Patent Number: 040574636
Section: claims

1. A method of operating a nuclear reactor having a reactive core including fissile material with an axial dimension and adjustable control means for controlling the reactivity within the core, comprising the steps of: monitoring a parameter representative of the power generated within the core at a first and second axial location;  obtaining from the core power parameters measured at said first and second axial locations a representation of the axial power distribution within the core; and  adjusting the control means to produce an axial power distribution to maintain a uniform and symmetric xenon distribution above and below substantially the center of the core over a substantial axial length of the core during normal reactor operation including load follow. 2. The method of claim 1 wherein the core is divided axially into upper and lower halves corresponding to said first and second monitored locations. 3. The method of claim 2 wherein the representation of the axial power distribution corresponds to the axial offset of the core. 4. The method of claim 3 wherein the adjusting step activates the control means to maintain the axial offset substantially equal to a predetermined target value throughout reactor operation including changes in reactor power. 5. The method of claim 4 wherein the control means includes control rods and the target value is obtained from the step of determining the axial offset at full power with equilibrium xenon and all control rods removed from the fuel region of the core. 6. The method of claim 4 wherein the target value is determined periodically. 7. The method of claim 6 wherein the interval between the determination of the target value is substantially equal to an equivalent full power month. 8. The method of claim 4 wherein the axial power distribution can vary during reactor operation to have a flux difference within a band of approximately plus or minus 5 percent of a predetermined value for the flux difference corresponding to the target value of the axial offset. 9. The method of claim 1 wherein the control means includes a plurality of elongated control rods comprising neutron absorbing material which are axially aligned with the core along the longitudinal control rod dimension and longitudinally movable into and out of the core with the longitudinal length of the rods at least substantially equal to the axis length of the core and a core cooling medium or moderator having a controlled variable concentration of a neutron absorbing element. 10. The method of claim 9 wherein the changes in reactor power are in part controlled to a desired level by variation of the concentration of the neutron absorbing element and wherein said adjusting step positions the control rods to maintain the substantially symmetric axial power distribution. 11. The method of claim 10 wherein the movement of the control rods within the core are approximately linearly proportional to changes in the power output of the core. 12. The method of claim 9 wherein the control means includes part length elongated control rods comprising neutron absorbing material which are axially aligned with the core along the longitudinal dimension and longitudinally movable within the core with the longitudinal length of the part length rods substantially less than the axial length of the core, wherein the control rods are adjusted for the reactivity control associated with changes in core power output and the adjusting step positions the part length rods to maintain the substantially symmetric axial power distribution. 13. The method of claim 12 wherein as a byproduct of the fission reaction of the fissile material the element xenon is created within the core having a neutron absorption property and wherein said adjusting step controls the concentration of the neutron absorbing element within the core to control the reactivity change due to xenon buildup or depletion corresponding to and associated with changes in core power output. 14. The method of claim 12 wherein the part length rod neutron absorbing capability is substantially equal to the control rod absorption capability of equivalent length. 15. The method of claim 12 wherein said adjusting step employs the part length rods for axial power distribution control for a power operating period approximately equal to less than 60 percent of every 30 equivalent full power days. 16. The method of claim 12 wherein the part length rods are inserted to a distance of approximately 30 percent from the bottom of the core at full power operation and the insertion linearly increases to approximately 10% from the bottom of the core at 50 percent core power.