Radioactive decay is a physical phenomenon that can be modeled by simple computational techniques, following the two main
schools of mathematical modeling: the deterministic school or the probabilistic school. Nuclear power is a proven technology and has the potential to generate virtually limitless energy with no greenhouse gas emissions during operations.Since no combination of other supply technologies is likely to fully replace nuclear's carbon abatement potential, success in overcoming technical, social and political barriers is vital. One reason for the difficulties in gaining the necessary public acceptance all over the world for nuclear power plants is the management of long-lived radioactive waste, such as spent nuclear fuel and the wastes arising from the reprocessing of spent fuel. In this work we focused on the deterministic school. The mathematical model is characterized by an initial value problem with a single or composed chain of radioactive decays according to the event of an atomic nucleus to decay to a daughter-atomic nucleus, that is stable or not. In this paper we describe a computer software modeling simple radioactive decays, decays to stable nuclei and directly coupled decay chains that we developed on a free platform. To achieve this goal, we used a matrix formulation of the Laplace transform and a diagonalization technique by means of similarity transformation, where we introduced a general form of constructing the diagonalizer matrix and its inverse, that are needed. We present numerical results for typical problems.