thorium cycle and reprocessing (Score 1)

In a pressurized light water reactor, the reactor-grade uranium fuel has to be replaced after 3% of the fuel has been burned up, leaving us with 97% useful fuel and some dangerous waste products. Since reprocessing was forbidden in the United States by President Carter, we've been making plans to vitrify and bury all of our leftover fuel in a way that would render it permanently inaccessible. Even if we reversed the ban on reprocessing, we would still be dealing with a lot of dangerous long-lived actinides that are highly radioactive but last longer than the human race has existed.

I recently read a detailed analysis of the thorium fuel cycle. Based on a probabilistic analysis of the decay product chain, it's believed that a practical thorium fuel cycle can be designed that burns up 97% of the fuel, leaving only 3% of the input material behind as nasty stuff. Thorium is also quite plentiful in comparison to uranium, and is isotopically pure unlike uranium.

Here are some interesting facts about the thorium fuel cycle from wikipedia:

There are several potential advantages to thorium-based fuels.

Thorium is estimated to be about three to four times more abundant than uranium in the earth's crust[3], 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, naturally occurring thorium consists of only a single isotope (232Th) in significant quantities. Consequently, all mined thorium is useful in thermal reactors.

Thorium-based fuels exhibit several attractive nuclear properties relative to uranium-based fuels. The thermal neutron absorption cross section (a) and resonance integral for 232Th are about three times and one third of the respective values for 238U; consequently, fertile conversion of the former is more efficient in a thermal reactor. Also, although the thermal neutron fission cross section (f) of the 233U is comparable to 235U and 239Pu, it has a much lower capture cross section () than the latter two fissile isotopes, resulting in fewer non-fissile neutron absorptions and improved neutron economy. Finally, the number of neutrons released per neutron absorbed () 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 [1].

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.[1]

Because the 233U produced in thorium fuels is inevitably contaminated with 232U, thorium-based used nuclear fuel possesses inherent proliferation resistance. Uranium-232 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.

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[ citation needed ], 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.