Tuesday, March 2, 2010

Is Nuclear Power Sustainable?

For nuclear power to be a viable solution to our future energy requirements, we need a plentiful supply of nuclear fuel which can be accessed without inordinate effort. There are occasional claims from anti-nuclear activists that there are insufficient rich ore bodies for an expansion of nuclear power, or that the energy used to mine and mill the fuel produces an excessive carbon footprint. The truth is that there are huge reserves of nuclear fuel available for exploitation in Earth's crust, and the carbon footprint associated with the nuclear fuel cycle is quite small now, and likely to shrink further in the future.

Current nuclear plants run on U-235 and sometimes on Pu-239 reprocessed from earlier fuel burns or recycled from decommissioned nuclear weapons. Natural uranium consists mainly of U-238, with U-235 making up only 0.7% of the total. In practice, this means that for roughly every 200 tonnes of natural uranium mined, only one ton of fissile U-235 fuel is produced (some U-235 is left behind by the enrichment process). It is possible to improve this figure by reprocessing used fuel to recover some of the unburned U-235 as well as the plutonium produced during the fuel burn, but this doesn't change the order-of-magnitude calculations on fuel availability for LWRs. As a rough rule of thumb, one tonne of fissile fuel can generate 1 GW.year (1GW.y) of energy as electricity. This is true of all three important fissile isotopes, U-235, Pu-239 and U-233.

How much generating capacity should we allow for? Demographic trends indicate the global population reaching about ten billion people mid-century and stabilising at that level. For that stability to be reached, people will need a certain minimum standard of living and energy consumption.

The average global electrical generating capacity for 2009 was about 2.2 terawatts (2.2TW, or 2.2 trillion watts). Divided among the estimated 6,790,000,000 humans alive as of July 2009, this provided 325 watts/person. Of that output, actual power consumed was 301 watts/person. For comparison, Chad currently consumes 1 watt/person, Ghana 27 watts/person, India 56 watts/person, China 293 watts/person, Poland 384 watts/person, South Africa 500 watts/person, Britain 646 watts/person, Germany 759 watts/person, France 780 watts/person, Netherlands 848 watts/person, Australia 1,176 watts/person, the United States 1,439 watts/person, and Sweden 1695 watts/person. These figures are for electricity consumption only.

If the global population plateaus at 10 billion, we will need a 50% increase in generating capacity by 2050, taking us up to 3.3TW, just to maintain the current per capita consumption. In reality there will be more development in the currently underdeveloped world, as well as in the First World. There is also likely to be greater electrification of the economy, with functions such as transportation, desalination, fertiliser production and other processes using electricity rather than fossil fuel combustion. It's not unreasonable to suggest that we may have around 5TW of electrical generating capacity by 2050.

If all this electricity were to come from nuclear power plants, we would need to burn 5000 tonnes of fissile fuel each year. Is this sustainable? Can we use more if necessary? Is this a permanent solution to the energy crisis, or is it at best a useful stopgap until a more sustainable power system can be developed? To answer these questions we need to understand the magnitude of the global nuclear fuel resource.

The table below was compiled for the January 1980 edition of Scientific American. Both it and the associated commentary are taken from the nuclearinfo.net site, reproduced here with the kind permission of Dr. Martin Sevior:

Uranium Distributions in the Earth's Crust
The following table is from Deffeyes & MacGregor, "World Uranium resources" Scientific American, Vol 242, No 1, January 1980, pp. 66-76.

type of deposit--------------------------------estimated tonnes-----estimated ppm

Vein deposits----------------------------------2 x 10^5--------------10,000+

Pegmatites, unconformity deposits-------2 x 10^6--------------2,000-10,000

fossil placers, sand stones------------------8 x 10^7--------------1,000-2,000

lower grade fossil placers,sandstones----1 x 10^8--------------200-1,000

volcanic deposits-----------------------------2 x 10^9--------------100-200

black shales-----------------------------------2 x 10^10-------------20-100

shales, phosphates---------------------------8 x 10^11-------------10-20

granites----------------------------------------2 x 10^12-------------3-10

average crust----------------------------------3 x 10^13-------------1-3

evaporites, siliceous ooze, chert-----------6 x 10^12-------------.2-1

oceanic igneous crust-----------------------8 x 10^11-------------.1-.2

ocean water----------------------------------2 x 10^10-------------.0002-.001

fresh water-----------------------------------2 x 10^6--------------.0001-.001

“The total abundance of Uranium in the Earth's crust is estimated to be approximately 40 trillion tonnes. The Rossing mine in Namibia mines uranium at an ore concentration of 300 ppm at an energy cost 500 times less than the energy it delivers with current thermal-spectrum reactors. If the energy cost increases in inverse proportion to the ore concentration, shales and phosphates, with a uranium abundance of 10 - 20 ppm, could be mined with an energy gain of 16 - 32. The total amount of uranium in these rocks is estimated to be 8000 times greater than the deposits currently being exploited.”

In his book 'Sustainable Energy — without the hot air', Professor David Mackay, Dept. of Physics, University of Cambridge gives an estimate of how much of the uranium resource could be considered accessible with conventional extraction techniques. Noting that phosphate deposits have been mined for their uranium content in America and Belgium prior to 1998, Professor Mackay combines the current proven economic uranium reserves and the phosphate reserves to reach a figure of 27 million tonnes of easily recoverable uranium. This figure has increased a bit since the figures used by Mackay were published and will likely increase again, so we'll round it up to 30 million tonnes for our calculations. He also notes recent development work on the recovery of uranium dissolved in seawater by Japanese researchers, reported to be achievable at US$100-300/kg UO3. This reserve is calculated to be 4.5 billion tonnes.

Allowing for 5TW of nuclear power generation using LWRs burning 1 tonne of U-235/GWe.y we need 5000 tonnes of U-235 per annum, which equates to 1,000,000 tonnes of natural uranium per annum. This would exhaust our 30 million tonne reserve in just thirty years. Assuming we can extract half the oceanic uranium, we have a 2,000 year supply. This would require 220 million tonnes of the adsorbent cloth, with a cross-sectional collection area of 24,000 square kilometres submerged under the sea. This is not impossibly huge, but it is substantial, and could arguably have a significant impact on marine ecosystems. Nonetheless it is conceivable that with oceanic uranium extraction we could run our civilisation at above its current per capita electricity consumption for a span of time as long as that which separates us from the Roman Empire using technology no more advanced than current LWRs.

But will this be sufficient? The world currently boasts 2.2 TW of electrical generating capacity, but the average ongoing global energy use is around 15TW. Most of this comes from the burning of fossil fuels for transport, heating, industrial processes, agriculture, primary industry and so forth. The bounty of fossil fuel bequeathed to us by nature will one day no longer be available, whether by failure to effectively compete, legislative fiat, or eventual depletion. Can nuclear power replace non-electric energy applications, and if so, can it be sustained?

Fortunately the answer to the first question is yes, nuclear power can certainly replace most if not all applications presently met by fossil fuels. Nuclear reactors produce great quantities of heat which can be used as process heat for chemical industries, desalination, synthetic fuel production, fertiliser production, district heating, and many other applications. It is even thought that nuclear power may assist in making many processes more efficient than they are today. For instance, a fleet of electric cars with batteries charged by nuclear power plants would be considerably more energy-efficient than an equivalent fleet of our current petrol and diesel cars.

The answer to the second question depends on just how much energy is needed to cover all reasonable demands which civilisation might place on its power source. It has sometimes been suggested that 10-11TW would be sufficient for a well-organised nuclear powered world of ten billion people. This may be so, although such a world may be a bit more economically constrained on average than developed nations are today. In such a future, the ocean uranium reserve referred to previously still suffices to run things for an equivalent span of time separating the present from Saxon England.

The historical pattern of human energy utilisation, however, speaks against any assumption of reduced power use. The norm has been the opposite, with people using ever more power per capita as time goes by. This trend has continued in spite of occasional advances in energy efficiency. Also, in the long run it is likely that the developing world will catch up to the developed, and the aspirations of billions of people will force energy consumption ever higher. Eventually we should expect that First World standards will be the norm for all people. While increasing electrification of the world economy may result in some efficiencies, there are likely to be new uses for power, possibly including energy-intensive geoengineering efforts to mitigate climate fluctuations. The per capita power consumption from all sources (electric and non-electric) for the United States is now around 10kW. Accepting this as a convenient working figure for standard per capita consumption, we find we need to plan for 100 terawatts of electrical power generation to ensure sufficient capacity for a world population of 10 billion.

One hundred terawatts requires the fissioning of 100,000 tonnes of fissile fuel each year. If we are to rely on LWRs, this will consume 20 million tonnes of natural uranium per annum. Lets see how our various uranium reserves stack up:

Our initial uranium reserve of 30 million tonnes now disappears in a year and a half. The ocean reserve, once again assuming 2 billion tonnes, is exhausted after a century. Referring to the World Uranium Resources table above, we see there are about 22 billion tonnes of natural uranium present in concentrations of 20 ppm or more. This reserve is sufficient for 1,100 years. It is probably not possible to supply LWRs from uranium at lower concentrations.

Extracting large quantities of uranium from such low grade feedstock will require ever more infrastructure to maintain supply. The ocean extraction system, for example, would need 11 billion tonnes of adsorbent cloth with a cross-sectional collection area of nearly half a million square kilometres. Light water reactor technology now encounters its limits. The LWR can be a good stopgap measure, but it is not the key to a truly sustainable future. It is capable of taking us to through the middle of this century, but must soon after yield its dominant position to the breeder reactor.

Breeder reactors are nuclear reactors which utilise part of their neutron flux to transmute non-fissile nuclei into fissile nuclei which can then be burned in the reactor. Uranium-238 can be bred into plutonium-239, and thorium-232 into uranium-233. This capability gives breeder reactors a much greater supply of fissile fuel than LWRs. Plutonium breeders can fission the entire supply of natural uranium. All naturally occurring thorium, with an abundance four times that of uranium, can be bred into uranium-233 and fissioned. It is instructive to see how this impacts nuclear fuel reserves.

Our original 30 million tonnes of uranium now provides our 100TW civilisation with 300 years of power. The 2 billion tonne ocean reserve is good for 20,000 years. The larger 22 billion tonne reserve of uranium above 20 ppm concentration can now provide 220,000 years of power. But this is not the limit of the breeder's potential. As well as greatly extending the usefulness of existing reserves, breeder reactors also unlock vast quantities of low grade ore for our use. To demonstrate this, we'll take a look at how the capabilities of the breeder applied to average continental crust can revolutionise the scale of the energy resources available to humanity.

Average continental crust contains 2.7 ppm (parts per million) of uranium and 9.6 ppm of thorium, totalling 12.3 ppm of fissile and fertile fuel. We want to extract 100,000 tonnes of fuel each year for our 100TW civilisation. How much average rock and dirt do we need to dig up on an annual basis for this? The answer is we need to excavate about 8.2 billion tonnes of earth. For comparison, about 6.8 billion tonnes of coal was mined worldwide in 2009. Out of this, about 5 billion tonnes went to electricity production, which produced 40% of the world's electrical power, about 0.9TW (these figures were derived from information on the World Coal Institute website). It should be noted that the density of coal varies from around 1.1 to 1.5 tonnes/m^3, but the average density of Earth's crust is 2.7 tonnes/m^3. While the mass of material mined from average crust to obtain our 100,000 tonnes of nuclear fuel is greater than the mass of coal mined in 2009, the volume of material disrupted would be smaller.

Once we have mined our 8.2 billion tons of perfectly ordinary and unremarkable rock and dirt, we need to extract the nuclear fuel. This could be done by grinding, chemical treatment, pyroprocessing or whatever is most suitable for the particular minerals in question. We may get a reasonable estimate to the upper bounds of the energy required for this process by assuming the ore is completely melted. The power required to melt the same mass of silicon (the second most common element in Earth's crust after oxygen) is about 723 GW.y. It is likely that the whole separation process could be accomplished with less than 1TW.y of energy. This operation corresponds to an extraction and milling rate of 260 tonnes of crust each second.

What is the size of the resource? Let's assume that only the portion of continental crust currently under dry land is exploited for its uranium and thorium content, to a depth of roughly four kilometres (the deepest mine currently operating is the TauTona mine in Carletonville, South Africa at 3,900m, and the Kola Superdeep Borehole in Russia is 12,262m). This represents a reserve of 20 trillion tonnes of fertile and fissile fuel, capable of powering our 100TW civilisation for 200 million years. This is the span of time separating us from the dawn of the Jurassic Period, when the supercontinent Pangaea was starting to break apart into Laurasia and Gondwana. Dinosaurs were just beginning to make their mark on the world, and the allosaurus, stegosaurus and diplodocus were yet to evolve.

It will be a very long time before whoever comes after us in the far distant future will need to worry about mining ordinary crust. The science is clear: There is more than enough high grade uranium ore in the short term to allow us to transition to a completely nuclear-powered economy during this century, and a supply of fuel for the breeder reactors of the future so vast as to leave no doubt that nuclear power is completely sustainable in any meaningful sense of the word for far beyond the probable lifetime of our civilisation, and indeed, of our species.