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

Archive for the ‘fast reactors’ Category

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.

[2]: From the World Factbook, 2008 ed. (jokes about the integrity of CIA’s intelligence aside…)

Fast spectrum reactors and supercriticality.

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I’ve been reading this little essay on the history and progress of India’s nuclear energy programs recently. I encourage you to have a look, if you’re interested.

It’s a bit critical of India’s interest in Breeder reactor technology, suggesting that fast-neutron-spectrum “breeder” reactor technology has been tried, and rejected, throughout most of the nations of the world with significant experience in nuclear engineering.

There’s one particularly interesting claim made here:

“If the operating system failed to insert control rods fast enough, the increased reactivity would, in turn, heat up the sodium further; this chain could ultimately cause a fuel meltdown into a supercritical configuration and a small nuclear explosion. “

Helen Caldicott’s Nuclear Madness also says essentially the same thing:

“Once out of control, a fission reaction in a breeder could cause not only a meltdown but also a fully fledged nuclear explosion.”

A sufficiently enormously supercritical configuration would indeed have the potential to be the makings of a nuclear explosion – that’s how a nuclear fission bomb works, by compressing fissile material together into a massively supercritical mass.

Technically, a supercritical mass is any mass of fissile material with an overall neutron multiplication ratio, k > 1. k < 1 represents an ordinary, sub-critical mass, and k = 1 represents a, just marginally, critical mass.

k > 1 in the core of a fission reactor represents an unrestrained, massive power excursion – which usually results in the explosion of the reactor, if this condition is sustained for any significant length of time.

Such an event results in a significant release of thermal energy – in the presence of a water coolant, the resulting rapid production of steam pressure contributes significantly to the explosive force of such an event.

A significant amount of direct neutron and gamma radiation is released from such a critical assembly.

Such power excursions were responsible for the destruction of the reactor in the Chernobyl reactor disaster and the destruction of the experimental SL-1 reactor in the US in 1961.However, in all practical cases, it is not easily conceivable that the neutron multiplication factor would be great enough, and that the fissile material would stay together long enough, without destroying itself, and without reducing neutron multiplication via thermal expansion or doppler broadening, for a fully fledged nuclear weapon-style to occur. This seems to me to be quite implausible.

Any nuclear engineers out there care to share their thoughts?

Written by Luke Weston

August 20, 2007 at 1:31 am