Development of early core anomaly detection using nuclear instrumentation

not registered; not registered

Neutron flux monitoring can be an effective method for the early detection of reactivity anomalies in a FBR core. This development required that the normal range of reactivity fluctuations at power should be well characterized. An analysis of Reactor noise showed that the low frequency power fluctuation was influenced by the coolant temperature fluctuation, and that the high frequency power fluctuation was caused by control rod vibration. But spectral resolution was not adequate to determine the normal range of power fluctuation quantitatively. Also, a transfer function for the coolant temperature to reactor power ratio was modeled with the time constants of the thermal expansion of the core support plate, coolant temperature measurements, and so on. This was necessary to clearly understand the cause of the normal power fluctuations. The calculated values of simulated reactor power were compared with typical power ratio data from JOYO and the comparison was good in the low frequency range. Hence, it is clear that the dynamic characteristics of reactor power are caused by coolant temperature fluctuations. The high frequency power fluctuation, which is caused by control rod vibration, can be separated from the normal reactor power fluctuation. This analysis has shown that the normal range of reactor power fluctuations can be quantitatively determined accurately, and that neutron flux monitoring can be applied to detect reactivity anomalies early in a FBR core.