Patent Number: 
Section: claims

1. A mixed oxide nuclear fuel assembly employable for a thermal neutron/light water reactor comprising: a plurality of nuclear fuel rods arranged in a matrix in a channel box and means to allow flowing light water acting as both a moderator and a coolant in spaces between said nuclear fuel rods, said mixed oxide nuclear fuel assembly being positionable in a reactor core, wherein:  20 through 40% of the quantity of said nuclear fuel rods are mixed oxide nuclear fuel rods, each of which contains fissionable plutoniums having only one kind of enrichment grade selected from a range of 5 through 15 weight %, and  80 through 60% of the quantity of said nuclear fuel rods are UO 2  fuel rods containing U isotopes at least including U 235 . 2. A mixed oxide nuclear fuel assembly employable for a thermal neutron/light water reactor in accordance with  claim 1 , wherein: claim 1 20 through 25% of the quantity of said nuclear fuel rods are mixed oxide nuclear fuel rods. 3. A mixed oxide nuclear fuel assembly employable for a thermal neutron/light water reactor in accordance with  claim 2 , further comprising: claim 2 a plurality of gadolinium rods containing U isotopes at least including U 235  and gadolinium, said gadolinium rods being positionable at an area where the effects of said moderator are strong. 4. A mixed oxide nuclear fuel assembly employable for a thermal neutron/light water reactor in accordance with  claim 2 , wherein: claim 2 the enrichment grade of said U 235  is selected from a range of 1 through 5 weight %. 5. A mixed oxide nuclear fuel assembly employable for a thermal neutron/light water reactor in accordance with  claim 2 , wherein: claim 2 said mixed oxide nuclear fuel rods are positionable at an area where the effects of said moderator are weak. 6. A mixed oxide nuclear fuel assembly employable for a thermal neutron/light water reactor in accordance with  claim 2 , further comprising: claim 2 a water rod arranged in an area where effects of said moderator are weak. 7. A mixed oxide nuclear fuel assembly employable for a thermal neutron/light water reactor in accordance with  claim 1 , wherein: claim 1 each of said mixed oxide nuclear fuel rods contains fissionable plutoniums having only one kind of enrichment grade selected from a range of 10 through 15 weight %. 8. A mixed oxide nuclear fuel assembly employable for a thermal neutron/light water reactor in accordance with  claim 7 , further comprising: claim 7 a plurality of gadolinium rods containing U isotopes at least including U 235  and gadolinium, said gadolinium rods being positionable at an area where the effects of said moderator are strong. 9. A mixed oxide nuclear fuel assembly employable for a thermal neutron/light water reactor in accordance with  claim 7 , wherein: claim 7 the enrichment grade of said U 235  is selected from a range of 1 through 5 weight %. 10. A mixed oxide nuclear fuel assembly employable for a thermal neutron/light water reactor in accordance with  claim 7 , wherein: claim 7 said mixed oxide nuclear fuel rods are positionable at an area where the effects of said moderator are weak. 11. A mixed oxide nuclear fuel assembly employable for a thermal neutron/light water reactor in accordance with  claim 7 , further comprising: claim 7 a water rod arranged in an area where effects of said moderator are weak. 12. A mixed oxide nuclear fuel assembly employable for a thermal neutron/light water reactor in accordance with  claim 1 , wherein: claim 1 the enrichment grade of said U 235  is selected from a range of 1 through 5 weight %. 13. A mixed oxide nuclear fuel assembly employable for a thermal neutron/light water reactor in accordance with  claim 12 , further comprising: claim 12 a plurality of gadolinium rods containing U isotopes at least including U 235  and gadolinium, said gadolinium rods being positionable at an area where the effects of said moderator are strong. 14. A mixed oxide nuclear fuel assembly employable for a thermal neutron/light water reactor in accordance with  claim 12 , further comprising: claim 12 a water rod arranged in an area where effects of said moderator are weak. 15. A mixed oxide nuclear fuel assembly employable for a thermal neutron/light water reactor in accordance with  claim 1 , wherein: claim 1 said mixed oxide nuclear fuel rods are positionable at an area where the effects of said moderator are weak. 16. A mixed oxide nuclear fuel assembly employable for a thermal neutron/light water reactor in accordance with  claim 15 , further comprising: claim 15 a plurality of gadolinium rods containing U isotopes at least including U 235  and gadolinium, said gadolinium rods being positionable at an area where the effects of said moderator are strong. 17. A mixed oxide nuclear fuel assembly employable for a thermal neutron/light water reactor in accordance with  claim 15 , wherein: claim 15 the enrichment grade of said U 235  is selected from a range of 1 through 5 weight %. 18. A mixed oxide nuclear fuel assembly employable for a thermal neutron/light water reactor in accordance with  claim 15 , further comprising: claim 15 a water rod arranged in an area where effects of said moderator are weak. 19. A mixed oxide nuclear fuel assembly employable for a thermal neutron/light water reactor in accordance with  claim 1 , further comprising: claim 1 a plurality of gadolinium rods containing U isotopes at least including U 235  and gadolinium, said gadolinium rods being positionable at an area where the effects of said moderator are strong. 20. A mixed oxide nuclear fuel assembly employable for a thermal neutron/light water reactor in accordance with  claim 1 , further comprising: claim 1 a water rod arranged in an area where effects of said moderator are weak.