Neutron Noise Measurements of a Fast HEU Copper System

In addition to nuclear data validation and criticality safety research, criticality experiments offer unique opportunities for investigations into fundamental reactor physics phenomena and measurement techniques. Unlike conventional light water research reactors where kinetics parameters do not significantly deviate from one reactor to another, critical experiments with neutron lifetimes from tens of nanoseconds to tens of microseconds can be constructed on vertical lift machines. Kinetics measurements of new fast reactors will be crucial for the validation of computational methods. In this work, we present the results of neutron noise measurements of a fast highly enriched uranium–fueled copper-reflected critical system constructed on the Comet vertical lift assembly at the National Criticality Experiments Research Center. Fluctuations in the system’s neutron population were analyzed in the frequency domain by ³He proportional counters and organic scintillators in order to measure the prompt neutron decay constant α at delayed critical and four subcritical states. Agreement between the extrapolated α_DC values of the two detector systems was within 4%. Additionally, α values at delayed critical were examined as a function of time in the presence of small power drifts. By comparing to the linear channel, it is shown that these drifts did not cause a noticeable effect on α but did inversely vary with the signal amplitude. A consistent bias between the ³He and scintillator measurements of α and signal amplitude was observed and is believed to be caused by electronic noise or spacial effects. These measurement techniques will be essential in supporting the development and deployment of advanced fast reactor types through their ability to verify and validate computational methods.