reactor designs, thorium-based used nuclear fuel possesses inherent proliferation resistance. cannot be chemically separated from and has several decay products that emit high-energy gamma radiation. These high-energy photons are a radiological hazard that necessitate the use of remote handling of separated uranium and aid in the passive detection of such materials. The long-term (on the order of roughly to ) radiological hazard of conventional uranium-based used nuclear fuel is dominated by plutonium and other minor actinides, after which long-lived fission products become significant contributors again. A single neutron capture in is sufficient to produce transuranic elements, whereas five captures are generally necessary to do so from . 98–99% of thorium-cycle fuel nuclei would fission at either or , so fewer long-lived transuranics are produced. Because of this, thorium is a potentially attractive alternative to uranium in mixed oxide (MOX) fuels to minimize the generation of transuranics and maximize the destruction of plutonium. Disadvantages There are several challenges to the application of thorium as a nuclear fuel, particularly for solid fuel reactors: In contrast to uranium, naturally occurring thorium is effectively mononuclidic and contains no fissile isotopes; fissile material, generally , or plutonium, must be added to achieve criticality. This, along with the high sintering temperature necessary to make thorium-dioxide fuel, complicates fuel fabrication. Oak Ridge National Laboratory experimented with thorium tetrafluoride as fuel in a molten salt reactor from 1964 to 1969, which was expected to be easier to process and separate from contaminants that slow or stop the chain reaction. In an open fuel cycle (i.e. utilizing in situ), higher burnup is necessary to achieve a favorable neutron economy. Although thorium dioxide performed well at burnups of 170,000 MWd/t and 150,000 MWd/t at Fort St. Vrain Generating Station and AVR respectively, challenges complicate achieving this in light water reactors (LWR), which compose the vast majority of existing power reactors. In a once-through thorium fuel cycle, thorium-based fuels produce far less long-lived transuranics than uranium-based fuels, some long-lived actinide products constitute a long-term radiological impact, especially and . On a closed cycle, and can be reprocessed. is also considered an excellent burnable poison absorber in light water reactors. Another challenge associated with the thorium fuel cycle is the comparatively long interval over which breeds to . The half-life of is about 27 days, which is an order of magnitude longer than the half-life of . As a result, substantial develops