Mixer-settler and neutron monitoring method therefor

In the present invention, a flow of a nuclear fuel solution flowing in a mixer-settler is accurately measured by a neutron monitor to monitor the stability of the flow. A first neutron detector d1 is disposed near a mixer portion. A second neutron monitor d2 is disposed at a settler portion at a position sufficiently apart from the mixer portion. In the monitoring method, assuming a counter rate for each of the monitors as C1, C2, and the ratio between the counter rates as C1/C2=R, R=C1/C2 is determined at a standard condition, and normality of the flow is monitored by R(counter rate) or F=(C1 - C2/R)/(C1 + C2/R). If both of C1 and C2/R exceed a predetermined value, an alarm is outputted. In this case, F is an off set factor. Further, if one of them is abnormal, an alarm circuit is bypassed, to inspect the circuit by a neutron source. (I.S.)