Integral Fast Breeder Reactors

[ewmayer]

This is related to a topic which recently cropped up in a "how would you stimulate the economy" sub-discussion in the MET2010 thread in the Soapbox ... I suggested a massive "Green Energy Manhattan Project", with one of the key technologies being Thorium reactors, which have numerous and potentially massive advantages over conventional "third generation" reactors and few of the drawbacks - most especially, they are physically incapable of meltdown-style and loss-of-coolant failures like did in Chernobyl and Three Mile Island, they burn a vastly greater fraction of fissile and "activatable" material (including spent waste from conventional reactors), they produce a much smaller waste stream consisting only of short-lived (safe in a few centuries, rather a hundred millennia) isotopes, and they do not pose the kind of dual-use weapons-proliferation issues conventional reactor designs do.

A friend sent me this link to a piece on Australian National Radio (ABC)'s Science Show program, from July of last year, on this subject - fascinating stuff. The page contains the full transcript, plus a link to the original audio program - pick your format:

http://www.abc.net.au/rn/scienceshow...09/2629053.htm

And here is a Wikipedia Page on IFRs.

[ewmayer]

Some Additional links for the interested readers:

http://en.wikipedia.org/wiki/Molten-...tor_Experiment

http://en.wikipedia.org/wiki/Thorium_fuel_cycle

http://en.wikipedia.org/wiki/Molten_salt_reactor

http://en.wikipedia.org/wiki/Nuclear_fuel_cycle

The second of the above articles gives an excellent summary of the various nuclear reactions involved, and summarizes the pros and cons:

Advantages as a nuclear fuel

Thorium offers several potential advantages:

Thorium is estimated to be about three to four times more abundant than uranium in the Earth's crust[9], 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 nuclear properties relative to uranium-based fuels. The thermal neutron absorption cross section (\sigma_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 (\sigma_f) of the resulting 233U is comparable to 235U and 239Pu, it has a much lower capture cross section (\sigma_v) 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 (\eta) 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. 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.

The long term (on the order of roughly 10^3 to 10^6 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.
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Disadvantages as nuclear fuel

There are several challenges to the application of thorium as a nuclear fuel, particularly for solid fuel reactors.

Unlike uranium, natural thorium contains no fissile isotopes; fissile material, generally 233U, 235U, or plutonium, must be supplemented to achieve criticality. This, along with the high sintering temperature necessary to make thorium-dioxide fuel, complicates the fuel fabrication process. Oak Ridge National Laboratory experimented with thorium tetrafluoride as fuel in a molten salt reactor from 1964–1969, which was far easier to both process and separate from fuel poisons (contaminants that slow or stop the chain reaction.)

If thorium is used in an open fuel cycle (i.e. utilizing 233U in-situ), higher burnup is necessary to achieve a favorable neutron economy. Although thorium dioxide has performed well at burnups of 170,000 MWd/t and 150,000 MWd/t at Fort St. Vrain Generating Station and AVR respectively[1], challenges complicate achieving this burnup in light water reactors (LWR), which compose the vast majority of existing power reactors.

Another challenge associated with a once-through thorium fuel cycle is the comparatively long interval over which 232Th breeds to 233U. The half-life of 233Pa is about 27 days, which is an order of magnitude longer than the half-life of 239Np. As a result, substantial 233Pa builds into thorium-based fuels. Protactinium-233 is a significant neutron absorber, and although it eventually breeds into fissile 235U, this requires two more neutron absorptions, which degrades neutron economy and increases the likelihood of transuranic production.

Alternately, if solid thorium is used in a closed fuel cycle in which 233U is recycled, remote handling is necessary for fuel fabrication because of the high radiation dose resulting from the decay products of 232U. This is also true of recycled thorium because of the presence of 228Th, which is part of the 232U decay sequence. Further, although there is substantial worldwide experience recycling uranium fuels (e.g. PUREX), similar technology for thorium (e.g. THOREX) is still under development.

Although the presence of 232U makes it a challenge, 233U can be used in fission weapons, but this has been done only occasionally. The United States first tested 233U as part of a bomb core in Operation Teapot in 1955.[10] However, unlike plutonium, 233U can be easily denatured by mixing it with natural or depleted uranium. Another option is to judiciously mix thorium fuels with small amounts of natural or depleted uranium during fabrication to ensure that 233U concentrations at the end of cycle are acceptably low.

Though thorium-based fuels produce far less long-lived transuranics than uranium-based fuels,[7] some long-lived actinide products constitute a long term radiological impact, especially 231Pa.[8]

Advocates for liquid core and molten salt reactors claim that these technologies negate thorium's disadvantages. Since only one liquid core reactor using thorium has been built, it is hard to validate the exact benefits.[citation needed] A possible thorium disadvantage is its lack of relevance to the nuclear weapon industry.[dubious – discuss]