Experience of Xenon Oscillation During an Initial Physics Test of UCN Unit 5 in Korea

Initial core of Ulchin Nuclear Unit 3 (UCN3), which is one of earlier OPR1000 model, was 4 batches and designed as annual cycle after second cycle. The utility requested that UCN Unit 5 (UCN5), which is another of OPR1000 model, had capability of a longer cycle operation from second cycle. Therefore, KNF modified the number of batches from 4 to 3 for OPR1000 initial core, as well as, the number of burnable absorber, and the cutback length of the absorber. However, due to these changes, Xenon oscillation was slightly increased at 100% power during the physics test of UCN5, while that oscillation at 100% power in UCN3 had been gone down without any control rod motion. The xenon oscillation direction is related to axial stability index. The index of UCN3 increased from a slightly negative at BOC to positive at EOC, the index of UCN5 was positive even at BOC, which meant that the core does not go to be stable without the control rod motion. The core of UCN5 became the steady state by the insertion of control rods into the core. To meet the physics test condition, the oscillation was controlled by control rods immediately. After the happening, KNF optimized the cutback length of burnable absorber rods and applied to APR1400, which will keep being stable in xenon oscillation during physics test at the initial cycle.