Abstract: A device and method for determining the power of a pressurized water nuclear reactor, comprising measurement of the neutron power (15) and of the temperature of the primary fluid in the cold branch (1) and in the hot branch (1'), computing (2, 4, 6, 8, 12) the thermal power of the reactor from these temperature measurements (1, 1'), comparing (14) the thermal power signal (13) and the neutron power signal (16) and correcting (22) the neutron power signal (16). The device in addition comprises means for fast and precise determination of the secondary power of the reactor by comparison of a first signal of the secondary thermal power (13) established by means of the temperature measurements (1, 1') and a second signal of the thermal power (16') established by means of a simplified secondary heat balance (25).