Patent Number: 
Section: claims

1. A nuclear reactor core comprising:nuclear fuel comprising Pu-239;a neutron moderator which behaves as an Einstein oscillator and as the temperature of the reactor increases said neutron moderator increases the energy of thermal neutrons into a Pu-239 neutron absorption resonance; anda neutron absorbing element with neutron absorption of 0.3 eV added to said nuclear reactor core, whereinsaid neutron absorbing element is different from a burnable poison used in the nuclear reactor core,said neutron absorbing element was added upon fueling the nuclear reactor core in an amount calculated to suppress, at any time during the life of the fuel, a reactivity gain with temperature due to said neutron moderator increasing the energy of thermal neutrons into a neutron absorption resonance of said Pu-239. 2. The nuclear reactor core of claim 1, wherein said neutron moderator comprises ZrHx, where x is between 1.5 and 1.7. 3. The nuclear reactor core of claim 1, wherein said neutron moderator comprises YH2. 4. The nuclear reactor core of claim 1, wherein said neutron moderator comprises TiH2. 5. The nuclear reactor core of claim 1, wherein said neutron moderator comprises ThH2. 6. The nuclear reactor core of claim 1, wherein said Pu-239 is 0.4 weight percent or more of said nuclear fuel. 7. The nuclear reactor core of claim 1, wherein said Pu-239 is in the range of 0.1 to 1.0 weight percent of said nuclear fuel. 8. The nuclear reactor core of claim 1, wherein said neutron absorbing element is erbium. 9. The nuclear reactor core of claim 1, wherein said Pu-239 is in the range of 1.0 to 5.0 weight percent of said nuclear fuel. 10. The nuclear reactor core of claim 1, wherein said Pu-239 is in the range of 5.0 to 10.0 weight percent of said nuclear fuel. 11. The nuclear reactor core of claim 1, wherein said Pu-239 is in the range of 10.0 to 20.0 weight percent of said nuclear fuel. 12. The nuclear reactor core of claim 1, wherein said Pu-239 is in the range of 20.0 to 100.0 weight percent of said nuclear fuel. 13. The nuclear reactor core of claim 1, wherein said neutron absorbing element has an absorption cross section of at least 100 barns. 14. The nuclear reactor core of claim 1, wherein said neutron absorbing element has an absorption cross section of at least 500 barns. 15. The nuclear reactor core of claim 1, wherein said neutron absorbing element has an absorption cross section of great than 1000 barns. 16. A method of controlling a nuclear reactor comprising:providing nuclear fuel comprising Pu-239 in a core of the nuclear reactor;providing a neutron moderator which behaves as an Einstein oscillator and as the temperature of the nuclear reactor increases said neutron moderator increases the energy of thermal neutrons into a neutron absorption resonance of said Pu-239; andadding a neutron absorbing element with neutron absorption of 0.3 eV to said core, whereinsaid neutron absorbing element is different from a burnable poison used in the core,said neutron absorbing element is added upon fueling the nuclear reactor core in an amount calculated to suppress, at any time during the life of said nuclear fuel, a reactivity gain with temperature due to said neutron moderator increasing the energy of thermal neutrons into said neutron absorption resonance of said Pu-239.