  As nuclear power expands, technical, economic, political, and environmental
analyses of nuclear fuel cycles by simulators increase in importance. To date,
however, current tools are often fleet-based rather than discrete and
restrictively licensed rather than open source. Each of these choices presents
a challenge to modeling fidelity, generality, efficiency, robustness, and
scientific transparency. The Cyclus nuclear fuel cycle simulator framework and
its modeling ecosystem incorporate modern insights from simulation science and
software architecture to solve these problems so that challenges in nuclear
fuel cycle analysis can be better addressed. A summary of the Cyclus fuel cycle
simulator framework and its modeling ecosystem are presented. Additionally, the
implementation of each is discussed in the context of motivating challenges in
nuclear fuel cycle simulation. Finally, the current capabilities of Cyclus are
demonstrated for both open and closed fuel cycles.
