Generalized Perturbation Theory for Nonlinear Systems from the Importance Conservation Principle

On the basis of the importance conservation principle, a time-dependent generalized perturbation theory for nonlinear fields is developed to any order of approximation required by simple heuristic arguments. Applications are shown in the neutron-nuclide domain, relevant to reactor fuel cycle studies, and in the heat-energy neutron field, relevant to nuclear reactor design and safety analysis.