Thursday, February 23, 2012

Thorium Nuclear Power – A Lesson From Norway.

Norway holds a resource of 170,000 tonnes of thorium, which amounts to 15% of the world’s total of 1.2 million tonnes. There is far more thorium than that within the earth’s crust all told, averaging 8 ppm compared with around 2.8 ppm for uranium, but the above figures refer to richer ores, most commonly monazite sand which contains up to 12% of thorium. There is some opinion that thorium nuclear power might be a better environmental/energy-strategy for Norway than relying on carbon-capture which many consider to be uneconomic. However, the matter of thorium reactors is not straightforward. Professor Egil Lillestol of Bergen University has been pushing thorium for some years now, and thinks that Norway should set the trend in building a prototype accelerator-driven reactor in which a massive particle accelerator converts thorium-232 to uranium-233 by irradiating it with slow (spallation) neutrons generated by the impact of a 1.6 GeV proton beam on a lead target. The conversion is not direct, and involves the initial formation of thorium-233, which decays rapidly to protactinium-233, and then to uranium-233 over a period of about a month. Hence presumably reprocessing is involved in the final stage, since if the protactinium-233 is left in the reactor it will be at least partly converted to protactinium-234, which is not a useful fissile material.

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.

However, it is a very big accelerator that will be needed to do the job, and the estimated costs for the project are about 500 million Euros. There are various advantages cited for this type of reactor, including the claim that it can be stopped easily if things get out of hand, and that it produces less long-lived nuclear waste than the uranium-fuelled fission reactors that are currently in common use. However, there are a whole host of scientific and engineering challenges that need to be overcome, and even identified in the first place because nobody has ever built one of these reactors, and hence the plans are still only on the drawing board.

As I have already stressed, it is a very big accelerator that will be needed if the project has any chance of success, so big in fact that there are none with sufficient power anywhere in the world. Some of the suggestions include using molten lead as the coolant for the system, but the reactor would run at a temperature above 700 degrees C. when the material becomes corrosive. A number of countries (including the US, Russia, the UK, France and Japan) have entrenched firm investments in uranium based reactors, and will use them for as long as they can. There are sizeable quantities of uranium on the world market, although the price has recently soared. Nonetheless, there is likely to be resistance to the research and development of a brand-new technology based on thorium, in view of huge costs that will effectively be borne by the Norwegian taxpayer if they go it alone down this unlit path.

The immediate future doesn’t look optimistic for thorium, certainly with the untested accelerator-driven reactors, and yet two thorium reactors have been operated, which were of the far simpler molten-salt reactor kind. Thus it might prove more expedient to invest in this at least tried technology, which could extend the useful lifetime of nuclear power by hundreds of years. The reason is that converting thorium-232 to uranium-233 is a form of “breeder” technology meaning that practically 100% of the thorium can be processed ultimately into nuclear fuel, rather than just the 0.7% uranium-235 isotope that exists in naturally occurring uranium, and which requires enrichment before it can be used. Indeed, the 99+% of uranium-238 can be converted into plutonium-239 and this used in fuel-rods, but there are many negative connotations attached to plutonium, which is almost the “p-word” for the nuclear industry: i.e. unmentionable, certainly in the tabloid press. There are serious issues of terrorism – dirty bombs at the very least, if not an out and out A-bomb detonation involving plutonium. The word alone would swathe a city and the world with fear. Uranium-233 made from thorium is harder to conceal than plutonium, since it is always contaminated with uranium-232, a strong gamma-ray emitter, and accordingly quite easily detected “in a suitcase” than plutonium which is principally an alpha-particle emitter and far more readily hidden.

There is no doubt that we will see a rise in nuclear power and for a number of reasons – cutting CO2 emissions, and securing energy supplies. Most of current thinking is based around using uranium as the fuel to drive it, but thorium could prove a very useful supplement and might power a new generation of reactors when we are short of uranium and do need to “breed” fuel if it proves uneconomic to mine poor quality uranium ores. I maintain my reservations about how long other resources, e.g. oil and gas will last, with which to mine and process either uranium or thorium, but if the latter appears viable in the longer run, I suggest that molten salt (liquid fluoride) reactors would be a better approach than the far more complex (and as yet untested) accelerator-driven systems.

The latter are reminiscent in scale to the putative nuclear-fusion reactors, said to mimic processes in stars, e.g. the sun, of which a working model is not expected for at least another 60 years. No one should forget that we need to make our energy provisions against a backdrop of 10 – 20 years at best, as oil and then gas begin to run short (the “Oil Dearth Era”). We do not want to back a loser now, as it is a one-off bet with the future of civilization resting on the outcome of this particular race.

Related Reading. There is also a link at the top left hand corner of this blog.

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