The Digital Time-Series Data Acquisition System using first-in-first-out (FIFO) memory module has been developed for the monitoring of subcritical reactivity. The system records arriving time of the signal from a neutron detector, and time-series data are saved in a personal computer (PC). Using this system, the measurement of time-series data is carried out more efficiently than that using a conventional multichannel scaler (MCS) system, and it has been applied for the monitoring of subcritical reactivity of the reactor in a steady state using the conventional Feynmantechnique. In the present work, this system was applied for the monitoring of subcritical reactivity in an unsteady state using the difference filter technique. The test experiment was carried out with the research reactor of Atomic Energy Institute of Kinki University, Japan. From the experimental results, it was shown that the present system was useful for the continuous monitoring of varying subcritical reactivity by application of the difference filter technique.

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