Time-domain noise analysis techniques such as the Rossi-α, the variance-to-mean, and the interval-distribution methods can be used to measure fundamental reactor parameters in a wide variety of reactor systems, provided the power level of the system is not too high. Simple expressions have been derived that define the maximum power level (i.e., the “reactor noise threshold”) above which time-domain reactor noise techniques are likely to fail in subcritical, critical, and supercritical systems.