Answer:
For a quick indication of a nuclear reactor's power level, an instrument sensitive to the neutron flux is used. That is, it gives a signal proportional to the number of neutrons per second at the point the detection chamber is placed. By calculation, the signal can be related to the reactor power, but is not likely to be accurate, and chambers vary in sensitivity. Therefore to get an accurate figure the reactor power has to be obtained from thermal measurements, ie temperatures of the coolant and its flow rate, and the flux detector can then be calibrated to give a more accurate figure.