Monitoring method for nuclear power plant

In a BWR type nuclear power plant, a critical value to be allowed in view of the operation is determined relative to a minimum critical power ratio (MCPR) and a linear power density in order to keep fuel integrity. The present invention concerns monitoring for the MCPR. That is, CPR is calculated periodically based on each of process data inputted from the plant. An operation limit value is determined based on calculated CPR and each of process data used for the calculation. The operation limit value is corrected, as a function of the control rod position and the reactor flow rate upon CPR calculation and at present. Corrected operation limit value and a low power range monitor (LPRM) signal inputted from the plant are compared to monitor the minimum CPR, that is MCPR. With such procedures, MCPR can be monitored continuously. Further, since the operation limit value is corrected at an insertion position before and after the control rod withdrawal, the non-linear change of the MCPR due to withdrawal of the control rod can be evaluated appropriately. As a result, there is no more necessary to provide unnecessary margin. (I.S.)