Friday, November 26, 2010

What is uranium enrichment?

The great majority of civilian nuclear power reactors currently in service are thermal reactors. Thermal reactors can only use fissile nuclei as fuel (unlike fast reactors which can also use fissionable nuclei). Natural uranium consists of over 99% non-fissile U-238, with fissile U-235 being only 0.7% of the total. Uranium fuel for a thermal reactor using light water as a moderator must have at least 3% U-235. The process of producing uranium fuel with an increased portion of U-235 is known as enrichment. The primary problem with any procedure to alter the isotopic composition of a sample of any element is that chemical processes very rarely distinguish between different isotopes of the same element, so non-chemical methods are usually necessary. This has historically tended to make enrichment technology difficult, complex, expensive and energy-intensive, although the situation has improved somewhat.

Fundamental Economic Principles Of Uranium Enrichment:

There are two basic considerations governing the economics of uranium enrichment. These are the amount of separative work and the mass of natural uranium feedstock needed to produce a given quantity of enriched uranium at a given level of enrichment.

Separative Work:

The amount of separative work needed for uranium enrichment is measured in special units known as Separative Work Units (SWU). The SWU is a measure of the amount of work done in the process of seperating isotopes. They are usually expressed in units of kilogram SWUs, or tonne SWUs. An SWU will require different amounts of energy depending on the enrichment process used. A detailed description of the SWU can be found on the following site:

Natural Uranium Feedstock Mass:

In addition to the separative work units provided by an enrichment facility, the other important parameter to be considered is the mass of natural uranium required to yield a desired mass of enriched uranium. As with the number of SWUs, the amount of feedstock material required will depend on the level of enrichment desired and upon the amount of U-235 that ends up in the DU. Unlike the number of SWUs required during enrichment, which increases with decreasing levels of U-235 in the depleted stream, the amount of natural uranium needed will decrease with decreasing levels of 235U that end up in the DU.

Economic Consequences of the Interaction Between SWUs and Feedstock Mass:

Putting these two factors together in an illustrative example, in the enrichment of LEU for use in a light water reactor it is typical for the enriched stream to contain 3.6% U-235 (as compared to 0.7% in natural uranium) while the depleted stream contains 0.2% to 0.3% U-235. In order to produce one kilogram of this LEU it would require approximately 8 kilograms of natural uranium and 4.5 SWU if the DU stream was allowed to have 0.3% U-235. On the other hand, if the depleted stream had only 0.2% U-235, then it would require just 6.7 kilograms of natural uranium, but nearly 5.7 SWU of enrichment. Because the amount of natural uranium required and the number of SWUs required during enrichment change in opposite directions, if natural uranium is cheap and enrichment services are more expensive, then the operators will typically choose to allow more U-235 to be left in the DU stream whereas if natural uranium is more expensive and enrichment is less so, then they would choose the opposite.

Downblending and the Megatons To Megawatts Program:

Another factor which has influenced the economics of uranium enrichment over the past two decades is the practice of downblending. This is the production of LEU for reactor fuel by dilution of HEU sourced from nuclear warheads with depleted uranium. this has been done since 1995 under the auspices of the Megatons To Megawatts program mandated by the 1993 United States-Russia nonproliferation agreement, with 400 tonnes of Russian HEU processed into fuel for US reactors as of September 2010. This fuel has provided half the power currently generated by US civilian reactors. The program is scheduled to end in 2013, by which time a total of 500 tonnes of HEU from former Soviet warheads will have been consumed. The termination of this program will likely increase the demand for enrichment services around the world.

Enriched Uranium Grades:

Uranium can be enriched to any desired level depending on the amount of separative work performed. There are a number of recognised enrichment levels.

Depleted Uranium (DU):

DU is the uranium resulting from the tails of the enrichment process, so named because it is depleted in U-235 compared to natural uranium. DU usually has a U-235 content of 0.2-0.3%

Slightly Enriched Uranium (SEU):

SEU is a relatively new grade of enriched uranium with U-235 levels between 0.9% and 2%. SEU is sometimes used in preference to natural uranium for heavy water reactors, and has been proposed as the fuel type for the ACR-1000 reactor design, intended as an advanced variant of the CANDU series.

Low-Enriched Uranium (LEU):

LEU is uranium which has been enriched to less than 20% U-235 composition. Uranium fuel for LWRs is usually enriched to between 3% and 5%. Research reactors and medical isotope production reactors typically use higher levels of enrichment, around 12% or more. The OPAL reactor at Lucas Heights uses uranium enriched to just below 20% U-235.

Highly Enriched Uranium (HEU):

HEU is uranium enriched to above 20% U-235. It is used in some research and isotope production reactors, and is also used in nuclear power plants for naval vessels such as nuclear submarines and aircraft carriers, where the high enrichment levels allow for compact plant designs.

Weapons-Grade Uranium (WGU):

HEU enriched to 85% U-235 and above is also known as weapons-grade uranium. This is the grade of uranium used in the production of some nuclear weapons.

Enrichment processes:

The art of uranium enrichment extends back to the Second World War, when a number of approaches were rushed through development under the pressure of military demand. Some early techniques which were abandoned in favour of more effective methods were thermal diffusion, which exploited the tendency of lighter isotopes to diffuse toward warmer surfaces and heavier isotopes toward colder, and electromagnetic isotope separation, which used a powerful magnetic field to separate beams of uranium ions into different streams according to atomic mass. These techniques were largely abandoned in favour of gaseous diffusion, and more recently, the gaseous centrifuge process.

Gaseous Diffusion:

Gaseous diffusion was the first uranium enrichment process employed on a large scale in the civilian nuclear power sector. It was first employed at the Oak Ridge facility for the Manhattan Project during the Second World war, and was later used in a number of nations around the world for their civilian nuclear power programs. In this process, uranium is combined with fluorine in the form of Uranium Hexafluoride (UH6) gas and forced through semi-permeable membranes, producing a slight separation of molecules carrying U-235 and U-238. The process is based on Graham's Law, which describes the tendency of lighter molecules to escape from a container with a semi-permeable membrane wall faster than heavier molecules. By employing a cascade of many stages, any given degree of U-235 enrichment can eventually be reached.

Gaseous diffusion is an energy-intensive process, requiring on the order of 2,500 kilowatt hours to perform one kilogram SWU. Producing LEU for LWR fuel using gaseous diffusion requires approximately one fortieth of the amount of power eventually obtained from it. The gaseous diffusion process currently produces about 33% of the world's enriched uranium.

Gaseous Centrifuge Process:

The gas centrifuge process uses rotating cylinders to concentrate the heavier isotope around the rim and harvest the U-235-rich portion from near the rotation axis. This process is much more efficient than the older gaseous diffusion process, requiring as little as 2% of the energy to perform equivalent separative work. A gaseous centrifuge plant can perform one kilogram SWU for around 50-60 kilowatt hours. Gaseous centrifuge plants currently produce about 54% of the world's enriched uranium.

An improved centrifuge known as the Zippe centrifuge (named after Gernot Zippe, the German scientist credited with its invention), uses a temperature gradient along the length of the cylinder to enhance the separation of light and heavy isotopes. This type has been used by the commercial firm Urenco for uranium enrichment, and is also believed to have been used by Pakistan in its nuclear weapons program.

Laser Enrichment:

Some laser enrichment techniques have also been considered. The one currently receiving the most interest is the SILEX process, standing for Separation of Isotopes by Laser Excitation. The SILEX process is of special interest to Australia as the CSIRO played a large role in its development. SILEX is currently being developed by GE Hitachi Nuclear Energy in agreement with Silex Systems. Details of the process and performance are heavily classified, but it is said that SILEX offers an order of magnitude better performance over the gaseous centrifuge process.

Two other laser enrichment techniques have also been investigated. These are AVLIS (Atomic Vapour Laser Isotope Separation) and MLIS (Molecular Laser Isotope Separation). These techniques have encountered technical difficulties which have shut down most efforts to develop them.

Other Enrichment Techniques:

Some development work has been done on other processes, occasionally reaching the pilot plant stage, and in at least one case a full scale operational plant. The most significant of these has probably been the Helikon Vortex Separation Process developed by South Africa's UCOR corporation. UCOR operated a plant in from the 1970s through to 1991 using this technology to produce LEU for civilian nuclear fuel and WGU for South Africa's nuclear weapons program. The technique consists of forcing a stream of UH6 diluted with hydrogen tangentially into a shaped tube at near the speed of sound. The tube thus acts as a non-rotating centrifuge. The process uses large amounts of electricity, has substantial cooling requirements, and is therefore not considered economic for the production of reactor fuel. Over its operational lifetime the plant is said to have produced hundreds of kilograms of HEU. An alternative aerodynamic process has been used in a demonstration plant in Brazil which faced similar issues to the South African plant, and has also been shut down.

In addition to these aerodynamic approaches, chemical separation processes and plasma separation processes have also been tried. Most of these efforts have been shut down or scaled back, and none currently make a significant contribution to world uranium enrichment capacity.

Proliferation Issues:

Uranium enrichment is one of the most sensitive stages of the nuclear fuel cycle because of its potential to produce weapons-grade uranium. Uranium must be enriched to over 85% U-235 before it is suitable for nuclear weapons production, but it is still the same process used to create LEU for civilian nuclear fuel, although carried through to a far higher level. Enrichment capacity is therefore regarded as a potential nuclear proliferation threat, which is why Iran's uranium enrichment program has received so much sceptical attention from the international community.

Although the long-term nuclear future lies with breeder reactor technology, the main expansion in nuclear infrastructure over the next few decades will almost certainly be of the LWR type, requiring a reliable supply of LEU for reactor fuel on the global market. There are preliminary moves underway led by the International Atomic Energy Agency in cooperation with Russia, the United States and others to establish international uranium enrichment centres for the provision of LEU to the international market. This is intended to guarantee supply to the many nations now expressing interest in developing their own civilian nuclear power programs, thereby diminishing the justification for new players to develop their own national uranium enrichment capacity. Australia could potentially play a role in this system by establishing an international enrichment centre here, using either gaseous centrifuge technology or SILEX technology, enriching locally mined uranium to LEU levels and providing it to clients who have signed up to the program. In this way, Australia could play a major role in limiting the spread of nuclear weapons capability throughout the world during this particular historical phase of the use of nuclear power. Establishing such a centre would also assure Australia of local supply of LEU fuel for our own nuclear plants once we've established our domestic nuclear power industry.

Further information:

More details on the subject of uranium enrichment can be found on the following sites:

Friday, May 7, 2010

Mining Nuclear Fuel.


The fuel for nearly all current nuclear reactors ultimately derives from the world's uranium resource. Reactors using fuel sourced from thorium are also possible, and the present generation of LWRs can use uranium bred from thorium as fuel, but this technique is not in widespread use. The greater story of obtaining nuclear fuel is thus the story of uranium mining.

Current world uranium production:

In 2009, 50,572 tonnes of uranium was mined worldwide. Kazakhstan led production with 13,820 tonnes (27%), Canada produced 10,173 tonnes (20%) and Australia 7,982 tonnes (16%), although Australia has the largest commercially extractable reserves as currently defined.

Mining techniques:

There are three main techniques in use for uranium mining. These are open pit mining, underground mining, and in situ leach mining. In open pit mining, the overburden of material is blasted and excavated away to reveal the ore body, which is then blasted, excavated and removed with loaders and dumptrucks. Underground mining is carried out in the same manner as for other underground mines, with access tunnels, and drilling and blasting. In situ leach mining involves drilling boreholes down into an ore body, pumping a leaching fluid into the ore and then pumping the resulting solution to the surface to extract the uranium. The leaching fluid is sometimes a combination of hydrogen peroxide and sulphuric acid (Australia), sometimes high concentration sulphuric acid alone (Kazakhstan), or sometimes an alkaline solution (United States), depending on the nature of the ore body. The World Nuclear Association states that 7% of the world's uranium mined in 2009 was extracted as a by-product to other mining activity. That figure includes the uranium produced at Olympic Dam.

Health concerns:

The anti-nuclear movement often claims that uranium mining is a highly dangerous and environmentally hazardous practice. The examples used to support these claims generally come from the mid-20th century, such as the often quoted case of the Navajo uranium miners in the United States during the 1950s. Significant numbers of Navajo miners went on to develop small cell carcinoma in later life. This was determined to result from exposure to radon-222, a natural radioactive decay product deriving from uranium which was concentrated in poorly ventilated mine tunnels. Radon is often present in all kinds of underground mines because uranium is a common, widely distributed element in earth's crust. Underground coal mines often have elevated uranium concentrations and thus high radon levels. Proper ventilation and toxic gas removal systems are essential safety measures for all underground mining operations, not just uranium mines.

Workers in open pit mines are usually inside the sealed cabins of vehicles, and water is sprayed in the mine to settle dust particles. In situ leach mining protects workers from the risk of radon by never exposing them to it.

Uranium contamination of ground water is a possibility and needs to be managed. The health risk of ingesting uranium comes from the chemical toxicity (roughly similar to that of lead), and first manifests in mammals as kidney damage. Uranium is too weakly radioactive to present a radiotoxicity problem. Most experts consider the health impacts of uranium mining to be of the same order as mining for other heavy metals, such as gold, or lead.

It should be understood that the regulations governing the mining, refining, enrichment and milling of uranium are now so extensive and encompassing that uranium is very likely the most heavily regulated mineral in the world.

Resources and reserves:

According to the World Nuclear Association, as of 2007 the global known recoverable reserves of uranium were 5,469,000 tonnes. The largest national share of that reserve was Australia (1,243,000 tonnes, 23%), followed by Kazakhstan (817,000 t, 15%), Russia (546,000 t, 10%), South Africa (435,000 t, 8%), Canada (423,000 t, 8%), USA (342,000 t, 6%), Brazil (278,000 t, 5%), Namibia (275,000 t, 5%), Niger (274,000 t, 5%), Ukraine (200,000 t, 4%) and Jordan (112,000 t, 2%).

These reserves are presently extractable at a cost of US$130/kg or less. Uranium was extracted from phosphate deposits in the United States and Belgium prior to 1998, and could be again if the price of uranium rises enough. The uranium resource associated with the phosphate reserves is estimated to be 27 million tonnes. There are many similar low grade uranium deposits which could be tapped if the price of uranium rises much above the current level. The price of uranium is not a major factor in the price of nuclear power, and could increase many times over the current level before significantly impacting the cost of nuclear power for the end user.

Environmental footprint and comparisons with other energy sources:

What kind of environmental footprint does mining for nuclear fuel and related minerals have, and how does it compare with other electrical generation technologies? This powerpoint presentation by Dr. Per Peterson, professor and chair of nuclear engineering at Berkeley, presents figures on concrete and steel requirements for nuclear, wind, coal and combined cycle natural gas (see slide 11). Working through the numbers and adding fuel to the total, we arrive at the following figures for the mining needed to produce 1 megawatt.year of electrical energy (1MWe.y) for each technology:


676 tonnes (0.74t steel + 8.44t concrete + 666.7t U ore at 300ppm)


680 tonnes (123t steel + 557t concrete)


~5,500 tonnes (4.19t steel + 16.4t concrete + 5,500t coal)

Combined Cycle Natural Gas:

963 tonnes (0.147t steel + 2.88t concrete + 960t gas)

These numbers need some qualification for their proper significance to be appreciated. The final figures for the fossil fuel power sources only use the mass of fuel finally consumed. If the same method had been used for nuclear power, the mass of natural uranium mined as fuel would have been 0.2 tonnes, yielding a final figure for nuclear power of 9.38 tonnes, far below any of the others. Critics could justifiably point out that uranium ore is usually far more dilute than the coal or gas resources, and insisted that the mass of raw ore needed should be considered. The grade for the ore body at the Rossing mine in Namibia of 300ppm, one of the poorest ore bodies currently mined, was used to obtain the 676 tonne figure.

The figure for wind power is almost identical, but wind has severe and most likely unresolvable issues with variability and intermittency which render it all but useless for inclusion in the power grid of an industrial society.

The figure for coal power is dominated by mining for the fuel to the extent that mining for plant building materials falls into the noise. From the mining perspective, coal is the worst offender by far.

Natural gas requires much less fuel mass than coal, but suffers other problems associated with its extraction. Wide publicity has recently been given to the newly discovered shale gas reserves in the United States, which are said to represent a significant extension of global reserves. Experience is now showing that these reserves cannot be accessed without extensive damage to large areas of underground rock layers, with associated earth tremors and serious gas leaks resulting in significant environmental difficulties in the regions being exploited.

Finally, the figure for nuclear power assumes the use of light water reactor technology. This is the current standard in power reactor technology, and is likely to remain so for the next few decades, but following that it is likely that most new nuclear capacity will be breeder reactors. The figure for fuel mining for nuclear power above is based on the assumption that 200 tonnes of natural uranium is to be mined for 1 tonne of fissile fuel, but a well-designed fast breeder reactor can eventually consume all the natural uranium, so the mining requirement shrinks to 0.5% of that figure. This would bring down the fuel mining figure for nuclear power from 666.7 tonnes to 3.35 tonnes. The amount of structural steel and concrete required for a breeder reactor compared to a light water reactor may change somewhat, but even if it is ten times that of an LWR (which it won't be, and it may well be less), the total mining requirement remains far less than that of the competitors.

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 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.

Tuesday, February 23, 2010

How Do Nuclear Reactors Work?

Nuclear fission reactors are machines that facilitate a sustained fission reaction in suitable materials, usually for the purpose of electrical power production. Most nuclear fission reactors currently in use are uranium-235 burning thermal spectrum light water reactors, so I shall describe this type first, then look at the alternatives.

Light Water Reactors:

Light water reactors get their name from their moderating fluid, which is ordinary water. This is called “light water” because the water molecules have ordinary hydrogen atoms with one proton and no neutrons as the H2 component of H2O. Some nuclear reactors use heavy water as a moderator, which has deuterium (heavy hydrogen), with one proton and one neutron, as the H2 component. Heavy water reactors will be described later. There are two basic types of light water reactor in popular use, the Pressurised Water Reactor (PWR) and the Boiling Water Reactor (BWR). The PWR is the most common.

Pressurised Water Reactors:

Modern PWRs are built around certain basic components:

Reactor Core:

The reactor core is that part of the reactor where the nuclear fuel assemblies are located and the fission reaction takes place. The shell of the core is known as the pressure vessel in a light water reactor, as part of its function is to contain water and steam at high pressure.

Nuclear Fuel:

The fuel for a nuclear reactor generally consists of pellets of uranium oxide embedded within zirconium alloy (zircaloy) tubes, which are arranged into bundles called fuel assemblies. The uranium in the fuel has been enriched to consist of 3-5% fissile U-235 rather than the natural level of 0.7% U-235.


The moderator is a substance which slows neutrons emitted naturally by the uranium fuel to thermal velocities, enabling them to be captured by U-235 nuclei and initiate fission reactions. Moderator material needs a high proportion of very light elements in order to absorb the kinetic energy of the neutrons rather than just reflecting them with little loss of velocity. Hydrogen in water (H2O) is well suited to this purpose, and light water is used as the moderator in most current reactors.

Control Rods:

Control rods are long rods inserted between fuel assemblies for the purpose of absorbing neutrons before they can participate in a fission reaction, thus controlling the reaction rate. They are made of elements with high neutron absorption cross sections, such as silver, indium, cadmium, boron, and others.


The coolant is a fluid which keeps the temperature of the reactor core under control by removing heat. It also serves the critically important function of delivering this heat to a secondary cooling loop which drives a turbine to produce electric power. Because water is both a highly effective moderator and coolant, it serves both roles in a light water reactor.

-Primary coolant loop. The primary coolant loop pumps water in a continuous cycle through the reactor core, then through the steam generator of the secondary loop, then back into the core for the next cycle. The water is pressurised to remain liquid at operating temperature. There is no exchange of water between the primary loop and the secondary loop.

-Secondary coolant loop. The secondary coolant loop contains water which is boiled in the steam generator using heat from the primary loop. The steam is then used to drive a turbine for electricity production before being cooled by the tertiary loop and recondensed to flow through the steam generator again. There is no exchange of water with the primary or tertiary loop.

-Tertiary coolant loop. The tertiary loop is an open cycle which takes water from a source outside the plant and uses it to cool the secondary loop by condensing the steam after it has driven the turbine. This water has no direct contact with water in the secondary or primary loop. It is used once, then discharged into the environment, usually at a temperature a few degrees higher than it had when taken into the plant.

The basic features listed above show how a nuclear reactor works in broad outline; Fuel bundles are placed in the reactor core. Neutrons emitted naturally by the uranium fuel are slowed by the moderator to thermal velocity and then set off subsequent fission reactions which liberate more neutrons to continue the process. The energy released by these reactions heat the coolant, which transfers heat energy through the primary and secondary loops to drive a turbine for power production. The fission reaction rate is managed with the assistance of neutron-absorbing control rods. Excess heat is taken away by the tertiary cooling loop.

There are other important components to a nuclear plant not directly related to power generation.

Containment Structure:

Containment refers to the barriers built into the reactor to shield the environment from radioactive material. Modern light water reactors have multiple layers of containment. The first level is the ceramic structure of the uranium oxide fuel pellets which trap fission products within the lattice. The next is the zircaloy cladding for the fuel. The third is the reactor vessel and cooling system. The final containment is the steel-reinforced concrete structure known as the containment building in which the reactor is situated. This is the ultimate backup if all else fails, and is designed to withstand high internal pressure to contain the steam released by a breach of the pressure vessel. They are also now designed to withstand attacks from outside such as missile strikes and impacts of large aircraft.

Radwaste Facility:

Nuclear plants need a facility to store used fuel after it has been removed from the core. The used fuel contains highly radioactive fission products which must be sequestered from the environment for some time before it can be moved to permanent storage. The first unit in the radwaste system is the spent fuel pool. This is a pool about forty feet deep where the spent fuel is kept while it cools down for several years. Spent fuel initially has a high temperature, maintained by the decay of high-level fission products. Storage in the spent fuel pool helps to prevent melting, and also provides shielding from the high level radioactivity of the spent fuel.

After about a decade the shortest lived and most highly radioactive fission products have decayed away, and the spent fuel can be removed from the pool to be sealed in steel-reinforced concrete containers for above ground storage.

Boiling Water Reactors:

BWRs are similar to PWRs, but are operated at a lower pressure. Instead of maintaining the water in the pressure vessel as a pressurised liquid, the water in a BWR is allowed to partially boil and drive a steam turbine directly, without using a secondary coolant loop. Because the turbine is in direct contact with steam from the reactor core, extra containment precautions are taken in the turbine hall of a BWR.

In the United States, the civilian nuclear power plant fleet consists of 69 PWRs and 35 BWRs.

Alternative reactor types:

There are many possibilities for reactor design beyond the ordinary light water variety. We will look at these in turn in subsequent essays.

-Heavy water reactors.

-Fast breeder reactors.

-Gas-cooled reactors.

-Molten salt reactors.

-Aqueous reactors.

-Small modular reactors.

Wednesday, January 27, 2010

What Is Nuclear Fission?

Nuclear fission is a natural phenomenon where atomic nuclei split into smaller ‘daughter’ nuclei, releasing large amounts of energy in the process. Many atomic nuclei are fissionable, meaning that they can be split by being hit with neutrons. Out of these nuclei, three are of special interest to humanity because the manner in which they split releases enough free neutrons for a self-sustaining chain reaction to occur among neighbouring atoms of the same isotope. This makes it possible to build nuclear reactors for practical power production. The nuclei in question are two uranium isotopes, U-233 and U-235, and one plutonium isotope, Pu-239. These are called fissile isotopes.

Out of the three fissile isotopes, only U-235 occurs naturally on Earth, where it composes 0.7% of natural uranium. The other two can be produced artificially by exposing certain other nuclei to neutron radiation to transmute them into other isotopes which then undergo radioactive decay to become the desired fissile isotopes. This process of irradiating non-fissile nuclei in order to convert them into fissile nuclei is called breeding. Non-fissile nuclei which can be bred into fissile nuclei are called fertile. The precursor fertile isotopes are:

1) Thorium-232, which can be transmuted to Th-233, decaying to protactinium-233, then to U-233.

2) Uranium-238, which can be transmuted to uranium-239, which will then decay to neptunium-239, which ultimately decays to Pu-239.

Uranium-238 composes over 99% of natural uranium, which is roughly as abundant as tin in the earth's crust. Thorium is roughly four times as common as uranium, and consists of 100% Th-232.

Nuclear fission can be further classified according to the velocity of the neutrons initiating the process. Neutrons emitted from an atomic nucleus typically have a velocity of around 14,000 km/s, about 5% of the speed of light. These are known as fast neutrons, and nuclear reactors which utilise them are known as fast reactors. The chief advantages of fast reactors is that they liberate more neutrons per collision in the fission process, which enables more efficient fuel breeding, and the energy of the collision is high enough to fission most actinides such as U-238, rather than just U-235 as usual. The chief disadvantage is that fast neutrons have a very small capture cross-section for their target nuclei, thus reducing the likelihood of fission-inducing collisions.

It is possible to slow neutrons down by exposing them to elements with light nuclei. These absorb most of the momentum of the collisions until the neutrons are travelling at only a couple of kilometres per second. This velocity corresponds to room temperature. These are known as thermal neutrons, and nuclear reactors designed to make use of them are known as thermal reactors. The vast majority of current nuclear power plants are thermal reactors. The chief advantage of thermal fission reactions is that they have a much larger capture cross-section with their target nuclei than fast reactions, thereby increasing the likelihood of a fission event. The disadvantages are that they cannot initiate fission in non-fissile nuclei such as U-238, and they release insufficient neutrons to achieve a break-even breeding rate for converting U-238 to Pu-239. Thermal neutrons can achieve break-even breeding of U-233 from Th-232, but this property has not yet been widely exploited.