Thursday, May 01, 2008

Thorium / uranium ratios and atomic power

Thorium and uranium are the only two actinide elements that are stable enough to have survived since the formation of the solar system without being destroyed by radioactive decay. They are geochemically similar. Both are strongly lithophilic, meaning that they form oxides and dissolve into silicate melts, without entering into metallic or sulfide phases. Both elements are fairly refactory, meaning that they condense earlyish in the solar nebula. And they are both incompatible during mantle melting. This means that they do not fit into the crystal structure of mantle minerals, so when a planetary mantle undergoes partial melting, the vast majority of the U and Th dissolves into the melt, and relatively little remains in the mantle mineral residue. This is because Th and U are both large +4 cations under most conditions, and mantle minerals (made mostly of Mg, Si, Fe, Al, and Ca oxides) don’t have any crystallographic sites into which such ions can fit.

Of the two, Th is slightly more refactory and incompatible, but not much, so their ratio doesn’t change a whole lot. As a result, the solar ratio (about 4) is not that different to the ratio of the lunar crust, the Martian crust, the Earth’s crust, and the Earth’s mantle. For most meteoritic, igneous, and mantle rocks, Th/U is about 4.

This number seems to have been picked up on by the more extreme proponents of nuclear fission- generated electricity, as evidence that even if we run out of uranium, there is still lots more thorium. Technically, this is true. If we were to run the entire solar system through a refining mill with a 100% recovery percentage, then at the end of the project, we would have 4 times more thorium than uranium. But the real world doesn’t work like that.

Even though the crustal Th/U ratio is also about 4, we aren’t conceivably going to dig up the entire Earth’s crust either. So the bulk crustal (or planetary) Th/U ratio is not really relevant to anything other than science fiction. What is important is the ability of geological processes to concentrate these elements into a highly enriched deposit, which we can then mine for a reasonable cost and effort. And this is where U and Th start to differ.

Although these two actinides are geochemically similar under most conditions in the solar system, there is one key difference in their chemistry. In the presence of abundant oxygen, U can oxidize from +4 to +6. For the first two billion years of Earth’s history, this was irrelevant. But about 2.4 billion years ago, free molecular oxygen first started appearing in Earth’s atmosphere and surface waters. And this changed everything.

Thorium can’t form a +6 ion, because Th +4 has the same electron configuration as the noble gas radon- all the electron shells are closed. But uranium has 2 extra electrons to lose, given enough oxygen around to take them. And the hexavalent chemistry is quite different to the tetravalent. In the +4 valence, both U and Th are generally insoluble under most hydrologic conditions. But U +6 forms a uranyl ion (UO2++), which is highly soluble in most geologically reasonable waters.

As a result, for the last 2.4 billion years, uranium has been dissolving from oxidized rocks, flowing through aquifers with the groundwater, and then reprecipitating wherever a later chemical reaction consumes the oxygen in the water. What this means is that uranium can be- and is- concentrated by a geologic process which has no effect on thorium.

The result is that uranium forms deposits more frequently, and of higher grade, than does thorium, which is distributed much more evenly across a wide variety of rock types. Today the world has a uranium reserve of 4 million tonnes, with a resource maybe ten times larger. This is despite not actively exploring for the substance since the cold war ended. Thorium, strictly speaking, doesn’t have reserves at all; it is currently only recovered as a byproduct of rare earth element mining. But based on known occurrences of monazite (LREE)PO4, which can contain a few percent of Th), the estimated resource is about 1.5 million tons.

So although the bulk crustal concentration of thorium is higher than that of uranium, its simpler chemistry means that it does not get concentrated into mineral deposits as easily. So while claiming that Th is more abundant is technically correct, it isn’t the sort of technicality that one should base energy policy on.
[edit: typos]

16 comments:

djm said...

Great post - Thorium seemed interesting from a non-proliferation perspective, but I guess the devil really is in the details.

For comparison, what is the typical concentration of U in ore? Does the hexavalent chemistry make Th easier to "serendipitously" recover from other ore-processing operations?

EcoGeoFemme said...

awesome post!

Dr. Lemming said...

Thorium reactors actually enable proliferation.

For example, the only country that presently has a thorium reactor program is India. India uses much of their uranium in their nuclear weapons program, and because of this, most other countries won't sell uranium to them. So since India has a native thorium supply, by burning thorium in their reactors they can use their relatively small uranium deposits for vaporizing- er, deterring Pakistan.

Uranium grades can be as high as 20%, or as low as 400 ppm. With the lower grades, however, the deposits are usually very large, and/or polymetallic, so that they also recover other minerals such as vanadium, copper, gold, or phosphorus*.

*Not actually a metal.

Chris said...

Just for curious... what valuable elements can be profitably mined out of coal ash and/or smokestack scrubbings? I've heard there's enough uranium to be a significant contribution to above-ground anthropogenic radioisotope load, but I don't know where to get the numbers and I wouldn't trust them if I found them.

Thanks,
Chris

Chuck said...

Depends on the coal. I think there is a lignite deposit in America (TX or CO?) that was looked at as a potential U mine- in the 300ppm range. Burning that would only increase the concentration in the ash. Coal is such a variable substance that it is hard to constrain what it will have in it.

Search the enviro geochem literature- and don't forget K and Th either.

But yeah, if coal plants had to comply with the same radiological emissions standards as nuclear plants, they would go out of business.

Ed said...

http://www.thoriumenergy.com/
The above company claims to have veins of ore which can produce the isotope of thorium which is usable to produce fissle material (U2333)
I wondered what your opinion is since they may be large scale scam artists or they may be on the level. Thanks in advance for your comment

Chuck said...

Ed,
It looks like they have a small, highish grade Ree/Th deposit, and they are talking up the Th side of things to get the funds needed to either push them over into economic viability, or to raise funds for the next stage of their project.
Note that I am not a financial advisor, and making investment decisions based on comments on this blog will leave you as broke as I am.

Primordial Th is monoisotopic- 232Th. 232Th can grab a neutron and then beta decay to form 233U. But there is nothing isotopically anomalous about this particular deposit- geologically old Th can't have isotope anomalies, because there is only one isotope.

Ed said...

I was not considering investing. I was going to push for thorium reactors in the US and any legitimate company could provide much valuable info. That's the only reason I mentioned the scam part. I know of a company (called Silver Star Energy) that owns producing gas wells in Californis and a test well in Canadian oil sand territory. Since crude oil went from $23 to $120, their stock has gone from $2/sh to $0.30/sh. Turns out that even though the owners were competent geoligists, they were using the co as a "cash cow". They would issue a positive news release, then when their stock went up, they would sell large blocks of it. I guess they did rather well. Have you read about India's thorium reactor?

Dr. Lemming said...

The Indian reactor is based on the Canadian reactor design. If you want to get Thorium into the US, one approach would be to get a CANDU reactor approved for use in the USA.

Since Canada is currently the world's largest U exporter, I don't know how keen they are to burn Th, though.

A_N said...

India does not need U to "vaporize" pakistan ...

India needs nuclear weapons to "deter" China which is getting its U supply from, drum roll please, AUSTRALIA ...

everybody sees sins in others ...

It would seem that Australia also exports Tritium to the biggest war monger on this planet ...

Ironically, Australia enjoys a virtual nuke umbrella within ANZUS ...

:)

a_n said...

> The Indian reactor is based on the Canadian reactor design.

you would be closer to the truth if you claimed that every car in Australia is based on a Ford Model-T ...

Indian AHWR and Fast Breeders are not based on anything Canadian ... (Indian PHWR are loosely based on CANDU) ...

recently, Canada asked India for help with refurbishing one of its CANDU reactors because they realize that India is more advanced in that field ...

Indiain response? ... No Can-Do ... :)

please read up a bit before spitting out garbage on the internet ...

Dr. Lemming said...

Apparently I read the wrong things. Thank you for the correction on reactor design.

We will see how PM Rudd's new declaration on nuclear disarmament effects uranium sales to China...

alok_n said...

Dr. Lemming,

You are welcome ... meanwhile, here is an article to reflect upon ...


----------------
S.P. Seth, Sydney

If Prime Minister Kevin Rudd was banking on his specialist understanding of China as a Mandarin speaker to forge a new relationship with Beijing, it has obviously not worked so far.

His government initially sought to ingratiate with Beijing by snubbing Japan and India. Tokyo was not amused when it was left out of Rudd's recent major foreign trip to the United States, Europe and China.

The Rudd government also dumped the quadrilateral security dialogue to include the United States, Japan, India and Australia. Even worse, this was done at a joint press conference with the visiting Chinese foreign minister.

As Beijing was dead set against this viewing it as part of a containment policy, this decision seemed to give China a role in the formulation of Australia's foreign policy, at least when it concerned China. And not surprisingly, it wasn't regarded well by other dialogue partners.

India was also left out of the loop on the question of uranium supplies. The Howard government was favorably disposed on this issue as part of an emerging U.S.-India strategic nexus.

Beijing couldn't have asked for more from the new Australian government, confirming the widely held view that Kevin Rudd was biased towards China.

Apparently, Rudd believed that having proved his China credentials early on, he would now have some friendly license to express his honest views on China's human rights problem during the Tibetan unrest. And he did it quite frankly during his China visit, advising Beijing to hold dialogue with the Dalai Lama and his representatives on the question.

This was all happening in the midst of the Olympic torch relay when protests were staged in London, Paris and elsewhere against China's repression in Tibet. The protesters were also targeting the Chinese paramilitary security presence surrounding the torch.

At the time Prime Minister Rudd declared that the security of the Olympic torch relay would be handled only by the Australian police during its passing through Canberra. This apparently added to Beijing's displeasure, and it showed this by seeking to ignore Rudd's directive, thus creating a murky situation.

In other words, Prime Minister Rudd's special relationship with China looked like unraveling even before it got going.

At another level, Beijing was hoping it might get a sympathetic treatment from the new Rudd government on the pricing of resource materials (like iron ore) China is importing from Australia. In the last few years, prices of resource materials have soared because of growing demand, much of it from China.

One way of putting some control over the price is for China to have an equity stake in Australian corporations engaged in mining and exporting these materials. Beijing is now aggressively pushing to acquire such stake and control. But it is meeting some resistance, which it regards as discriminatory.

Writing in The Australian, Jennifer Hewett, its national affairs correspondent, has commented that, "The Rudd government is becoming extremely concerned about the prospect of ever-increasing Chinese investment in Australian resources companies."

It can't just be a sheer coincidence that an Australian operated gold mining company in China has, at about the same time, come under severe criticism on Chinese television and other media outlets for acquiring the company for almost nothing and for causing environmental degradation and other vile practices.

John Garnaut, Sydney Morning Herald's Beijing correspondent, reported in his paper on May 12 that, "The 30-minute tirade, which advocated even tougher restrictions on foreign investment in Chinese mines, was broadcast nationally twice last week and the transcript reprinted on more than 500 Chinese internet news and blog sites."

As it happens, there is a convergence of sorts between China's resentment over Prime Minister Rudd's criticism over Tibet, and the economics and politics of Australia's mining, investment and export of resource materials.

As columnist Ian Verrender has put it in the Sydney Morning Herald, "Soon after delivering his message in Mandarin to Beijing (during Rudd's China visit] about human rights concerns [in Tibet), he was confronted with accusations that Australia treated Chinese investment differently than money from other nations."

With its economic success and political power, China is in the midst of a national upsurge. It believes that the timing of the Tibetan unrest, to coincide with the Olympic torch relay, is a conspiracy against its coming of age as a great/super power, with the August Olympics as a spectacular backdrop.

And Australia's joining of the criticism of its human rights in Tibet has dented Rudd's credentials as China's friend.

In its courting of Beijing, the Rudd government sought to substitute China for the whole of Asia. Among the three pillars of Australia's foreign policy under his government (as spelled out in a signed article not long before Rudd became Prime Minister), while the first two would focus on "our alliance with the United States [and], our membership of the United Nations", the third pillar would comprise "a policy of comprehensive engagement with the Asia-Pacific region."

But so far, the engagement with Asia-Pacific would seem to suggest mainly China. Japan and India aside, having been given short shrift, Southeast Asia seems to have escaped notice of the Rudd government.

Critiquing then Prime Minister John Howard's Asia policy in his signed article, Rudd wrote, "In our own region, Australia has increasingly the look and feel of an outsider...(because) Mr Howard has emphasized Australia's differences from, rather than commonalities with, the region."

And Rudd promised that under his Labor Party government, Australia "will revert to a long tradition of engagement with the region..." with a view to "find Australia's security in Asia, not from it..."

With such scant notice taken so far of Southeast Asia, it is not surprising that Indonesia, the largest ASEAN country, has felt left out. Even more so because Indonesia has been routinely featured as Australia's important, if not the most important, neighbor.

The problem, though, is that even when Indonesia is recognized as Australia's important neighbor, Canberra doesn't really know how to give it a concrete shape in bilateral relations, other than the security aspect of it in some form or the other. And since the relationship has always lacked depth, it tends to languish. At the moment, though, it is in slumber.

The Rudd government was expected to energize the entire gamut of Australia's foreign relations with ASEAN countries. But the early signs do not look promising.

With his anticipated close relationship with Beijing, Prime Minister Kevin Rudd was hoping to become an interlocutor between China and the West. By virtue of that Australia would also gain a new respect in Asia, being China's buddy.

But it doesn't seem to be working like that. The Rudd government may need to rework its Asia policy by recognizing its different components and dealing with them in their own right rather than expecting them to fit in as part of Canberra's grand plan.

It is early days yet with the Rudd government having been in power for only some months. It might yet surprise us with a more broad-based Asia policy as it gets going.

The writer is a freelance writer based in Sydney and can be reached at SushilPSeth@aol.com

Dr. Lemming said...

In the future, could you please use a link instead of reposting entire article in comments?

In think that the whole mining/ foreign investment issues are complicated that many Chinese companies are partially or mostly state-owned. But that is a whole other topic.

As for K.Rudd's Asia policy, I'm happy to give him a bit of time- There's a lot of countries, and even more people down here, and he hasn't been around that long so far.

Anonymous said...

If Th doesn't dissolve in water, what processes do cause Th deposits?

Chuck said...

Thorium doesn't really form high grade deposits. That's the whole point. It is concentrated in certain minerals, but in general it is distributed through the crust much more evenly than U is.