Reactor stability monitoring device

The device of the present invention detects an instable phenomenon in a reactor core of a BWR-type reactor, to contribute to the improvement of safety and operation efficiency. Namely, a correlation dimension/lyapunov index calculation device takes signals, as samples, from detectors, and determines a correlational dimension and a lyapunov index based on time sequence data, and determines a fluctuation rate based on these values. A comparison calculator compares these values with data base which accumulates previously evaluated correlational dimension/lyapunov index corresponding to a reactor core stability. A stabilizing operation-judging device improves required stability based on the results of the comparison and calculation. The device of the present invention having such a constitution calculates a correlational dimension and a lyapunov index from the time sequence data taken as samples, and also determines a fluctuation rate together with the values, and compares them with reference values previously stored in the data base. Accordingly, the stability of the reactor core can accurately be monitored. (I.S.)