Thorium fuel cycle
This parallels the process in uranium breeder reactors whereby fertile 238U absorbs neutrons to form fissile 239Pu.
The thorium fuel cycle has several potential advantages over a uranium fuel cycle, including thorium's greater abundance, superior physical and nuclear properties, reduced plutonium and actinide production,[1] and better resistance to nuclear weapons proliferation when used in a traditional light water reactor[1][2] though not in a molten salt reactor.
[3][4][5] Concerns about the limits of worldwide uranium resources motivated initial interest in the thorium fuel cycle.
Rubbia's proposal offered the potential to incinerate high-activity nuclear waste and produce energy from natural thorium and depleted uranium.
He first researched thorium reactors while working at NASA, while evaluating power plant designs suitable for lunar colonies.
[15] A 2011 MIT study concluded that although there is little in the way of barriers to a thorium fuel cycle, with current or near term light-water reactor designs there is also little incentive for any significant market penetration to occur.
However, the 231Pa (with a half-life of 3.27×104 years) formed via (n,2n) reactions with 232Th (yielding 231Th that decays to 231Pa), while not a transuranic waste, is a major contributor to the long-term radiotoxicity of spent nuclear fuel.
[20] 232U has a relatively short half-life (68.9 years), and some decay products emit high energy gamma radiation, such as 220Rn, 212Bi and particularly 208Tl.
Thorium is estimated to be about three to four times more abundant than uranium in Earth's crust,[21] although present knowledge of reserves is limited.
Thorium-based fuels also display favorable physical and chemical properties that improve reactor and repository performance.
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 103 to 106 years) 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.
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.
[22] 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 233U, 235U or plutonium, must be added to achieve criticality.
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 233U in situ), higher burnup is necessary to achieve a favorable neutron economy.
As only two liquid-core fluoride salt reactors have been built (the ORNL ARE and MSRE) and neither have used thorium, it is hard to validate the exact benefits.
[6] Additionally from Energy Information Administration, "Spent Nuclear Fuel Discharges from U. S. Reactors", Table B4: Dresden 1 Assembly Class.
