuel rods comprising a cladding containing a plurality of nuclear fuel pellets stacked end-to-end within said cladding, wherein substantially each fuel pellet of substantially all of the fuel rods is coated with a layer of a refractory material interposed between the fuel pellet and the cladding of sufficient thickness to effectively prevent direct contact between the pellet and the cladding, said layer comprising a burnable poison having an isotopic composition adjusted to reduce excess reactivity in said reactor core, and wherein said core is divided into a plurality of regions, each of said regions being characterized by fuel rods having fuel pellets of a different enrichment, wherein said poison isotopic composition is adjusted in accordance with the enrichment of each of said regions. 22. A method of controlling excess reactivity and preventing interaction between fuel pellets and fuel rod cladding in a nuclear reactor core having an initial excess reactivity comprising the steps of: