The invention improves detecting that a control cluster has fallen into the reactor of a nuclear fission power station. Variations are monitored firstly in the nuclear power (3) and secondly in at least one parameter (1, 2) external to the reactor and having an influence on reactor control. A cluster is considered as having fallen when a rapid drop (12j) is detected in said power while no variation (12a, 12b, . . . 12h) is simultaneously detected in an external parameter that could have given rise to said power drop. Application to producing electricity.
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