Wednesday, June 07, 2006

Thorium instead of Uranium?

It may well turn out that thorium is the better nuclear fuel as compared with uranium, since it offers the advantages that: (1) it is present in around 3 times the abundance of uranium on Earth, overall, (2) it can be bred into the fissile nuclear fuel uranium-233, (3) far less plutonium and other transuranic elements are produced than is the case from uranium fuel, (4) the thorium fuel cycle might be used to consume plutonium, thus reducing the nuclear stockpile while converting it into useful electrical energy.
Thorium is a naturally occuring element discovered in 1828 by the famous Swedish chemist Berzelius, who named it after Thor, the Norse god of thunder. It is found in soils at an average of 6 parts per million (ppm), and in most rocks. In higher concentrations, thorium occurs in several kinds of mineral, of which the most common is monazite which contains around 12% of it. Other than negligible amounts of a few highly radioactive isotopes, thorium occurs exclusively as thorium-232. The world total of economically extractable thorium is estimated at around 1.2 million tonnes, and Australia and India top the list with 300,000 and 290,000 tonnes of it respectively. Interestingly, Norway has 170,000 tonnes of thorium, which adds to the large energy reserves of this country in terms of gas, oil and coal. Perhaps Norway will be less hard hit by Peak Oil than will other nations, although given its largely northern location, should the Atlantic Conveyor slow down and cool the northern hemisphere, it will need to burn more fuel than more southern lands. For comparison, I note that the U.S.A. has 160,000 tonnes and Canada 100,000 tonnes of thorium.
Although thorium-232 is not fissile in itself, it can be converted to a fissile fuel in the form of uranium-233 via the absorption of slow neutrons. Hence as is the case for uranium-238, thorium-232 is "fertile" and may be bred into a nuclear fuel, which in the former case is plutonium-239. I am told that there is one essential contrast between the two elements thorium-233 and plutonium-239, namely that while the latter may be used as "weapons grade plutonium" in a nuclear weapon, uranium-233 is a far less effective nuclear explosive, and has not been therefore used as such.
Since 100% of the thorium fuel can be converted into nuclear fuel compared with only the 0.7% of natural uranium that is fissile uranium-235, which is enriched by centrifugation or gaseous diffusion of uranium hexafluoride, there is an obvious advantage. It might be argued that the rest of the 100% of uranium (238) can be converted to plutonium in a similarly effective manner, but this requires fast neutrons in a fast breeder reactor: a technology with many disadvantages, both in terms of handling the toxic plutonium, pyrophoric liquid sodium, or its even more reactive alloy with potassium, and that these kinds of reactors are less controlable and in a worst case scenario could go-off like a bomb! Thus, in avoiding the latter method, around 40 times as much energy might be extracted from thorium than from an equivalent quantity of uranium. Even on the basis of the "known" 1.2 million tonne resource of thorium, a simple sum indicates that it could provide 40 times the sustainability of current available uranium fission based nuclear power for: [1.2 million (tonnes of thorium)/3.5 million (tonnes of uranium)] x 40 x 50 years (i.e. the current estimate based on uranium) = ca. 700 years. Even if we made all our electricity from thorium, around a quarter of that would be possible, i.e. just short of two centuries worth, and so if governments are intent on nuclear expansion to get around global warming, thorium may well prove a better alternative to uranium.
I recommend to the interested reader an excellent blog that I have been referred to (http://thoriumenergy.blogspot.com/) which details the potential use of thorium as a nuclear fuel, and to which I have posted a link, above.
Having visited Prague recently, I was struck by the efficiency of their public transport system: trams and the metro. If sufficient electricity to power such systems might be provided from thorium, whatever is possible from water (hydro, tidal stream, wave), solar power and wind power, and they were employed generally in the world, we might just break the hold on us of our dependence on oil in the present enormous quantities required for transportation, at least on a localised level. Current levels of aviation would remain unsustainable, however.

3 comments:

Kirk Sorensen said...

There are several reasons why U-233 is unattractive for nuclear weapons. One is that it doesn't produce as many neutrons in fast fission as Pu-239. Another is that its properties in very fast fission (such as a nuclear detonation) are poorly understood.

But the biggest deterrent is that U-233 is inevitably contaminated with U-232 during its formation. It is highly impractical to separate them. And U-232 has a short half-life (~80 years) and a decay chain that includes the strong gamma emitter Tl-208. A few months after the U-233 is isolated from parent materials, the decay chain of U-232 begins to set up and the strong 2.3 MeV gammas of Tl-208 would irradiate the weapon, its electronics, as well as providing an easily-detected alert to the world that U-233 was present in a location.

In contrast, the alpha decay of U-235 and Pu-239 are rather easily shielded, making clandestine transport of these weapons much easier, and well as allowing long-term storage with relatively little damage to the electronics of the device. With all these drawbacks, it is not surprising that U-233 has not been utilized in operational nuclear weapons. This inherent physical resistance to proliferation is a powerful argument for the adoption of thorium as a basic nuclear energy source.

Willem van der Merwe said...

I 200% agree, if one consider that two HTR`s were successflly operated on U233 from thorium, the ten percent of waste compared to conventional uranium-route,  it seems that there is a conducted negation of all the superior qualities of thorium over uranium,

Willem van der Merwe said...

I 200% agree, if one consider that two HTR`s were successflly operated on U233 from thorium, the ten percent of waste compared to conventional uranium-route,  it seems that there is a conducted negation of all the superior qualities of thorium over uranium,