In response to the accident at the Fukushima Daiichi nuclear power station in Japan, the U.S. Nuclear Regulatory Commission (NRC) and Department of Energy agreed to jointly sponsor an accident reconstruction study as a means of assessing severe accident modeling capability of the MELCOR code. MELCOR is the state-of-the-art system-level severe accident analysis code used by the NRC to provide information for its decision-making process in this area. The objectives of the project were: (1) collect, verify, and document data on the accidents by developing an information portal system; (2) reconstruct the accident progressions using computer models and accident data; and (3) validate the MELCOR code and the Fukushima models, and suggest potential future data needs. Idaho National Laboratory (INL) developed an information portal for the Fukushima Daiichi accident information. Sandia National Laboratories (SNL) developed MELCOR 2.1 models of the Fukushima Daiichi Units 1, 2, and 3 reactors and the Unit 4 spent fuel pool. Oak Ridge National Laboratory (ORNL) developed a MELCOR 1.8.5 model of the Unit 3 reactor and a TRACE model of the Unit 4 spent fuel pool. The good correlation of the results from the SNL models with the data from the plants and with the ORNL model results provides additional confidence in the MELCOR code. The modeling effort has also provided insights into future data needs for both model development and validation.

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This report clarifies many technical issues being analyzed by the Advanced Fuel Cycle Initiative (AFCI) program, including Inert Matrix Fuel (IMF) versus Mixed Oxide (MOX) fuel, single-pass versus multipass recycling, thermal versus fast reactors, the need to recycle Np-Pu-Am to meet established AFCI objectives, the borderline case of Cm, the potential need for transmutation of technetium and iodine, and the value of separating cesium and strontium. This report represents the first attempt to calculate a full range of metrics, spanning all four AFCI program objectives [DOE2005a, DOE2005, DOE2006] -waste management, proliferation resistance, energy recovery, and systematic management/economics/safetyusing a combination of "static" calculations and a system dynamic model, DYMOND.[Moisseytsev2001, Yacout2005a] (In late FY2006, DYMOND is being replaced with the VISION model.) In many cases, we examine the same issue both dynamically and statically to determine the robustness of the observations. All analyses are for the U.S. reactor fleet. This is a technical report, not intended for a policy-level audience. A wide range of options are studied to provide the technical basis for identifying the most attractive options and potential improvements. No single fixed strategy guarantees optimal performance at all times in all possible futures. Instead, the objective in the next few decades should be to cost-effectively develop the tools to deal with the circumstances at that point in time. Technical maturity and readiness to deploy were outside the scope of this report.Many dynamic simulations of option deployment are included. There are few "control knobs" for driving or piloting the fuel cycle system into the future, even though it is dark and raining (uncertain) and controls are sluggish with slow time response: what types of reactors are built, what types of fuels are used, and the capacity of separation and fabrication plants. Driving responsibilities are distributed among utilities, government, and regulators, compounding the challenge of making the entire system work and respond to changing circumstances. We identify approaches that would increase our ability to drive the fuel cycle system: (1) have a recycle strategy that could be implemented before the 2030-2050 approximate period when current reactors retire so that replacement reactors fit into the strategy, (2) establish an option such as multi-pass blended-core IMF as a downward plutonium control knob and accumulate waste management benefits early, (3) establish fast reactors with flexible conversion ratio as a future control knob that becomes available as fast reactors are added to the fleet, and (4) expand exploration of blended assemblies/cores and targets, which appear to have advantages and agility.Results suggest multi-pass full-core MOX appears to be a less effective way than multi-pass blended core IMF to manage the fuel cycle system because it requires higher TRU throughput while more slowly accruing waste management benefits. Single-pass recy...