product decay to continue the chain reaction. Because the delayed neutrons evolve slowly, this makes the reactor very controllable. In an LFTR, the presence of fission products in the heat exchanger and piping means a portion of these delayed neutrons are also lost. They do not participate in the core's critical chain reaction, which in turn means the reactor behaves less gently during changes of flow, power, etc. Approximately up to half of the delayed neutrons can be lost. In practice, it means that the heat exchanger must be compact so that the volume outside the core is as small as possible. The more compact (higher power density) the core is, the more important this issue becomes. Having more fuel outside the core in the heat exchangers also means more of the expensive fissile fuel is needed to start the reactor. This makes a fairly compact heat exchanger an important design requirement for an LFTR. Waste management – About 83% of the radioactive waste has a half-life in hours or days, with the remaining 17% requiring 300-year storage in geologically stable confinement to reach background levels. Because some of the fission products, in their fluoride form, are highly water-soluble, fluorides are less suited to long-term storage. For example, cesium fluoride has a very high solubility in water. For long term storage, conversion to an insoluble form such as a glass, could be desirable. Uncertain decommissioning costs – Cleanup of the Molten-Salt Reactor Experiment was about $130 million, for a small 8 MW(th) unit. Much of the high cost was caused by the unexpected evolution of fluorine and uranium hexafluoride from cold fuel salt in storage that ORNL did not defuel and store correctly, but this has now been taken into consideration in MSR design. In addition, decommissioning costs don't scale strongly with plant size based on previous experience, and costs are incurred at the end of plant life, so a small per kilowatthour fee is sufficient. For example, a GWe reactor plant produces over 300 billion kWh of electricity over a 40-year lifetime, so a $0.001/kWh decommissioning fee delivers $300 million plus interest at the end of the plant lifetime. Noble metal buildup – Some radioactive fission products, such as noble metals, deposit on pipes. Novel equipment, such as nickel-wool sponge cartridges, must be developed to filter and trap the noble metals to prevent build up. Limited graphite lifetime –