The DOE Nuclear Energy Advanced Modeling and Simulation (NEAMS) program is developing simulation capabilities to leverage U.S. leadership computing facilities in the design of next-generation reactors. One of the active areas of research is in the Advanced Fuel Cycle Initiative (AFCI), which is examining a closed nuclear fuel cycle based on a new generation of fast neutron reactors speciﬁcally designed for the transmutation of spent nuclear fuel to address nuclear waste management concerns. Partitioning and transmu- tation of transuranic elements from nuclear spent fuel is considered as a way of reducing the burden of geological disposal. Advanced simulation is viewed as critical in bringing fast reactor technology to fruition in an economic and timely manner and the DOE has recently established area-speciﬁc campaigns to look at open questions in closing the fuel cycle. Analysis of fast reactor cores is one the areas of interest and the thermal-hydraulic performance—pressure drop and mixing induced by the coolant ﬂow—ﬁgures prominently in design questions.