A “back-of-the-envelope” method is presented for estimating neutron-induced radionuclide populations. The method uses energy-averaged neutron-reaction cross sections as base data and accounts for nonequilibrium nuclide formation by any sequence of transmutations; it provides for cases in which nuclides in transmutation sequences may be produced in more than one way or may decay or react to produce more than one product nuclide, and it accounts for both constant physical removal from circulating fluids and the severe depletion of parent nuclei. Evaluation in a series of time steps is not required; the calculation is done only for the time of interest. Estimates of neutron-induced radionuclide populations are made from the sum of population contributions corresponding to the most significant transmutation sequences by which the radionuclide is formed. Transmutation sequences are defined in such a way that population contributions corresponding to them can be evaluated from either exact analytic solutions or from a simple approximate procedure that always yields an upper bound to population contribution.
