The traveling wave reactor is far from proven. In fact, one has never been built or operated on any scale, and there is significant contention for how difficult it would be to implement and control a TWR. However, there are a number of reactor designs that can consume the bulk of the actinides and fission products produced by fission of [SUP]235[/SU]U leaving only short-lived isotopes that can be stored until they decay. Such ‘burner’ reactors produce more energy than a once-through [SUP]235[/SU]U and do not require the reprocessing effort and risks of breeder reactors, though they still typically require some degree of separation to gain efficient use of the primary fuel. The Integral Fast Reactor, Liquid Metal Cooled Fast Reactor, Molten Salt Reactor, Liquid Fluoride Thorium Reactor, and Subcritical Fission-Fusion Reactors all have a substantial proof of concept, with several having been operated at useful power production levels and tested in various regimes for stability, and all provide a substantial degree of burnup and efficient use of nuclear fuel; many can also use [SUP]232[/SUP]Th as a fuel, as well as different formulations of mixed oxide (MOX) without having to physically reconfigure the reactor.