[en] Starting from a class of parabolic systems considered in the literature for describing the operation of fast nuclear reactors, two kinds of modified systems are investigated: one accounts for retarded neutrons, the other one involves a small neutron feedback. It is shown that these systems, while equivalent to the original ones for any finite value of the neutron velocity, behave in a substantially different way in the limiting case of diverging neutron velocity. In particular, considerin retarded neutrons or neutron feedback terms avoids the appearance of undamped oscillations, which characterizes the behavior of the original systems in the same limiting case. (author)