Patent Number: 
Section: claims

1. A method for controlling an axial power distribution in a nuclear reactor having a core and a control rod, comprising:(a) calculating an axial offset of a current power distribution (AOP) using Expression (1), an axial offset of a power distribution capable of providing a current xenon concentration distribution under an equilibrium condition (AOX) using Expression (2), and an axial offset of a power distribution capable of providing a current iodine concentration distribution under an equilibrium condition (AOI) using Expression (3),AOP=(PT−PB)/(PT+PB)  Expression (1).AOX=(PTX−PBX)/(PTX+PBX)  Expression (2)AOI=(PTI−PBI)/(PTI+FBI)  Expression (3)WhereinPT is a relative power in an upper half of the core,PB is a relative power in an lower half of the core,PTX is a relative power in the upper half of the core capable of providing a current xenon concentration under an equilibrium condition,PBX is a relative power in the lower half of the core capable of providing a current xenon concentration under an equilibrium condition,PTI is a relative power in the upper half of the core capable of providing a current iodine concentration under an equilibrium condition, andPBI is a relative power in the lower half of the core capable of providing a current iodine concentration under an equilibrium condition;(b) calculating a parameter DAOPX (=AOP−AOX) and a parameter DAOIX (=AOI−AOX);(c) displaying a trajectory having a plot point which plots the parameter DAOPX on X axis and the parameter DAOIX on Y axis, whereby the plot point forms an ellipse having a major axis;(d) determining if the axial offset of the current power distribution (AOP) exceeds an allowable range;(e) setting off alarm when the axial offset of the current power distribution (AOP) exceeds the allowable range and the plot point has deviated from the major axis; and(f) moving the control rod after the alarm is set off so that the plot point is guided back to the major axis, whereby the axial power distribution is controlled, and whereby the xenon oscillation is controlled at the same time. 2. A nontransitory computer readable medium having a computer program, which when executed by a computer, causes the computer to perform the method according to claim 1. 3. The method according to claim 1, further comprising in step (f) calculating a distance on the X axis from the deviated plot point to the major axis, calculating a movement volume of the control rod based on the distance, and giving an operation signal of inserting the control rod by the movement volume to a control rod driving mechanism. 4. The method of claim 1, wherein the ellipse has an origin on the major axis, and wherein plot point is guided to the major axis excluding the origin. 5. A system for controlling an axial power distribution in a nuclear reactor having a core and a control rod, comprising:(a) an axial offset calculation unit that calculates an axial offset of a current power distribution (AOP) using Expression (1), an axial offset of a power distribution capable of providing a current xenon distribution under an equilibrium condition (AOX) using Expression (2), and an axial offset of a power distribution capable of providing a current iodine distribution under an equilibrium condition (AOI) using Expression (3),AOP=(PT−PB)/(PT+PB)  Expression (1)AOX=(PTX−PBX)/(PTX+PBX)  Expression (2)AOI=(PTI−PBI)/(PTI+PBI)  Expression (3)WhereinPT is a relative power in an upper half of the core,PB is a relative power in an lower half of the core,PTX is a relative power in the upper half of the core capable of providing a current xenon concentration under an equilibrium condition,PBX is a relative power in the lower half of the core capable of providing a current xenon concentration under an equilibrium condition,PTI is a relative power in the upper half of the core capable of providing a current iodine concentration under an equilibrium condition, andPBI is a relative power in the lower half of the core capable of providing a current iodine concentration under an equilibrium condition;(b) a parameter calculation unit that calculates a parameter DAOPX (=AOP−AOX) and a parameter DAOIX (=AOI−AOX);(c) a trajectory display unit that displays a trajectory having a plot point, wherein the plot point plots the parameter DAOPX on X axis and the parameter (DAOIX) on the Y axis, and forms an ellipse having a major axis;(d) an allowable range excess determination unit that determines if the axial offset of the current power distribution (AOP) exceeds an allowable range;(e) an alarming unit that sets off alarm when the axial offset of the current power distribution (AOP) exceeds the allowable range and the plot has deviated from the major axis; and(f) a control rod moving unit that moves the control rod after the alarm is set off so that the plot point is guided back to the major axis, whereby the axial power distribution is controlled, and whereby the xenon oscillation is controlled at the same time. 6. The system according to claim 5, characterized in that the control rod moving unit further calculates a distance on the X axis from the deviated plot point to the major axis, calculates a movement volume of the control rod based on the distance, and gives an operation signal of inserting the control rod by the movement volume to a control rod driving mechanism. 7. The system of claim 5, wherein the ellipse has an origin on the major axis, and wherein the plot point is guided to the major axis excluding the origin.