Thorium Fuel Cycle - Advantages As A Nuclear Fuel

Advantages As A Nuclear Fuel

Thorium is estimated to be about three to four times more abundant than uranium in the Earth's crust, although present knowledge of reserves is limited. Current demand for thorium has been satisfied as a by-product of rare-earth extraction from monazite sands. Also, unlike uranium, mined thorium consists of a single isotope (232Th). Consequently, it is useful in thermal reactors without the need for isotope separation.

Thorium-based fuels exhibit several attractive properties relative to uranium-based fuels. The thermal neutron absorption cross section (σ
a) and resonance integral (average of neutron cross sections over intermediate neutron energies) for 232Th are about three times and one third of the respective values for 238U; consequently, fertile conversion of thorium is more efficient in a thermal reactor. Also, although the thermal neutron fission cross section (σ
f) of the resulting 233U is comparable to 235U and 239Pu, it has a much lower capture cross section (σ
γ) than the latter two fissile isotopes, providing fewer non-fissile neutron absorptions and improved neutron economy. Finally, the ratio of neutrons released per neutron absorbed (η) in 233U is greater than two over a wide range of energies, including the thermal spectrum; as a result, thorium-based fuels can be the basis for a thermal breeder reactor.

Thorium-based fuels also display favorable physical and chemical properties which improve reactor and repository performance. Compared to the predominant reactor fuel, uranium dioxide (UO₂), thorium dioxide (ThO₂) has a higher melting point, higher thermal conductivity, and lower coefficient of thermal expansion. Thorium dioxide also exhibits greater chemical stability and, unlike uranium dioxide, does not further oxidize.

Because the 233U produced in thorium fuels is inevitably contaminated with 232U, thorium-based used nuclear fuel possesses inherent proliferation resistance. 232U can not be chemically separated from 233U and has several decay products which 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. 233U can be denatured by mixing it with natural or depleted uranium, requiring isotope separation before it could be used in nuclear weapons.

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. A single neutron capture in 238U is sufficient to produce transuranic elements, whereas six captures are generally necessary to do so from 232Th. 98–99% of thorium-cycle fuel nuclei would fission at either 233U or 235U, 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.