Modelling the worldwide nuclear reactor park including all supply chain details, i.e. the nuclear fuel cycle, demands for an integrated nuclear energy system model which also includes feedback loops representing physical feedbacks within the system as well as, and most prominently, socio-political feedbacks in the decision-making on the various available deployment pathways for nuclear energy. Argonne National Laboratory (ANL) started in 2000 with the development of such integrated nuclear energy system models, i.e. DYMOND and more recently DANESS. These models are based on system dynamics modelling used in various industry sectors and allowing to model the full mass-flow chain of time-varying mixes of nuclear reactor plants and associated fuel cycle options. Several other sub-models may then be coupled to the mass-flow kernel to calculate heat loads, economics, life cycle inventory, and several other parameters and feedback decision-making loops important in the assessment of nuclear energy futures.