Thorium: Proliferation warnings on nuclear 'wonder-fuel'

Thorium is being touted as an ideal fuel for a new generation of nuclear power plants, but in a piece in this week's Nature, researchers suggest it may not be as benign as portrayed.

The element thorium, which many regard as a potential nuclear "wonder-fuel", could be a greater proliferation threat than previously thought, scientists have warned.

Writing in a Comment piece in the new issue of the journal, Nature, nuclear energy specialists from four British universities suggest that, although thorium has been promoted as a superior fuel for future nuclear energy generation, it should not be regarded as inherently proliferation resistant. The piece highlights ways in which small quantities of -233, a material useable in , could be produced covertly from thorium, by chemically separating another isotope, protactinium-233, during its formation.

The chemical processes that are needed for protactinium separation could possibly be undertaken using standard lab equipment, potentially allowing it to happen in secret, and beyond the oversight of organisations such as the (IAEA), the paper says.

The authors note that, from previous experiments to separate protactinium-233, it is feasible that just 1.6 tonnes of thorium metal would be enough to produce 8kg of uranium-233 which is the minimum amount required for a nuclear weapon. Using the process identified in their paper, they add that this could be done "in less than a year."

"Thorium certainly has benefits, but we think that the public debate regarding its proliferation-resistance so far has been too one-sided," Dr Steve Ashley, from the Department of Engineering at the University of Cambridge and the paper's lead author, said.

"Small-scale chemical reprocessing of irradiated thorium can create an isotope of uranium – uranium-233 – that could be used in nuclear weapons. If nothing else, this raises a serious proliferation concern."

Thorium is widely seen as an alternative nuclear fuel source to uranium. It is thought to be three to four times more naturally abundant, with substantial deposits spread around the world. Some countries, including the United States and the United Kingdom, are exploring its potential use as fuel in civil nuclear energy programmes.

Alongside its abundance, one of thorium's most attractive features is its apparent resistance to nuclear proliferation, compared with uranium. This is because thorium-232, the most commonly found type of thorium, cannot sustain nuclear fission itself. Instead, it has to be broken down through several stages of radioactive decay. This is achieved by bombarding it with neutrons, so that it eventually decays into uranium-233, which can undergo fission.

As a by-product, the process also produces the highly radiotoxic isotope uranium-232. Because of this, producing uranium-233 from thorium requires very careful handling, remote techniques and heavily-shielded containment chambers. That implies the use of facilities large enough to be monitored.

The paper suggests that this obstacle to developing uranium-233 from thorium could, in theory, be circumvented. The researchers point out that thorium's decay is a four-stage process: isotopically pure thorium-232 breaks down into thorium-233. After 22 minutes, this decays into protactinium-233. And after 27 days, it is this substance which decays into uranium-233, capable of undergoing nuclear fission.

Ashley and colleagues note from previously existing literature that protactinium-233 can be chemically separated from irradiated thorium. Once this has happened, the protactinium will decay into pure uranium-233 on its own, with little radiotoxic by-product.

"The problem is that the neutron irradiation of thorium-232 could take place in a small facility," Ashley said. "It could happen in a research reactor, of which there are about 500 worldwide, which may make it difficult to monitor."

The researchers note that from an early small-scale experiment to separate protactinium-233, approximately 200g of thorium metal could produce 1g of protactinium-233 (and therefore the same amount of uranium-233) if exposed to neutrons at the levels typically found in power reactors for a month. This means that 1.6 tonnes of thorium metal would be needed to produce 8kg of uranium-233. They also point out that protactinium separation already happens, as part of other .

Given the need for access to a research or power reactor to irradiate thorium, the paper argues that the most likely security threat is from potential wilful proliferator states. As a result, the authors strongly recommend that appropriate monitoring of thorium-related nuclear technologies should be performed by organisations like the IAEA. The report also calls for steps to be taken to control the short-term irradiation of thorium-based materials with neutrons, and for in-plant reprocessing of thorium-based fuels to be avoided.

"The most important thing is to recognise that thorium is not a route to a nuclear future free from proliferation risks, as some people seem to believe," Ashley added. "The emergence of technologies will bring problems as well as benefits. We need more debate on the associated risks, if we want a safer nuclear future."

The researchers are: Dr Stephen F. Ashley and Dr. Geoffrey T. Parks from the University of Cambridge; Professor William J. Nuttall from The Open University; Professor Colin Boxall from Lancaster University; Professor Robin W. Grimes from Imperial College London.

Copies of the comment piece in this week's Nature are available on request. Interviews with Dr Steve Ashley can also be arranged by contacting Tom Kirk.

Explore further

UK stays cautious over thorium as nuclear fuel

More information: Nuclear energy: Thorium fuel has risks, DOI: 10.1038/492031a
Journal information: Nature

Provided by Open University
Citation: Thorium: Proliferation warnings on nuclear 'wonder-fuel' (2012, December 5) retrieved 22 August 2019 from
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Dec 05, 2012
"thorium-232 breaks down into thorium-233" is an error -- it must absorb a neutron for this to occur

Dec 05, 2012
So is this a method of producing weapons grade uranium without the use of centrifuges?
Thorium can be purified chemically, irradiated, then the protactinium-233 chemically seperated. You then have nearly pure protactinium-233. This can be allowed to decay into uranium-233, then chemically purified.

Dec 05, 2012
The "trick" to this is you must develop "high frequency" Pa separation to avoid forming 234Pa, which eventually gives rise to U232. Developing Pa separation is not the difficulty, it is the frequency of the separation of the Pa, with no losses, to guarantee that the gigantic cross section of 233Pa cannot absorb another neutron, which is quite difficult, and has never been demonstrated. The claim has never been that it is impossible to produce pure U233. The only claim I am aware of is that it is much easier, massively cheaper, & vastly more reliable, to produce high purity 239Pu by diverting research reactor material through the well established path of U238 irradiation [where there is no problem similar to U232 contamination].

Dec 05, 2012
Just checked wikipedia, since we all know it is always right.

On U-232 impurity:
Production of 233U (through the irradiation of thorium-232) invariably produces small amounts of uranium-232 as an impurity, because of parasitic (n,2n) reactions on uranium-233 itself, or on protactinium-233.

On U-232 decay:
The decay chain of 232U quickly yields strong gamma radiation emitters:
232U (α, 72 years)
228Th (α, 1.9 year)
224Ra (α, 3.6 day, 0.24 MeV)
220Rn (α, 55 s, 0.54 MeV)
216Po (α, 0.15 s)
212Pb (β−, 10.64 h)
212Bi (α, 61 s, 0.78 MeV)
208Tl (β−, 3 m, 2.6 MeV)
208Pb (stable)

... and in weapons:
The hazards are significant even at 5 parts per million. Implosion nuclear weapons require U-232 levels below 50 PPM (above which the U-233 is considered "low grade"

Dec 05, 2012
I forgot it is 233Pa being subject to parasitic [n,2n] reactions and forming 232Pa, which then decays to Uranium 232.

Dec 05, 2012
of course there is also the thorium purity issue with 230Th plus n => 231Th minus b => 231Pa plus n => 232Pa minus b => 232U

Dec 05, 2012
Thorium is cleaner and safer than uranium and plutonium, but still based on neutrons. Aneutronic fusion is that is far the cleanest and safest nuclear option, almost no neutron emissions, no risk of weapon proliferation.

Dec 05, 2012
The book, THORIUM: energy cheaper than coal, has a lengthy discussion of the difficulties of perverting commercial liquid fluoride thorium reactors to make weapons. It concludes that a nation intent on becoming a nuclear power would not bear the cost and technical risk of inventing and manufacturing a new weapon but choose the well-known technology route trod by North Korea, India, and Pakistan -- centrifuge uranium enrichment or plutonium production with frequent fuel changes in a graphite or heavy water reactor. In fact the single fluid denatured molten salt reactor version is more proliferation resistant than today's light water reactors. Note that Iran has just removed fuel rods from its new LWR after only 2 months instead of 18, sacrificing power generation to harvest high purity weapons-grade plutonium before it is degraded to reactor-grade plutonium. The world needs to look at the big picture of energy demands, environmental degradation, and conflict risks.

Dec 05, 2012
I am sure everyone had fun pitching their pet theory, but how about reality (often discarded, ignored or painted over with someone's favorite color). Consider that Thorium is abundant and naturally concentrated on the earth's surface. Thorium is easy to identify, easy to collect, easy concentrate and easy to separate from other minerals and elements. Thorium is available to anyone willing to collect it -- in thousands of places and on every continent. In fact, you could easily train an illiterate child to collect, concentrate and chemically separate (purify) Thorium from common beach sand. So, if Thorium does have any weapons potential there is nothing you could do about it anyway.

Dec 05, 2012
And now with the ability to easily concentrate deuterium: it may be possible for countries like Iran to produce thermonuclear weapons.

Dec 05, 2012
How about reality: Consider that Thorium is abundant and naturally concentrated on the earth's surface. Thorium is easy to identify, easy to collect, easy concentrate and easy to separate from other minerals and elements. Thorium is available to anyone willing to collect it -- in thousands of places and on every continent. In fact, you could easily train an illiterate child to collect, concentrate and chemically separate (purify) Thorium from common beach sand. So, if Thorium does have any weapons potential there is nothing you could do about it anyway.

Dec 06, 2012
To burn 96% of the 170,000 tons of nuclear weapons and
LWR spent fuel while producing CO2 free thermal and
electrical power trumps the miniscule proliferation
risk of the LFTR. Also, exploiting Thorium in this way
will help unleash many of the Rare Earth Elements
to the market place.

Dec 06, 2012
For this situation to occur as the article states, they'd have to have a reactor that separates out the Pa-233.
That idea was thought of during the MSRE at Oak Ridge in the 60's, but with the more recent concerns, reactors that does not need to separate it are favored.

The whole point in separating it was to help the neutron economy by preventing parasitic absorptions. You don't need to remove the Pa-233 for that though, increasing the volume of the thorium blanket around the core does the job just fine, and makes sure any U-232 stays together with the U-233.
The added cost for a bigger blanket is very small.

A reactor like that can't be used to create usable U-233 for weapons without significant modifications, which would be blatantly obvious to inspectors.

On a final note, only a single nuclear weapon using U-233 have been tested, the MET blast of Operation Teapot, and it was a mixed Pu-239/U-233 core with ratios that are unknown.
The yield was significantly less than expected.

Dec 06, 2012
Why doesn't carry "Nuclear energy: Radical reactors"
in 05 December 2012 Nature by M. Mitchell Waldrop ??

Dec 06, 2012
Fears of proliferation have kept this country burying more than half it's nuclear fuel in mountains or storing it on site as "waste" for decades.

Newsflash, if a country reaches a certain technological level and they want nuclear weapons they ARE GOING TO GET THEM....period. The only way you're going to stop it is to invade and occupy the country for years...or periodically bomb them back into the stone age.

This fear mongering proliferation argument is getting VERY thin. It's used by people who have a problem with nuclear power generation. It's about a policy they don't like, not about more countries getting weapons...because ANYONE with an ounce of foresight knows they'll get them anyway. You're going to have increased proliferation of nuclear weapons no matter WHAT policy you think is going to save you from it. The only question is are you going to allow yourself to benefit from the use of thorium reactors and stop burying nuclear fuel in mountains for eons or are you not....

Dec 06, 2012
This study was funded by uranium mining and fuel rod assembly companies. It might be getting money from the uranium waste disposal companies but they dont exist because the waste is simply kept onsight in fue waterl pools waiting for disposal or disaster....whichever comes first.

Dec 06, 2012
Simply told that any point in the Th breeding process up to and including Tea Pot type U233 be required to small 'spike' any blend with 'poison' U238 to render purified weapon grade useless.
Easy requirement for any NPT nation.

Dec 07, 2012
Has any state actually used these backdoors to create nuclear weapons? Even though it is possible to use nuclear waste to create nuclear weapons, I understand that no one has ever bothered to do it. These proliferation fears belong in the realm of science fiction. Every nuclear armed state has elected to design reactors specifically to produce weapons usable uranium or plutonium, and none have used nuclear waste to produce weaponry.

Dec 07, 2012
I am surprised that nobody has used an obvious way to make Pu 239. Pipe a solution of U238 (nitrate or sulfate) through a reactor core or expose it to some other intense neutron source. Pick out the Np from the solution with a ion exchange resin, then as it decays to Pu 239, sort that out of solution with ion exchange and concentrate to virtually pure Pu 239. The result should have virtually no Pu 240 in it.

Dec 09, 2012
Is it not the case that humans are already capable of the alchemical holy grail of transmuting lead into gold, albeit at a cost that negates the purpose? Therefore if cost is not a factor surely no nuclear fuel will ever be 'proliferation safe' in the hands of a determined State.

just askin'

Dec 09, 2012
yeah the *actual* wonder fuel... is illegal, because humans are insane and hate themselves. hemp

Dec 10, 2012
The lead author wrote: "Small-scale chemical reprocessing of irradiated thorium can create an isotope of uranium – uranium-233 – that could be used in nuclear weapons. If nothing else, this raises a serious proliferation concern."

Does this author not realize the process needs no reactor? Why hide the fact that one could buy a standard medical proton accelerator and use a simple spallation target to irradiate common Th232 with neutrons to produce as much Pa233 & U233 as desired?

What is this paper's motive?

Where were the editors of this site?

I've conversed with one of the authors to get clarification of their purpose in simply restating well-known physics, as if it presents unknown danger for Thorium use in civilian power.

What the authors fail to mention is that the Th molten-salt reactor (MSR) is where Thorium provides the most safety & efficiency benefits. In such a reactor, fissile salt (U233) constitutes ~1/30 of the salt content. How to get a bomb's worth at >700C?

Jan 27, 2013
Thank You DRAlexC
What was the point of the article? What is the motive? Hey you English talk about it, the Chinese will do it. Remember this, Cheap Power equals Global Power...The country that has one will have the other

Jan 27, 2013
The "Comment piece in the new issue of the journal, Nature" is being publicly critiqued for its errors, so will be getting a copy of that shortly. Why the editors here have decided to proffer "comment piece" as Physics is a mystery..

The authors, one whom I know, have received critical comments already -- here is the first set, to be expanded on to Nature...
I'm surprised that this is stated as above. For example, a molten-salt reactor needs only a ton of U233 to operate at 1GWe for a year, running a city. The amount of Thorium needed if the reactor is breeding U233 internally, is also a ton. The rate of Pa233 & U233 production is about 1kg per hour, to run a city. If they were trying to extract Pa or U from the reactor for weapons, they would have to do so over more than 8 hours to meet your condition, while the reactor essentially stops. If he/she were trying to go undiscovered, the theft rate would have to be so slow that over a day would be required, yet...

Jan 27, 2013
...(since this site seems more concerned with "wit" than facts, here's the continuation)...
yet the loss of reactor power would easily be detected by operators.

If your intent is to show that Th can be converted to Pa & U233 outside a reactor, as by proton-accelerator and neutron production via spallation, then that makes even less sense, since Th and medical accelerators are easily obtained.

So, I think your article needs some broadening/correction to address the real situation re proliferation. Thorium & Pa aren't the issue at all, so that concentration is misleading. It would be far easier for a group to simply steal spent LWR fuel, and process it using chemical & laser enrichment, etc. to get U235 & Pu239.

Since these processes have been available worldwide and Th is abundant, why no U233 bombs? Do the authors think Nobel folks like Seaborg, Wigner, etc. were fools?
Dr. A. Cannara
http colonslashslash tinyurl dotcom/7o6cm3u

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