On some perturbation theory singularities for reactivity effects calculations

It is shown in this paper that traditional formulas for reactivity effects calculation based on diffusion approximation small-perturbation theory sometimes need updating. The corresponding specifications are provided. These are caused by the unbalance of neutron "leakages" between subdomains (zones, cells, fuel subassemblies, etc.) of the reactor as a result of perturbation. The unbalance occurs because of the jumps of relative increments of diffusion coefficients on the subdomain interfaces.