Thursday, December 21, 2006

Thorium - Letter to The Independent.

As I reported two postings ago "Thorium gets Good Press over Uranium", I sent a letter to "The Independent" newspaper in response to an article they published on using thorium as a source of energy for the future. That article focussed on Accelerator Driven Systems (ADS), and my point was that thorium could instead be used in a Liquid Fluoride Reactor, to particular advantage, notably avoiding the huge energy cost of running an accelerator system capable of producing an intensely powerful proton beam with which to produce neutrons by spallation, in a lead target. One such accelerator is the "ring-zyklotron" at the Paul Scherrer Institute in Switzerland, where I have carried out particle beam experiments over many years. The power is huge, and for a beam current of 1.5 milliamps with an accelerating potential of 600 million electron volts (MeV), we have a proton beam running at a power of 900 kilowatts.
Beams of charged particles are steered and focussed down "beam-lines" (steel-pipes pumped out to a hard vacuum) using dipole magnets for bending the beam, and quadrupole magnets (a pair of dipoles in pairs... sort of!) which act as "lenses" to focus the beam. Of course, depending on the polarity of a dipole magnet, the beam is bent (steered) either to the right or to the left, say. After one set-up, a magnet was set the wrong polarity, with the result that instead of the beam passing smoothly down the middle of the tube, it was brought into the wall of it, which immediately vapourised, drilling a hole about the size of the beam-spot - say about a circle 3 cms in diameter - straight through the side of the beam line. I was surprised at this until I worked out the power, and then realised that 900 kilowatts on a piece of metal about an inch across...well, what else would it do but punch a hole straight through it?! Particle beams are not "clean", either, and are contaminated with all kinds of radioisotopes from the "target", which took some time to be cleaned-up before the accelerator and its beam-lines were deemed safe for experimenters to return there!

As the "Inde" have not published that first letter - understandably as there are more immediately pressing issues such as the Ipswich murders in the press, I sent them a modified letter to see if the essential message could be got across with fewer technical details than in the original one. Since the Ipswich case, social policy aspects and the many features pertaining to the Middle East continue to absorb most of the space in the "Letters" section, I'm not sure if they will publish this one either! In any case, I have copied the text below:

I applaud Helen Brown's article "What Energy Crisis" (Wednesday,December 13) extolling the potential virtues of thorium as a nuclear fuel. However, thorium does not "require an accelerator-driven system" (ADS). Such accelerators need huge amounts of electricity to run them, as particle accelerators always do, but in this case are required to produce a beam of protons of such intensity that until 10 years ago the prevailing technology meant that it could not have been done. Rather like nuclear fusion, a working ADS reactor is some way off, and may never happen. However, thorium can be effectively utilised in a liquid-fluoride reactor, where the nuclear materials are present in the form of a solution of fluoride salts. This kind of reactor permits the continuous reprocessing of its nuclear material relatively easily (certainly as compared to solid fuel based nuclear reactors) which is critical in the way thorium is actually used. Details of this and related matters are at

Professor Chris Rhodes

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