Neutron Noise, Pulses, and Waves

Experience with Subcriticality Oeter-nmination by Noise Techniques in the FTR Engineering-Mockup Critical Experiment Program, Carles , CoAn ( ANL  

Noise methods have been applied to the measurement of subcriticality in the FTR Engineering Mocknp critical experiment program as a cross-check on subcriticality measurements by rod drop. Initial efforts used the two-detector cross power spectral density (CPSD) method.  

The system is calibrated by exposure of one scintillator to white noise In the form of a Co source. Both multifilter inputs are connected to the one detector output, and measurements are taken as described above. For each band, the reading taken on a reactor run is divided by the white-noise reading, calibrating out equipment frequency response. 

With this equipment it was found possible to measure roll-off frequencies to 1 or 2% in 1 h, from hear critical down to 2% subcritical. However, the CPSD method gave unsatisfactory results apparently because of departures from point kinetics in the reactor behavior. Near critical, observed roll-off frequency was up to 10% greater with the detectors in the center of the core than with the detectors in the reflector. Furthermore, measured sub-erlticalltles dlsagreSd sharply with rod-drop results, beingTow by as much as 30% of the latter. 

Another method was developed and has given results agreeing with rod-drop measurements within 0.1% Ak. Like the methods of Selfritz and Albrecht it measures coherence between signals from two neutron detectors, but unlike their methods it uses the same equipment as for the CPSD measurements. While the CPSD at a given frequency is indicated by the difference between the two above-mentioned scaler readings, the coherence is indicated by their ratio. 

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