Oscillation power range monitor system and a method of operating a nuclear power plant

An oscillation power range monitor system designed to monitor oscillation of power of a nuclear reactor and output a reactor trip signal to shut down the nuclear reactor automatically if the oscillation is found abnormal, the system comprising:

• a plurality of OPRM units, each of the OPRM units receiving and processing cell-output set signals coming from any selected LPRM detectors of LPRM strings arranged at the four corners of a cell of a lattice, the LPRM strings being arranged in a reactor core in the form of the lattice on the horizontal plane of the reactor core, each LPRM strings having a plurality of LPRM detectors configured to detect a neutron flux and a gamma-ray flux and having neutron flux sensitivity more decreasing than gamma-ray flux sensitivity over a period for measuring the neutron flux, wherein each of the OPRM units has:
• a cell average calculating unit receiving cell-output set signals and averaging the cell-output set signals, thereby calculating a cell average value;
• a time average calculating unit calculating a time average cell value that is temporal average of the value calculated by the cell average calculating unit in a prescribed time period;
• a normalized cell value calculating unit calculating a normalized cell value that is a ratio of the average cell value to the time average cell value;
• a trip determining unit outputting a reactor trip signal if amplitude or growth rate of oscillation of the normalized cell value has exceeded a prescribed value, or period of oscillation of the normalized cell value has exceeded a prescribed condition;
• and
• a signal and prescribed value adjusting unit adjusting relation between the normalized cell value and the specific value, thereby compensating for deterioration in the neutron flux sensitivity of any LPRM detector in order to keep outputting the reactor trip signal.