Monday, December 18, 2006

Thorium gets Good Press over Uranium!

"The Independent" newspaper, published an article last week (Wednesday, December 13th) to which I responded with a letter, pointing out that an accelerator-driven system (ADS) is not the only way that thorium can be used to generate nuclear power, but that it could be used very advantageously in Liquid Fluoride Reactors. The letter has not been printed as yet, and so I have copied the text at the bottom of this posting. Such accelerators need huge amounts of electricity to run them, as particle accelerators do, but these 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, the working ADS technology is some way off, and may never happen, although professor Egil Lillestol of Bergen University in Norway is pushing that the world should use thorium in such ADS reactors. Using thorium as a nuclear fuel is a laudable idea, as is amply demonstrated in the blog "Energy from Thorium" (http://thoriumenergy.blogspot.com/) to which there is a link on this blog (above left). However, the European Union has pulled the plug on funding for the thorium ADS programme, which was directed by Professor Carlo Rubbia, the Nobel Prizewinner, who has now abandoned his efforts to press forward the programme, and instead concentrated on solar energy, which was another of his activities. Rubbia had appointed Lillestol as leader of the CERN physics division almost two decades ago, in 1989, who believes that the cause is not lost.
Thorium has many advantages, not the least being its greater abundance than uranium. It is often quoted that there is three times as much thorium as there is uranium. Uranium is around 2 - 3 parts per million in abundance in most soils, and this proportion rises especially where phosphate rocks are present, to anywhere between 50 and 1000 ppm. This is still only in the range 0.005% - 0.1% and so even the best soils are not obvious places to look for uranium. However, somewhere around 6 ppm as an average for thorium in the Earth's crust is a reasonable estimate. There are thorium mineral deposits that contain up to 12% of the element, located at the following tonnages in Turkey (380,000), Australia (300,000), India (290,000), Canada and the US combined (260,000)... and Norway (170,000), perhaps explaining part of Lillestol's enthusiasm for thorium based nuclear power. Indeed, Norway is very well endowed with natural fuel resources, including gas, oil, coal, and it would appear, thorium.
An alternative technology to the ADS is the "Liquid Fluoride Reactor" (LFR), which is described and discussed in considerable detail on the http://thoriumenergy.blogspot.com/ blog, and reading this has convinced me that the LFR may provide the best means to achieve our future nuclear energy programme. Thorium exists naturally as thorium-232, which is not of itself a viable nuclear fuel. However, by absorption of relatively low energy "slow" neutrons, it is converted to protactinium 233, which must be removed from the reactor (otherwise it absorbs another neutron and becomes protactinium 234) and allowed to decay over about 28 days to uranium 233, which is fissile, and can be returned to the reactor as a fuel, and to breed more uranium 233 from thorium. The "breeding" cycle can be kicked-off using plutonium say, to provide the initial supply of neutrons, and indeed the LFR would be a useful way of disposing of weapons grade plutonium and uranium from the world's stockpiles while converting it into useful energy. The LFR makes in-situ reprocessing possible, much more easily than is the case for solid-fuel based reactors. I believe there are two LFR's working already, and if implemented, the technology would avoid using uranium-plutonium fast breeder reactors, which need high energy "fast" neutrons to convert uranium 238 which is not fissile to plutonium 239 which is. The LFR is inherently safer and does not require liquid sodium as a coolant, while it also avoids the risk of plutonium getting into the hands of terrorists. It is worth noting that while uranium 235 and plutonium 239 could be shielded to avoid detection as a "bomb in a suitcase", uranium 233 could not, because it is always contaminated with uranium 232, which is a strong gamma-ray emitter, and is far less easily concealed.

The Independent article claims that thorium "...produces 250 times more energy per unit of weight" than uranium. Now this isn't simply a "logs versus coal on the fire" kind of argument, but presumably refers to the fact that while essentially all the thorium can be used as a fuel, the uranium must be enriched in uranium 235, the rest being "thrown away" and hence wasted as "depleted" uranium 238 (unless it is bred into plutonium). If both the thorium and uranium were used to breed uranium 233 or plutonium 239, then presumably their relative "heat output" weight for weight should be about the same as final fission fuels? If this is wrong, will someone please explain this to me as I should be interested to know?
However, allowing that the LFR in-situ reprocessing is a far easier and less dangerous procedure, the simple sums are that contained in 248 million tonnes of natural uranium, available as a reserve, are 1.79 million tonnes of uranium 235 + 246.2 million tonnes of uranium 238. Hence by enrichment 35 million tonnes (Mt) of uranium containing 3.2% uranium 235 (from the original 0.71%) are obtained. This "enriched fraction" would contain 1.12 Mt of (235) + 33.88 Mt of (238), leaving in the other "depleted" fraction 248 - 35 Mt = 213 Mt of the original 248 Mt, and containing 0.67 Mt (235) + 212.3 Mt (238). Thus we have accessed 1.79 - 0.67 = 1.12 Mt of (235) = 1.12/224 = 4.52 x 10*-3 or 0.452% of the original total uranium. Thus on a relative basis thorium (assuming 100% of it can be used) is 100/0.452 = 221 times as good weight for weight, which is close to the figure claimed, and a small variation in enrichment to a slightly higher level as is sometimes done probably would get us to an advantage factor of 250!

However, plutonium is a byproduct of normal operation of a uranium-fuelled fission reactor. 95 to 97% of the fuel in the reactor is uranium 238. Some of this uranium is converted to plutonium 239 and plutonium 241 - usually about 1000 kg forms after a year of operation. At the end of the cycle (a year to 2 years, typically), very little uranium 235 is left and about 30% of the power produced by the reactor actually comes from plutonium. Hence a degree of "breeding" happens intrinsically and so the practical advantage of uranium raises its head from 1/250 (accepting that figure) to 1/192, which still weighs enormously in favour of thorium!

As a rough estimate, 1.4 million tonnes of thorium (about one third the world uranium claimed, which is enough to last another 50 years as a fission fuel) would keep us going for about 200/3 x 50 = 3,333 years. Even if we were to produce all the world's electricity from nuclear that is currently produced using fossil fuels (which would certainly cut our CO2 emissions), we would be O.K. for 3,333/4 = 833 years. More thorium would doubtless be found if it were looked for, and so the basic raw material is not at issue. Being more abundant in most deposits than uranium, its extraction would place less pressure on other fossil fuel resources used for mining and extracting it. Indeed, thorium-electricity could be piped in for that purpose.
It all sounds great: however, the infrastructure would be huge to switch over entirely to thorium, as it would to switch to anything else including hydrogen and biofuels. It is this that is the huge mountain of resistance there will be to all kinds of new technology. My belief is that through cuts in energy use following post peak oil (and peak gas), we may be able to produce liquid fuels from coal, possibly using electricity produced from thorium, Thorium produces less of a nuclear waste problem finally, since fewer actinides result from the thorium fuel cycle than that from uranium. Renewables should be implemented wherever possible too, in the final energy mix that will be the fulcrum on which the survival of human civilization is poised.

Here is a copy of the text of my letter to The Independent:
:

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", it can be utilised to particular advantage in a liquid- fluoride reactor (a specific example of a molten salt reactor), where the nuclear materials are present in the form of fluoride salts dissolved in a solution of other fluoride salts. This kind of reactor permits the continuous reprocessing of its nuclear material relatively easily; certainly as compared to nuclear reactors which use solid fuel. This factor is critical in the way thorium is actually used, because the material must first be converted to protactinium by neutron irradiation from a fissile kick-starter element (e.g. plutonium), and then isolated from neutrons (by removal from the reactor) allowing this to decay to uranium-233. The U-233 is then introduced to the reactor to undergo nuclear fission and consequent energy production.

Professor Chris Rhodes.




10 comments:

Kirk said...

Hi Chris,

You're absolutely right on your energy calculations--a kg of thorium and a kg of uranium-238, if each fully converted to U-233 and Pu-239 respectively, and each fully fissioned, would produce the same amount of energy.

But currently, we only use a tiny fraction of the energy available in uranium, and to access the remainder requires fast-spectrum reactors and plutonium breeding, which will inevitably pose a proliferation challenge.

Thorium, on the other hand, can be fully consumed in thermal-spectrum reactors, and the presence of U-232 makes U-233 unsuitable for weapons.

Your letter looks excellent, however I would recommend changing the line about how the reaction is "kickstarted" with plutonium. Plutonium, in many ways, is the worst fuel to start a "clean" thorium fuel cycle on, and by "clean" I mean a thorium fuel cycle that produces no long-lived transuranic elements (plutonium, americium, curium, etc.)

The best fuel to start a fluoride reactor is uranium-233, of which there is about 1000 kg available worldwide from different thorium programs. "Electrobreeding" of U-233 from virgin thorium might be practical if one was only breeding the "start-charge" of U-233 for a fluoride reactor. Based on some calculations done on the forum, about 64 MW*yr of electricity would be necessary to produce 1000 kg of U-233, which in turn would be used to start a 1000 MW reactor. Thus the reactor could "pay back" the energy required to breed its starting fuel in about 3 1/2 weeks, after which it would run on a pure thorium input.

Barring avaiable U-233, highly-enriched uranium (>90% U-235) would be the next best start fuel. Significant quantities of HEU are available from decomissioned nuclear weapons, and use in a liquid-fluoride reactor is far preferable to "downblending" and use in a conventional solid-uranium reactor.

Anonymous said...

Excuse me for the off-topic, but I could not find other means to contact you. Is the full-text RSS feed of your blog available?

Thanks,
Anton.

energybalance said...

Hi Kirk,
glad you picked-up on my request for comments/further infomation. As I thought it is a direct weight for weight comparison that exists between "fissile" fuels, but the awfully wasteful uranium enrichment process that is responsible for the huge advantage for thorium. This must amount to collosal quantities of U-235 being thrown away, or used in military weapons/armaments along with the intrinsic 99% or so of uranium-238. I might well put some of your information into another posting on thorium as the mood takes me (or what is is the recent press!).
But yes, I am here to defend the faith insofar as government policy dictates a nuclear component, and if that were to be based around thorium, all the better on a whole host of scores!

energybalance said...

Dear Anton,
there is an RSS link in the top left hand section of the blog. Try that and if it doesn't get you what you want, get back to me and I will try to help.

Chris.

Anonymous said...

Chris,

That RSS link, as well as the others specified in the 'link rel="alternate"' sections of the blog's HTML, only provide the summaries of the posts. What I would like to ask is a full-text feed, which provides complete entries.

I know that it is possible to do that in blogs hosted on blogger.com/blogspot.com - there is a number of blogs that provide just that.

It is in fact a matter of a considerable controversy. Some bloggers intentionally disable full-text feeds since they prefer their readers to actually show up on their sites to attract ad-based revenue, instead of reading the complete entries in a number of existing feed-reader programs. On the other hand, a number of blog readers are alienated by the concept, and would rather prefer to have a single point to read various blogs from. I know people who for that reason plainly refuse to subscribe to (and read) blogs without full-text feeds. Not being one of them, I nevertheless prefer to read blogs in on place.

So my question was, is it possible to activate the full text feed feature on the Energy Balance, or whether it is inactive by your choice.

Thanks,
Anton.

energybalance said...

Hi Anton,
I'm not out to make money on this blog, but since I am "effectively" self-employed these days and running it at my own expense, the odd "dollar in the box" that comes when someone clicks on an advert is welcome. I make practically nothing from it, and anyway it is the issues that matter, and that's why I started writing it.
The full-text feed is inactive through the code that we use, and I agree it is controversial which is the preferred style, but many people including myself would rather have the title and first few lines coming-up than the full text, to avoid being "awash" in information.
So "Energy Balance" intends to keep its present format! I hope that will not deter too many of my current readers - and the "spike" is truly massive for the number of hits over the past 2 days! I'm glad it is being read at least, by yourself and others.
Do come back on these points if you wish.

Regards,

Chris.

Anonymous said...

Chris,

I respect your choice although it is a bit disappointing. Just one small note, though: the existence of a full-text feed does not mean the unavailability of a summaries-only feed - and even when the underlying platform does not support having both at the same time, the summaries are still there within the full-text feed, and most reader software allows one to select between "summaries only" and "complete entries".

Anton.

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Chris Rhodes said...

Glad to have been of help!

Regards,

Chris Rhodes