Reactor void measuring instrument

Output signals of neutron detectors disposed in a reactor core are fed to an averaging circuit to obtain an average value of the output signal levels. The output of this circuit and an output of a neutron monitor disposed external to the core are fed to a comparator to obtain the ratio of the latter output to the former. The output of the comparator is fed to a calculator for calculating the ratio of said ratio to that previously obtained at a time when the reactor is operated at a certain low level of power. Since the thus calculated ratio in the calculator depends on to what extent voids have developed in the reactor core, this quantity can be read out from a meter accepting output from the calculator providing that the meter is previously calibrated based on the relation of the voids with the calculated ratio in the calculator. This measuring instrument has a high response permitting speedy measurement of the extent and distribution of voids. (Ohno, Y.)