[en] Purpose: To enable to obtain the output of compensation signals for boric acid removal and injection rate accurately corresponding to the actual position of control rods in a heavy water moderated BWR type reactor. Constitution: A time in which a power conditioning control rod reaches from a reference position to an upper or lower limit position is measured and a predetermined compensation signal for borid acid removal and injection is outputted only when the time is longer than a predetermined time. Further, the time in which the signal is being outputted is set such that it is not longer than a predetermined time. Thus, if the control rod reaches the upper or lower limit position by the reactivity applied in a transient manner, no compensation signal is outputted. Further, if the control rod does not return within a predetermined of time after the generation of the compensation signal, it is juged abnormal and the controlling operation is interrupted at that point. In this way, increase of the abnormal power rise or power down can be avoided. (Ikeda, J.)