Various Operating Regimes of a Subcritical System as a Function of Subcriticality in One-Group Theory

We present a model of a subcritical neutron multiplying system coupled to an external time-dependent neutron source, within one-group diffusion theory. Within this scheme we show that the problem is fully solvable without any other approximation. Then, we are able to show that all the known results about such systems can be consistently derived. In such a way we are able to discuss various specific aspects that distinguish a subcritical system from a critical one without relying on any other approximations or assumptions. Moreover, we prove that a subcritical system has very different working regimes as the level of subcriticality varies.