breeding, it is questionable if this is possible when other requirements are to be met. The thorium fuel cycle has very few spare neutrons. Due to limited chemical reprocessing (for economic reasons) and compromises needed to achieve safety requirements like a negative void coefficient too many neutrons may be lost. Old proposed single fluid designs promising breeding performance tend to have an unsafe positive void coefficient and often assume excessive fuel cleaning to be economic viable. Still much development needed – Despite the ARE and MSRE experimental reactors already built in the 1960s, there is still a lot of development needed for the LFTR. This includes most of the chemical separation, (passive) emergency cooling, the tritium barrier, remote operated maintenance, large scale Li-7 production, the high temperature power cycle and more durable materials. Startup fuel – Unlike mined uranium, mined thorium does not have a fissile isotope. Thorium reactors breed fissile uranium-233 from thorium, but require a small amount of fissile material for initial start up. There is relatively little of this material available. This raises the problem of how to start the reactors in a short time frame. One option is to produce U-233 in today's solid fueled reactors, then reprocess it out of the solid waste. An LFTR can also be started by other fissile isotopes, enriched uranium or plutonium from reactors or decommissioned bombs. For enriched uranium startup, high enrichment is needed. Decommissioned uranium bombs have enough enrichment, but not enough is available to start many LFTRs. It is difficult to separate plutonium fluoride from lanthanide fission products. One option for a two-fluid reactor is to operate with plutonium or enriched uranium in the fuel salt, breed U-233 in the blanket, and store it instead of returning it to the core. Instead, add plutonium or enriched uranium to continue the chain reaction, similar to today's solid fuel reactors. When enough U-233 is bred, replace the fuel with new fuel, retaining the U-233 for other startups. A similar option exists for a single-fluid reactor operating as a converter. Such a reactor would not reprocess fuel while operating. Instead the reactor would start on plutonium with thorium as the fertile and add plutonium. The plutonium eventually burns out and U-233 is produced in situ. At the end of the reactor fuel life, the spent fuel salt can be reprocessed to recover the bred U-233 to start up new LFTRs.