The Nuclear Fuel Cycle Simulation System (NFCSS) is a scenario based computer simulation tool that can model various nuclear fuel cycle options in various types of nuclear reactor. It is efficient and accurate in identifying, for example, the nuclear mineral resources and technical infrastructure needed for the front end of the nuclear fuel cycle; the amounts of used fuel, actinide nuclides and high level waste generated for a given reactor fleet size; and the impact of introducing recycling of used fuel on mineral resource savings and waste minimization. Since the first publication on the NFCSS as IAEA-TECDOC-1535 in 2007, there have been significant improvements in the implementation of the NFCSS, including a new extension to thorium fuel cycles, methods to calculate decay heat and radiotoxicity, and demonstration applications to innovative reactors.