Patent Number: 
Section: claims

1. A method for improving design or operation of a nuclear reactor having at least one control rod that falls by gravity toward a core of the nuclear reactor, the method comprising:determining an actual insertion time value when the control rod falls by gravity inside the nuclear reactor;performing a simulation of a control rod falling by gravity toward a core of the nuclear reactor, said performing including:three-dimensionally modeling an inside wall of the nuclear reactor, the control rod accommodated inside the nuclear reactor, a guide tube guiding the control rod, an impact absorption tube located at the bottom of the guide tube with a diameter smaller than the diameter of the guide tube, a first flow hole located in the inside wall of the guide tube, and a second flow hole located at the bottom of the impact absorption tube;configuring a flow field by dividing the inside of the nuclear reactor into a plurality of cells, wherein the flow field is configured by a variable grid system comprising a plurality of variable cells having a configuration that changes as the control rod changes location in the path along which the control rod passes, and by an aligned grid system comprising a plurality of fixed cells which maintain their configuration regardless of the location change of the control rod;calculating a first simulation estimated value for the insertion time until the control rod is inserted into the impact absorption tube, wherein calculating the first simulation estimated value is determined by analyzing a thermal-hydraulic phenomenon using three-dimensional computational fluid dynamics (CFD) codes; andcalculating a second simulation estimated value, wherein the second simulation estimated value is determined by changing a size of one or more of the variable cells and/or a size of one or more of the fixed cells, when the first simulation estimated value is compared with the determined actual insertion time value, and an error between the first simulation estimated value and the actual insertion time value is determined to be out of a reference range; andusing the second simulation estimated value to change the size of the one or more variable cells and/or a size of one or more of the fixed cells, when the second simulation estimated value is compared with the determined actual insertion time value, and an error between the second simulation estimated value and the actual insertion time value is determined to be out of a reference range. 2. The method of claim 1, further comprising calculating a hydraulic drag of the control rod caused by water filling inside of the nuclear reactor when the control rod falls. 3. The method of claim 1, wherein said analyzing includes consideration of friction force between water and the control rod and weight of the control rod when the control rod falls. 4. The method of claim 1, wherein the reference range is set in such a way that the error between the estimated value and the actual value lies within 5% of the reference range. 5. The method of claim 1, wherein the thermal-hydraulic phenomenon comprises a pressure change, a temperature change, and a density change in the water around the control rod when the control rod falls.