Abstract: The present invention seeks to obtain reliable detection of the fall of a control cluster into the core of a pressurized water nuclear reactor without giving rise to the risk of an erroneous detection causing an untimely emergency reactor stop. Four acquisition units (U1, U2, U3, U4) corresponding to the four quadrants of the reactor core receive neutron flux signals (F1), heat flux signals (P1), and cluster position signals (Z1A, Z1B, . . . ) from the various quadrants and process them to generate corresponding alarm signals. These units, together with a combination circuit (14, 18, 22), provide a cluster fall signal (24) in the presence of at least two alarm signals including at least one cluster position alarm signal. The invention is applicable to electricity power generation.

Abstract: The low alarm threshold (B) of the overheating ratio of a nuclear reactor is determined in such a manner that the higher the position of the temperature-regulating rods, the lower the value determined for said threshold. The invention is applicable, in particular, to pressurized water nuclear reactors used for producing electrical power.