[en] During the nuclear physics test and power operation of Yonggwang Unit 4 Cycle 4 which is Korean Standard Nuclear Power Plant, the unexpected Fxy increment measured by fixed incore detector system was observed and the reason was analyzed. The maximum Fxy occurred at In-Core Instrument (ICI) number -16 (full core box -59) Level 1, and the main reason was the highly measured incore detector signal. CECOR consistency test, box power comparison test, Fxy list-up test, and the evaluation of the core power distribution result in much difference between the predicted and the measured. The evaluation of the ICI depletion history, sensitivity and background signal, however, shows that there are no problems of incore detector system in operability. From the result of evaluation, even though ICI no 16 Level 1 is very suspect, there is no clear evidence for detector failure, and it is considered that the Fxy increment was induced by the Rhodium detector unusual signal. The objective of this study is to guide to setup the procedures for the determination of Rhodium detector failing and the Rhodium detector operability through the experience of Fxy anomaly in order to keep the core maintaining an operating thermal margin