[en] In a fuel reprocessing plant operating at steady-state with respect to SNM concentrations, flow-rates and inventory, it appears possible to estimate the in-process inventory of SNM by observation and correlation of input and output isotopic compositions of the SNM. Variations of input isotopic concentration may be deliberately introduced as an arbitrarily shaped history of tracer SNM solution having a different isotopic composition from the inventory. If, in normal operations, significant fluctuations in isotopic composition occur in the input to the plant, these fluctuations can be used instead of a deliberately injected tracer. Optimal filter methods are proposed for correlating the isotopic composition input fluctuations with those that comprise the output response. In contrast with the tracer-step displacement method for performing a dynamic inventory, the temporal response methods described do not indicate in-process material at a discrete instant of time, but rather the steady-state inventory during the measurement period. The expected advantage of the temporal response analysis over the step-displacement method is the relative ease with which the former can be executed, and consequently, less disturbance to plant operations. (author)