The near real-time material accountancy (NRTMA) method might be applied for safeguarding of the chemical process area of future reprocessing plants. Experimental data are not yet available for testing the capability of the NRTMA method but can be simulated using a digital computer. The mathematical modeling of the plutonium-bearing components of reprocessing plants is discussed, and results obtained by simulation models are presented. Particular attention is given to the long-term net fluctuations of plutonium inventories in hard-to-measure components such as the solvent extraction contactors. Comparing the variance of these inventories with the measurement variance for plutonium contained in feed, analysis, and buffer tanks, it is concluded that direct or indirect periodic estimation of contactor inventories would not contribute significantly to improving the quality of closed material balances over the process material balance area.