Patent Number: 
Section: claims

1. A method for operating a pressurized water reactor operation, comprising: preparing a fuel assembly including a fresh fuel rods that operate for a preset first operation time and once-used fuel rods that operate for a second operation time longer than the first operation time; creating an operation schedule of the pressurized water reactor based upon the first operation time of the fresh fuel rods and the second operation time of the once-used fuel rods; and operating the pressurized water reactor by repeating the operation schedule,wherein the operating the pressurized water reactor by repeating the operation schedule includes: operating the pressurized water reactor for a first cycle of a predetermined period of time by using a fuel assembly including a first set of the fresh fuel rods in a first region and a first set of the once-used fuel rods in a second region; after the first cycle operation is completed, discharging the first set of the once-used fuel rods to the outside of the pressurized water reactor; loading, after the first cycle operation is completed, a first portion of the first set of the fresh fuel rods into the second region in the fuel assembly; discharging a second portion of the first set of fresh fuel rods to the outside of the pressurized water reactor; loading a second set of the fresh fuel rods into the first region and operating the pressurized water reactor for a second cycle of a preset period of time; loading, after the second cycle operation is completed, a second set of fresh fuel rods into the second region; and a third set of the fresh fuel rods into the first region and operating the pressurized water reactor for a third cycle of a preset period of time. 2. The method of claim 1, wherein the second fuel rods have uranium enrichment that allows a long period of operation of 24 months. 3. The method of claim 2, wherein the uranium enrichment ranges from 4.7 to 4.95 wt. %. 4. The method of claim 1, wherein the first cycle and the second cycle are 24 months, and the third cycle is 18 months. 5. The method of claim 1, wherein the first cycle, the second cycle, and the third cycle form one operation schedule, and the pressurized water reactor operates by repeating the one operation schedule.