Patent Number: 050892100
Section: claims

1. In a boiling water reactor core having a plurality of vertically upstanding fuel bundles; each fuel bundle containing longitudinally extending sealed rods with fissile material therein; a surrounding channel for containing water flow within each said fuel bundle from water exterior of said fuel bundle; water moderator flowing in the confining channel from the bottom of said fuel bundle to the top of said fuel bundle for producing steam and moderating neutrons to a thermal energy state for producing continuing nuclear reaction in said fissile material;  said water moderator exterior of said confining channels for defining a core bypass zone having relatively high concentrations of water for moderation of neutrons from high energy state neutrons to a thermal energy state for the continuation of said nuclear reaction;  a plurality of said fuel rods including a component of fissile material including recovered plutonium and uranium distributed over an axial extent of said fuel assembly;  at least some of said fuel rods containing a component of neutron absorbing material for controlling excess reactivity imparted by said recovered plutonium;  the improvement in said distribution of fissile material and neutron absorbing material comprising:  said fissile material including a mixture of uranium and recovered plutonium in rods of said fuel bundle at locations other than the corners of said fuel bundle; and,  neutron absorbing material being located in rods of said fuel bundle at rod locations adjacent the corners of said fuel bundles whereby said neutron absorbing material has decreased shielding from said plutonium and maximum exposure to thermal neutrons for shaping said cold reactivity shutdown zone in said fuel bundle.  at least some of said fuel rods containing a component of neutron absorbing material for controlling excess reactivity imparted by said recovered plutonium; the improvement in the distribution of fissile material and neutron absorbing material comprising;  said fissile material including a mixture of uranium and recovered plutonium in rods of said fuel bundle at locations other than the corners of said fuel bundle; and, said neutron absorbing material being located in rods of said fuel bundle at rod locations in the corners of said fuel bundle; said neutron absorbing material having an axial distribution characterized by an enhancement in the axial zone of said fuel assembly, designated the cold shutdown zone, corresponding to at least a portion of the axial region of said core where the cold reactivity peaks, the aggregate amount of neutron absorbing material in said cold shutdown zone of said fuel assembly being greater than the aggregate amount of neutron absorbing material immediately above and below said cold shutdown zone whereby the cold shutdown reactivity is reduced relative to the cold shutdown reactivity in zones immediately above and below said cold shutdown control zone, said cold shutdown control zone having an axial extent measured from the bottom of the fuel assembly in the range between 60% and 90% of the height of said fissile material in said fuel assembly. 2. The invention of claim 1 and wherein said rods containing said neutron absorbing material do not contain fissile material including a mixture of uranium and recovered plutonium. 3. The invention of claim 1 and wherein said rods at the corners of said fuel bundle having a weighted axial distribution to define exclusively a "cold shutdown reactivity zone" of said fuel bundle. 4. The invention of claim 1 and wherein said fuel rods containing a component of fissile material including recovered plutonium and uranium include partial length rods. 5. In a boiling water reactor having a plurality of vertically upstanding fuel bundles; each fuel bundle containing longitudinally extending sealed rods with fissile material therein; a surrounding channel for containing water flow within each said fuel bundle from water exterior of said fuel bundle; water moderator flowing in the confined channel in the bottom of said fuel bundle to the top of said fuel bundle for producing steam and moderating neutrons to a thermal energy state for producing continuing nuclear reaction in said fissile material; said water moderator exterior of said confining channels for defining a core by-pass zone having relatively high concentration of water for moderation of neutrons from a high energy state to a thermal energy state for the continuation of said nuclear reaction; each rod including a component of fissile material distributed over an axial length of said fuel assembly; 6. The invention of claim 5 and wherein said fuel bundle includes an 8.times.8 array of fuel rods. 7. The invention of claim 5 and wherein said fuel bundle includes a 9.times.9 array of fuel rods. 8. The invention of claim 5 and wherein at least some of the rods at locations other than the corner of said fuel bundle are partial length rods. 9. The invention of claim 5 and including a fuel bundle having one corner rod placed in said bundle without neutron absorbing material being disposed in said corner rod.