An original technique for deriving the closed form solution of the multigroup system of time- and space-dependent neutron diffusion equations is reported and applied to a nonuniform multiplying structure of particular interest in cylindrical geometry. The problem of evaluating the time eigenvalues, the dynamic eigenstates, and the asymptotic power behavior of mixed fuel cores, where localized variations of the delayed neutron yield of the fuel occur, is analyzed on a rigorous basis and solved for significant sample geometries. The results provide a sound basis for establishing the region of a core where a significant amount of plutonium will induce the “minimum damage” to the overall dynamic characteristics of the reactor. These results also provide the “more suitable spatial distributions” to be assigned to a limited quantity of uranium to improve the dynamic performance of a nonuniform core, basically fueled with plutonium or mixed fissionable materials. Hence, it can be stated, on a rigorous a priori basis, the conditions where the plutonium energy release can be made as safe as possible.