AbstractApproximate methods, which are simple and quick to use, are presented for finding the response of a critical and a subcritical reactor, to ramp inputs ofδk. The solutions are based on one equivalent group of delayed neutrons. Various methods of selecting the equivalent delayed neutron decay constant are discussed and it is shown that a one-group model will always be conservative if a particular method of selection is used. It is also demonstrated, that the approximate solutions are more accurate than the well-known approximation proposed by Newson.