The present paper presents, a probabilistic simulation package, Tensit, used for migration calculation of radionuclides from a deep repository for spent nuclear fuel to the biosphere. The package is based on pre-existing codes (Matlab, Simulink and the probabilistic engine @Risk) and is capable of performing migration calculations for the near field, far field and the biosphere for a high level waste repository. In addition to the compartment based models based on transfer factors, the code is also able to handle the more complex ecosystem models (based on flow of carbon and nutrients) either separately or in combination with the compartment models. Comparisons with other codes used for compartment based biosphere modelling as well as example of simple models, more advanced ecosystems and connected ecosystems Landscape model are shown in the paper.