Physical Insights

An independent scientist’s observations on society, technology, energy, science and the environment. “Modern science has been a voyage into the unknown, with a lesson in humility waiting at every stop. Many passengers would rather have stayed home.” – Carl Sagan

Posts Tagged ‘nuclear fuels

Rethinking nuclear power.

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ABC Unleashed has recently featured an article by environmentalist Geoffrey Russell; Rethinking Nuclear Power.

It’s worth reading.

I like the idea of closing down uranium mines, and using existing stocks of mined uranium efficiently.

Uranium mining is far less environmentally intensive than mining coal, of course, but it’s basically inevitable that all mining is fairly environmentally intensive, and it’s always an appealing prospect if we can mine less material (whilst still maintaining our energy supplies and our standards of living, of course.)

I have to admit, when I first saw Geoff’s claim that we could completely eliminate uranium mining, I was skeptical. So I took a more detailed look.

A nuclear reactor which is efficiently consuming uranium-238 and driving a relatively high efficiency engine (typically, a Brayton-cycle gas turbine) will require approximately one tonne of uranium input for one gigawatt-year of energy output. This high efficiency use of U-238 could be best realized something like an IFR or a liquid-chloride-salt reactor (the latter is essentially the fast-neutron uranium fueled variant of a LFTR). This figure of one tonne of input fertile fuel per gigawatt-year is also comparable for the efficient use of thorium in a LFTR.

There are about one million tonnes of already mined, refined uranium in the world, just sitting around waiting to be put to use, which is termed so-called “depleted uranium”.

According to one source, the exact worldwide inventory of depleted uranium is 1,188,273 tonnes [1].
The total electricity production across the world today is about 19.02 trillion kWh [2].

Therefore, total worldwide stocks of depleted uranium, used efficiently in fast reactors, could provide every bit of worldwide electricity production for about 550 years.

That’s not forever, but it’s a surprisingly long time. And that’s just “depleted uranium” stocks; not including the stocks of HEU and plutonium from the arsenals of the Cold War, and not including the large stockpile of uranium and plutonium that exists in the form of “used” LWR fuel.

I know some thorium proponents aren’t going to like this; but there’s a strong case to be made here that uranium-238 based nuclear energy has a clear advantage over thorium, simply became of these huge stockpiles of already-mined uranium, for which there exists no comparable thorium resource already mined. The 3,200 tonnes of thorium nitrate at NTS is tiny compared to the uranium “waste” stockpile, but they’re both really useful energy resources which can replace the need for more mining.

Any type of breeder or burner reactor utilising 238U, or 232Th, as fuel requires an initial charge of fissile material to “kindle” it; however this requirement for fissile material is quite small; and personally, I think the inventories of HEU and weapons-grade plutonium recovered from the gradual dismantlement of the arsenals of the Cold War are perfectly suited to this purpose – destroying those weapons materials, whilst putting them to a valuable use.

Then again, with the means to completely replace the use of coal and fossil fuels in a way that requires very little or no uranium mining, I really hope the rest of the world keeps buying that iron and copper and bauxite. Alternatively, we’re going to have to start developing a more technologically based economy in this country to make up the reduction in exports of these commodities – perhaps developing and selling reactor technology?

Developing uranium enrichment technology, such as SILEX, is of limited usefulness because the relatively inefficient thermal-neutron fission of 235U, and hence the need for enrichment, will not supply any large portion of world energy demand in a sustainable fashion over the long term. The small amount of 235U in nature is of limited significance, over the long term.

Alternatively, perhaps a shake up of agriculture, using extensive desalination to supply fresh water requirements, might be used to replace Australia’s income from coal and uranium. I’m not sure.

Tip ‘o the hat to Barry at Brave New Climate for pointing out this article.

[1]: http://www.wise-uranium.org/eddat.html
[2]: From the World Factbook, 2008 ed. (jokes about the integrity of CIA’s intelligence aside…)

Barack Obama, nuclear energy and Yucca Mountain.

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Putting aside, for now, rhetoric like “OMG Obama will kill nuclear energy”, one of the only “anti-nuclear” positions that president-elect Obama has actually made overtly clear on the issue of nuclear energy is that he is opposed to the opening / licensing of the geological repository for radioactive waste disposal at Yucca Mountain.

However, it should be realized that “No Yucca Mountain facility” is not saying “No to nuclear energy”.

I really don’t think a Yucca Mountain style geological disposal facility is a prerequisite for the continuation or the expansion of nuclear energy in the United States for the foreseeable future.

Nuclear energy works just fine at present with no Yucca Mountain, and it will continue to work in future, even without Yucca Mountain going ahead.

If Obama’s position on Yucca Mountain causes the government, the nuclear energy industry, and the public to pause for a moment, step back, and ask if branding all that used fuel containing uranium, plutonium and other useful, valuable material as “waste” and sending it to geological disposal at Yucca Mountain is really a sensible proposal, then I really don’t think that’s a bad thing.

In fact, if Obama was to back efficient utilisation of these nuclear materials as the alternative to disposal at Yucca Mountain, then I wouldn’t expect to see a great deal of opposition to such a plan, from nuclear-literate parties, at all.

The used nuclear fuel removed from a conventional LEU-fueled light-water reactor is about 25 tonnes per gigawatt per year – the equivalent of less than two 15-tonne dry storage casks per reactor per year – something that is clearly not difficult to deal with.

If the uranium and plutonium comprising 97% of the nuclear fuel is recovered and re-used, and the remaining 3%* is put into dry storage casks, then just one storage cask provides enough capacity to store the material for one reactor for twenty years. Of course, of that 3% of fission product material, half the fission products aren’t even radioactive at all, or they have extremely short half-lives. Many fission products, both radioactive and not radioactive, are valuable, exotic and useful materials, with specialised, useful and interesting applications. The assumption that all such fission products would be treated as “waste” is, therefore, especially pessimistic.

* (You can account for the minor actinides (Np, Am, Cm, Cf) in either category. They constitute a very small amount of mass either way. Such actinides, like Pu and U, can be fissioned in a nuclear reactor as sources of energy, and like many fission products, they can also be used for specialised technological and scientific applications, such as the production of 238Pu from 237Np, and the use of Cf and Am:Be as neutron sources.)

Still, it seems a real shame to waste all that money that we’ve already spent on YM if it’s not going to be used. I’m not sure off the top of my head how far underground the tunnels at Yucca Mountain are, but perhaps it could be used as a deep underground laboratory, or something, just as the Waste Isolation Pilot Plant is?

Still, there are approximately 50,000 tonnes of used nuclear fuel already in the United States, the result of the last 50 years of nuclear energy. Opponents of nuclear energy are quick to point that out, but under a scenario similar to that elucidated above, with the separation of easily usable plutonium and uranium, the significantly radioactive fission product materials only constitute 1500 tonnes, or 100 DSCs worth. Until a geological repository is built, or those fission products are put to productive uses, that’s only one additional storage cask that need be stored at every power reactor in the country.

In the foreseeable future, with no Yucca Mountain, dealing with nuclear byproduct materials, storing them safely and securely on site, is not impractical, and it’s not intractable, and it’s not unsafe. There is nothing here which impedes or prevents a revival of nuclear energy generation.

Of course, under the Nuclear Waste Policy Act, the government will have to compensate nuclear utilities for the costs of this storage. No, this does not mean handing out government money to nuclear utilities – it means giving the Nuclear Waste Fund money that is supposed to go to Yucca Mountain back to the nuclear utilities in order to pay for the management of the existing 50,000 tonnes (approx.) of used fuel (and/or processing thereof), and more importantly, it ought to mean not requiring nuclear utilities to pay any more money – more correctly, not requiring nuclear electricity customers to pay any more money – for the Nuclear Waste Fund, until we know that a geological repository for radioactive waste is going ahead. Otherwise, what exactly are they paying for?

Thorium Energy Independence and Security Act of 2008

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This interesting legislation has been introduced in the US Senate today by Senator Orrin Hatch and Senator Harry Reid, with the intention to amend the Atomic Energy Act of 1954 to provide support for nuclear power generation using thorium nuclear fuel cycles.

It’s said that the thorium deposit in Lemhi Pass, Idaho contains 600,000 tonnes of thorium. Australia’s total identified thorium resources are put at 452,300 tonnes which Geoscience Australia estimates are extractable at less than US$80 per kilogram of thorium.

Just considering the Lemhi Pass thorium and Australia’s thorium reserves, alone, we have 1,052,300 tonnes of thorium available – not to mention all the uranium.

A thorium nucleus has a mass of 232 amu, obviously. Let’s assume that the energy ultimately yielded from each nucleus is 200 MeV, and the thorium is transmuted, and its energy harnessed via U-233 fission, with an overall efficiency within the reactor of 75%, and a further 50% of the energy is lost in a Brayton-cycle engine. Then, we can work out that one tonne of thorium gives about exactly one gigawatt-year of energy.

Current world electricity demand is estimated at a total of about 16,330 TWh. At current consumption, then, this 1,052,300 tonnes of thorium could supply all the world’s electricity needs, all of it, for an astonishing 565 years.

That’s with no use of deuterium or lithium, and effectively no use of natural uranium, or accumulated plutonium.

Food for thought, or thorium for thought, isn’t it?

The text of the bill follows

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Written by Luke Weston

October 3, 2008 at 9:08 am

Posted in nuclear fuels, thorium

Tagged with ,

Researchers develop filter for nuclear waste

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Apologies for the thread title, we have this essentially inevitable problem where newspaper headlines aren’t always the best description of science and technology.

http://www.theaustralian.news.com.au/story/0,25197,24370728-2702,00.html

Australian researchers say they have created a low-cost material to filter and safely store nuclear waste. The potential breakthrough for the environment was made by a team of scientists from Queensland University of Technology, led by Associate Professor Zhu Huai Yong from the School of Physical and Chemical Sciences.

(Tip to NEI Nuclear Notes for finding the story.)

Prof Zhu said the discovery was particularly important as the world increased its reliance on nuclear energy.

“You have to keep nuclear waste somewhere for hundreds of years,” Dr Zhu said.

“Water is used to cool nuclear reactors and during the mining and purification of nuclear material, so waste water is a big problem.

“For example, there is a lake in the United States filled with millions of gallons of nuclear waste water.”

But if the waste was stored conventionally in lakes or steel containers, there was a danger it could leak and pollute the land around it.

Well, I’m not so sure about what he’s talking about here. I seriously hope they’re not talking about water used at nuclear generating stations as “nuclear waste”, for example – clearly Lake Anna in Virginia, for example, does not contain millions of gallons of “nuclear waste”, or anything of any radiological significance at all, for that matter.

Alternatively, perhaps they’re talking about “tailings” waste from uranium mining and extraction? I’m not sure, but in any case, I’m not particularly concerned about waste from uranium mining and extraction, since it contains naturally occurring radionuclides such as radium which occur naturally in the ground, in exactly the same amounts that they’re naturally present in the earth, which can be put right back into the earth.

Professor Zhu said the QUT team had discovered how to create nanofibres, which are millionths of a millimetre in size and can permanently lock away radioactive ions by displacing the existing sodium ions in the fibre.

“We have created ceramic nanofibres which attract and trap radioactive cations (positively charged ions), possibly forever,” he said.

“The ceramic material can last a very long time, much longer than the radioactivity of a radioactive ion.”

Ceramic was also more chemically stable than metal, could last much longer and was much cheaper to make than steel.

The ceramic nanofibres were made from titanium dioxide, a mineral found abundantly in Australia and used to colour white paint.

The fibres were mixed with caustic soda and heated in a laboratory oven to make the ceramic material.

The nanofibres, which are up to 40 micrometres in length, look like white powder to the human eye, Prof Zhu said.

“The fibres are in very thin layers, less than one nanometre in width, and the radioactive ions are attracted into the space between the layers,” he said.

“Once the ceramic material absorbs a certain amount, the layers collapse to lock the radioactive ions inside.”

It sounds like they’ve created an artificial ceramic nanomaterial based on the well-studied chemistry of perovskite materials, in this case saturated with Na+ ions, forming something like Na2Ti3O7 or Na2TiO3, perhaps.

Perovskite materials can contain all kinds of different anions – positively charged ions, usually metals – bound up inside their crystal structure. Strontium titanate and barium titanate are both well-studied materials with interesting properties. Strontium titanate is a common chemical form in which radioactive 90Sr is used when it’s packaged in sealed radioactive sources, such as the very large 90Sr sources that form the basis of 90Sr radioisotope thermoelectric generators, because it’s essentially a completely inert, insoluble solid ceramic material from which the radionuclide cannot leach, dissolve or otherwise escape.

These perovskite-type materials, and their ability to “lock up” essentially any alkali metal or alkaline earth metal, as well as transition metals, actinides and just about any metal, really, inside their crystal structure is well studied technology with regards to the disposal of radioactive waste and radioactive byproduct material, both fission products and plutonium-contaminated defence wastes, and indeed, these materials are the fundamental basis of the use of Synroc type materials for radioactive waste immobilisation.

Now, it’s difficult to speculate, based on nothing more than a newspaper report, but I’m guessing that where such a perovskite material is turned into a nanomaterial, with nanoscale structure, and loaded with sodium ions, you end up with something that is kind of like an interesting inorganic ion-exchange material. (Ion-exchange materials are of course an important part of nuclear chemistry, both for recovering and recycling nuclear materials, and decontaminating waste materials.)

If you take some waste water that is contaminated somehow, say, with nuclear fission products like 90Sr or 137Cs, (and providing it’s not economical to recover such materials for useful purposes), I suppose you can simply run it through a bed of this material, and those ions (Sr2+ and Cs+, assuming that they’re in soluble forms) become permanently substituted into the perovskite. Then, this material can be sintered, and you have the radionuclide contamination converted straight into a chemically inert, insoluble ceramic. Unlike an ion-exchange material, however, once those cations are in the material, the idea is that they will be permanently retained there.

To paraphrase Bernard Cohen, you can take this material, if you want to dispose of it, turn it into a rock, then take the rock and put it in the rock’s natural habitat, deep inside the earth.

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