click to enlarge3.1 Mass to energy
While people until relatively recently must have thought they were converting mass to energy when they burned wood to cook meals and to keep warm, any student today would be aware that this was not the case. One form of carbon compound (the solid wood) was simply being converted to another (a colourless gas) which blew away. The hydrogen involved with the original compound also dispersed as water vapour. No measurable mass was lost, although energy was released. However, during the 20th century, as our understanding of nuclear physics developed, it was suggested that mass could in fact be turned into energy. This is what happens in a nuclear reactor, using atoms of particular metals such as uranium.
Uranium is 1.7 times more dense than lead, and is composed of atoms which have in their nucleus 92 protons (positively-charged) and about 140 neutrons (uncharged). One of the types of uranium atoms, or one of the uranium "isotopes" as they are called, has 143 neutrons. This uranium-235 (U-235) isotope is remarkable because when its nucleus is hit by a slow neutron (also known as a "thermal" neutron) the atom can split in two and release a lot of energy as heat. This is called nuclear "fission", and U-235 is thus a "fissile" isotope. In Einstein's terms some mass is lost and converted to energy. At the same time several fast neutrons are emitted from the split nucleus. If these are slowed by a moderator such as graphite or water they can cause other U-235 atoms to split, thus giving rise to a chain reaction*. See also Figure 14.
* What happens in a nuclear reactor is much more fully described in the UIC publication Physics of Uranium.
The other main isotope of natural uranium, U-238 is not itself fissile in conventional reactors but each atom can capture a neutron, indirectly to become fissile plutonium-239. It is thus "fertile". Pu-239 behaves similarly to U-235 except that its neutron yield is slightly greater than that of U-235. About one third of the energy from a commercial nuclear reactor comes from fission of the plutonium produced in the reactor.
The reactor core is loaded with uranium oxide fuel. In CANDU reactors, natural uranium (0.7% U-235) is used, while for light water reactors it is enriched to 3-4% U-235 (see also Section 4.2). In both cases the uranium oxide is typically in the form of ceramic pellets of UO2, assembled inside zircalloy or stainless steel tubes and surrounded by coolant and moderator. The moderator slows down the fast neutrons from the nuclear fission chain reaction so that they are more likely to cause ongoing fission. The slow neutrons cause further fission in U-235 atoms. Each such fission typically releases about 200 MeV, or 3.2 x 10-11 Joule, (contrasting with 4 ev or 6.5 x 10-19 J per molecule of carbon dioxide released in the combustion of carbon).
Commercial nuclear power generation involves containing and controlling the fission reactions so that the heat can be used to make steam which in turn generates electricity. The nuclear fuel cycle is described in Section 4.2.
Figures 9A and 9B show two different types of reactors used for generating electricity. In the core the uranium undergoes fission so that a lot of heat is released. The control rods shown regulate the rate of the reaction, and therefore the heat yield, by absorbing some of the moving neutrons.
In the Pressurised Water Reactor (Figure 9A) the core is surrounded by ordinary water and is enclosed in a very thick steel pressure vessel. The water, under high pressure, serves as both coolant and moderator. It is circulated to a heat exchanger (steam generator) where water in a separate circuit is turned into steam.
Figure 9B shows the Canadian-designed and built CANDU reactor. Instead of being in a pressure vessel, the fuel is in a number of pressure tubes within a reactor vessel called a calandria. Pressurised water or heavy water flows through the pressure tubes and conveys the heat to a steam generator. Heavy water at low pressure fills the calandria, surrounding the pressure tubes and acts as moderator.
In both cases this all occurs in a big concrete or steel containment structure. The steam is fed to a turbine generator, much the same as those installed in coal-fired power stations. The uranium-fuelled core of a nuclear power reactor simply takes the place of a boiler or furnace burning coal (or other fossil fuel) to generate the steam.
Figure 9A Pressurised water reactor (PWR)
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Figure 9B CANDU pressurised heavy water reactor (PWHR)
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Table 5:
| Nuclear power plants in commercial operation | ||||||
| Reactor type | Main Countries | Number | GWe | Fuel | Coolant | Moderator |
|
Pressurised Water Reactor (PWR) |
US, France, Japan, Russia |
263 |
237 |
enriched UO2 |
water |
water |
|
Boiling Water Reactor (BWR) |
US, Japan, Sweden |
92 |
81 |
enriched UO2 |
water |
water |
|
Gas-cooled Reactor (Magnox & AGR) |
UK |
26 |
11 |
natural
U (metal), |
CO2 |
graphite |
|
Pressurised Heavy Water Reactor 'CANDU' (PHWR) |
Canada |
38 |
19 |
natural UO2 |
heavy water |
heavy water |
|
Light Water Graphite Reactor (RBMK) |
Russia |
17 |
13 |
enriched UO2 |
water |
graphite |
|
Fast Neutron Reactor (FBR) |
Japan, France, Russia |
3 |
1 |
PuO2 and UO2 |
liquid sodium |
none |
|
TOTAL |
439 |
361 |
||||
GWe
= capacity in thousands of megawatts.
Source: IAEA April 2004, Nuclear Power Reactors in the World (at 31/12/03).
For reactors under construction: see paper Plans for New reactors Worldwide
Table of the World's Nuclear Power Reactors
In 2001 nuclear electricity generation was 2544 billion kilowatt hours, more than all electricity generated worldwide in 1961, and an increase of 4% on 2000 and 19.4% over the previous seven years. The reasons for the overall growth are several:
First, and most obviously, capacity is steadily increasing as new reactors come on line, as suggested by Table 6. At the end of 2002 there were 440 nuclear power reactors with a capacity of over 355 GWe operating in 31 countries, with 30 power reactors (25 GWe) under construction in ten countries.
Secondly, increased nuclear capacity in some countries is resulting from the uprating of existing plants. Power reactors in USA, Belgium, Sweden, Spain, Switzerland and Germany, for example, have had their generating capacity increased.
Thirdly, capacity or load factors are improving everywhere, so that more kilowatt hours come from the installed capacity. More than two thirds of the nuclear plants outside Russia and Ukraine in the last few years have had load factors over 75%, up from average 67% in 1992. Many countries average over 80% load factor. US nuclear power plant performance, at over 85%, has moved into the top bracket. Recently the annual improvement in US reactor performance was equivalent to putting 2-3 large new power station units on line each year. To put it another way, the US increase from 65% load factor in 1980s to 90% today is equivalent to adding 23,000 MWe capacity.
Fourthly, plant lives are being extended. Most nuclear power plants originally had a nominal design lifetime of 30 to 40 years, but engineering assessments have established that many plants can operate longer. Extending reactor operating life by replacing major components is often an attractive and cost-effective option for utilities. In USA and Japan most reactors had confirmed life-spans of 40 years, but many have now been cleared to operate for 60 years. When the oldest commercial nuclear power stations in the world, Calder Hall and Chapelcross in the UK, were built in the 1950s, it was assumed that they would have a useful lifetime of 20 years. They are now authorised to operate for 50 years.
New reactor start-ups seem likely to exceed the decommissioning of old reactors for several years at least, though most of the new reactors will be in the Asian region.
Uranium is ubiquitous on the earth.
It is a metal approximately as common as tin or zinc, and it is a constituent of most rocks and even of the sea. Some typical concentrations are: (ppm = parts per million) .
| High-grade orebody | 2% U, 20,000 ppm U |
| Low-grade orebody | 0.1% U, 1,000 ppm U |
| Granite | 4 ppm U |
| Sedimentary rock | 2 ppm U |
| Average in earth's continental crust | 2.8 ppm U |
| Seawater | 0.003 ppm U |
An orebody is, by definition, an occurrence of mineralisation from which the metal is economically recoverable. It is therefore relative to both costs of extraction and market prices. At present neither the oceans nor any granites are orebodies, but conceivably either could become so if prices were to rise sufficiently.
Measured resources of uranium, the amount known to be economically recoverable from orebodies, are thus also relative to costs and prices. They are also dependent on the intensity of exploration effort. Changes in costs or prices, or further exploration, may alter measured resource figures markedly. Thus, any predictions of the future availability of any mineral, including uranium, which are based on current cost and price data and current geological knowledge are likely to be extremely conservative.
With those major qualifications Table 7 gives some idea of our present understanding of uranium resources. It can be seen that Australia has a substantial part (about 27 percent) of the world's low-cost uranium, and Canada 15 percent.
TABLE 7.
| tonnes U | percentage of world | |
|---|---|---|
| Australia | 863,000 | 28% |
| Kazakhstan | 472,000 | 15% |
| Canada | 437,000 | 14% |
| South Africa | 298,000 | 10% |
| Namibia | 235,000 | 8% |
| Brazil | 197,000 | 6% |
| Russian Fed. | 131,000 | 4% |
| USA | 104,000 | 3% |
| Uzbekistan | 103,000 | 3% |
| World total | 3,107,000 |
Presently known resources of uranium are enough to last for half a century,
- considering only the lower cost category, and with it used only in conventional reactors. This represents a higher level of assured resources than is normal for most minerals. Further exploration and higher prices will certainly, on the basis of present geological knowledge, yield further resources as present ones are used up. A doubling of price from present contract levels could be expected to create about a tenfold increase in measured resources.
Widespread use of the fast breeder reactor (see 4.2) could increase the utilisation of uranium sixty-fold or more. This type of reactor can be started up on plutonium derived from conventional reactors and operated in closed circuit with its reprocessing plant. Such a reactor, supplied with natural uranium for its "fertile blanket", very quickly reaches the stage where each tonne of ore yields 60 times more energy than in a conventional reactor.
See also paper on Supply of Uranium
Reactor Fuel Requirements
The world's power reactors, with combined capacity of 350 GWe, require some 75,000 tonnes of uranium oxide concentrate from mines (or stockpiles) each year. While this capacity is being run more productively, with higher capacity factors and reactor power levels, the uranium fuel requirement is increasing but not necessarily at the same rate. The factors increasing fuel demand are offset by a trend for higher burnup of fuel and other efficiencies, so demand is steady. (Over the 18 years to 1993 the electricity generated by nuclear power increased 5.5-fold while uranium used increased only just over 3-fold.) It is likely that the annual uranium demand will grow only slightly to 2010.
Fuel burnup is measured in MW days per tonne U (MWd/t), and many countries are increasing the initial enrichment of their fuel (eg from 3.3 to 4.0% U-235) and then burning it longer or harder to leave only 0.5% U-235 in the fuel. This might mean that burnup is increased from 33,000 MWd/t to 45,000 MWd/t. On the other hand low uranium prices mean that enrichment plants are being operated so as to reduce energy requirements and leave more U-235 in the tails*.
*Increasing the tails assay from 0.25% to 0.30% U-235 for 3.5% enriched fuel means increasing the input from 7.0 to 7.8 kg per kilogram of enriched output.
Reprocessing of spent fuel from conventional light water reactors (see 5.2) also utilises present resources more efficiently, by a factor of up to 1.3 overall. At present the (reactor-grade) plutonium arising from reprocessing is used in fresh mixed oxide fuel (MOX), with depleted uranium from enrichment plants. Another factor which may similarly affect uranium demand is a fuel cycle now being developed by Korea and Canada which allows spent light water reactor fuel to be used as CANDU fuel, without chemical reprocessing.
CANDU plants currently operate on natural uranium fuel (0.7% U-235) with burnup of some 7500 MWd/tonne. These plants can be fuelled with slightly enriched uranium fuel (up to 1.2% U-235), increasing burnup to above 20,000 MWd/tonne without significant physical modifications. This will be done as uranium prices significantly increase.
The net result from all this is a small reduction in the amount of uranium required ex-mine to fuel each kilowatt-hour produced.
See also paper on Uranium markets
3.4 Energy inputs to Nuclear Electricity
Any electricity generation requires some energy inputs in mining, concentrating and transporting the fuel, manufacturing and constructing the plant, and dealing with the wastes. Energy use in mining and transport is closely related to quantities involved, and any comparison therefore favours uranium. On the other hand the capital-intensive nature of the nuclear fuel cycle is reflected in the plant, and the greater energy inputs to it.
The main energy input to the nuclear fuel cycle for reactors requiring enriched fuel may be in enriching uranium (see 4.2), which can be very energy-intensive. The following figures consider a 1000 MWe reactor run at 80% and therefore generating 7000 GWh/yr. This would require about 153 tonnes of natural uranium each year, which might be enriched to produce 20 tonnes of uranium fuel at 3.5% U-235. After conversion to UF6 this would need 4.2 GWh of electricity to enrich it in a modern centrifuge plant or up to 200 GWh in an older diffusion plant*. Then there is fuel fabrication as well as construction and operation of the reactor to include. The total energy inputs to the nuclear fuel cycle represent 1.7% or up to nearly 5% of the energy output depending on enrichment process used. Mining, at Ranger, uses energy equivalent to 0.05% of the mine's output ifused in a light water reactor.
*At a tails assay of 0.30% U-235 in the enrichment plant, 4.3 SWU per kg of 3.5% enriched product is required, @ 50 kWh/SWU for the modern centrifuge plant or up to 2400 kWh/SWU for the older gaseous diffusion plant. The 20 tonnes of enriched fuel would require input of 153 tonnes of natural uranium (180 t U3O8) at this tails assay, or 137 tonnes at 0.25% U-235, which has been the norm. See also section 4.2 and UIC briefing paper # 57.
This energy input needs to be seen in the light of the contrasting energy outputs from coal and nuclear. Running the 1000 MWe power station for a year at 80% capacity, assuming 33% thermal efficiency and using the data in Table 3, would require 2.5 million tonnes of the best coal (3.1 million tonnes of average domestic Australian black coal) or 153 tonnes of natural uranium.
In the case of Canadian reactors, enrichment of the fuel is not required but heavy water has to be made, and this requires substantial energy input. In a sense, the water moderator and primary coolant are enriched rather than the fuel. However, this "enrichment" is required only once, and the heavy water stays in use indefinitely.
3.5 Nuclear Weapons as a source of fuel
An increasingly important source of nuclear fuel is the world's nuclear weapons stockpiles.
Since 1987 the United States and countries of the former USSR have signed a series of disarmament treaties to reduce the nuclear arsenals of the signatory countries by approximately 80 percent by 2003.
The weapons contain a great deal of uranium enriched to over 90 percent U-235 (ie about 25 to 100 times the proportion in reactor fuel). Some weapons have plutonium-239, which can be used in diluted form in either conventional or fast breeder reactors.
Uranium
The surplus of weapons-grade highly enriched uranium (HEU) has led to an agreement between the USA and Russia for the HEU from Russian warheads and military stockpiles to be diluted for delivery to USEC and then used in civil nuclear reactors. Under the 'swords for ploughshares' deal signed in 1994, the US Government will purchase 500 tonnes of weapons-grade HEU over 20 years from Russia for dilution and sale to electric utilities, for US$ 11.9 billion.
Weapons-grade HEU is enriched to over 90% U-235 while light water reactor fuel is usually enriched to about 3-4%. To be used in most commercial nuclear reactors, military HEU must therefore be diluted about 25:1 by blending with depleted uranium (mostly U-238), natural uranium (0.7% U-235), or partially enriched uranium.
The contracted HEU is being blended down to 4.4% U-235 in Russia, using 1.5% U-235 for this. The 500 tonnes of weapons HEU will result in about 15 000 tonnes of low-enriched (4.4%) uranium over the 20 years. This is equivalent to about 152 000 tonnes of natural U, more than twice annual world demand.
The purchase and blending down will be done progressively. Up to 1999 it was at the rate of 10 tonnes per year (equivalent to approximately 3 700 tonnes of uranium oxide production per year). Since 2000 the dilution of 30 tonnes per year of military HEU is displacing about 10,600 tonnes of uranium oxide mine production per year, representing about 15% of the world's reactor requirements.
In addition, the US Government has declared 174 tonnes of highly-enriched uranium (of various enrichments) to be surplus from its military stockpiles, and this is being blended down to about 4300 tonnes of reactor fuel. In the short term the military uranium is likely to be blended down to 20% U-235, then stored. In this form it is not useable for weapons.
Plutonium
Disarmament will also give rise to some 150-200 tonnes of weapons-grade plutonium. This will be either:
The US Government has declared 38 tonnes of weapons-grade plutonium to be surplus, and is exploring both options for it. There is wide support for burning it as a mixed oxide fuel in conventional reactors, and this is the priority. Meanwhile the USA is developing a "spent fuel standard", which means that plutonium should never be more accessible than if it is incorporated in spent fuel.
However, Europe has a well-developed MOX capacity and Japan is developing its use. This suggests that weapons plutonium could be disposed of relatively quickly. Input plutonium would need to be about half reactor grade and half weapons grade, but using such MOX as 30% of the fuel in one third of the world's reactor capacity would remove about 15 tonnes of warhead plutonium per year. This would amount to burning 3000 warheads per year to produce 110 billion kWh of electricity, - enough for two thirds of Australia's needs.
Over 35 reactors in Europe are licensed to use mixed oxide fuel, and 20 French reactors are using it or licensed to use it as 30% of their fuel. CANDU reactors are well suited to burn MOX fuel, and development of this is planned, using US-supplied MOX.
Russia intends to use its plutonium as a fuel, burning it in both conventional and fast neutron reactors. If all the plutonium were used in fast neutron reactors in conjunction with the depleted uranium from enrichment plant stockpiles,* there would be enough to run the world's commercial nuclear electricity programs for several decades without any further uranium mining.
*When uranium is enriched for a conventional reactor about seven times more depleted uranium is produced than the enriched product. If uranium is enriched to 93% U-235 for a weapons programme about 200 times more depleted uranium than enriched product is produced. All this, comprising a very large proportion of all uranium ever mined, is "fertile" material and thus potential fast breeder fuel.
See also paper on Miltary warheads as nuclear fuel
Most of this book is concerned with uranium as a fuel for nuclear reactors. However, thorium can also be utilised as a fuel for CANDU reactors or in reactors specially designed for this purpose. The thorium fuel cycle has some attractive features, and is described further in Section 4.5.
Neutron efficient reactors, such as CANDU, are capable of operating on a thorium fuel cycle, once they are started using a fissile material such as U-235 or Pu-239. Then the thorium (Th-232) captures a neutron in the reactor to become fissile uranium (U-233), which continues the reaction.
Thorium is about three times as abundant in the earth's crust as uranium. Australian mineral sands, especially in Victoria and Western Australia, contain considerable quantities of thorium.
See also paper on Thorium
Along with the electricity production focus of this booklet, it is relevant to note that in addition to over 470 commercial reactors operating or under construction, there are some 280 research and/or isotope production reactors operating in 54 countries. These are mostly much smaller than those used for electricity production, but they nevertheless need fuel and produce wastes. Apart from actual research, they are used to produce medical isotopes and other radioactive sources for industry.
See also paper on Research reactors
Nuclear energy is particularly suitable for vessels which need to be at sea for long periods without refuelling, or for powerful and fast submarine propulsion. Following the end of the Cold War, there are still some 150 ships powered by more than 200 small nuclear reactors. Most of these are submarines, but they range from icebreakers to aircraft carriers. Their reactors are pressurised water types with special fuel and design which enables them to go at least ten years between refuelling.
The nuclear-powered submarines are able to maintain submerged speeds of up to 25 knots for weeks on end, which revolutionised their role. The navies of USA, Britain, France, Russia and China use nuclear-powered vessels.
Many nuclear-powered submarines were decommissioned in the 1990s due both to obsolescence and arms reductions. In the USA, after defuelling, the reactor compartments are simply cut away from the rest and are sent to low-level waste disposal sites (see chapter 5). In Russia however there are notorious problems apparently due to political and economic constraints. In the UK at this stage obsolete nuclear-powered vessels are simply defuelled.
See also paper on Nuclear-powered ships
3.9 Other applications of nuclear energy
Apart from marine propulsion and research reactors, a few nuclear plants (totalling about 5 MW thermal) are being used for non-electric applications. However, the potential is great in areas such as desalination and the petroleum industry, for refining and for enhancing extraction of oil from the ground and from tar sands. A major future use is likely to be for hydrogen production, initially by electrolysis but ultimately by thermochemical means at high temperatures.
Water cooled reactors can provide heat up to 300°C, and other experimental types such as the High Temperature Gas Reactor and Molten Salt Reactor, to more than 900°C. There is considerable experience in cogeneration, using heat as a by-product of electricity generation, in many countries.
See also papers on The Hydrogen Economy andDesalination
3.10 Accelerator-driven systems
The essence of a conventional nuclear reactor is the controlled fission chain reaction of U-235 and Pu-239. This depends on having a surplus of neutrons to keep it going (a U-235 fission requires one neutron input and produces on average 2.43 neutrons). However, without such a surplus, a nuclear reaction can be sustained by input of neutrons produced by spallation from heavy element targets bombarded by protons in a high-energy accelerator.
If the spallation target is surrounded by a blanket assembly of nuclear fuel, such as fissile isotopes of uranium or plutonium (or thorium which can breed to U-233), there is a possibility of sustaining a fission reaction. This is described as an Accelerator-Driven System (ADS).
In such a subcritical nuclear reactor the neutrons produced by spallation would be used to cause fission in the fuel, assisted by further neutrons arising from that fission. One then has a nuclear reactor which could be turned off simply by stopping the proton beam, rather than needing to insert control rods to absorb neutrons and make the fuel assembly subcritical. The fuel may be mixed with long-lived wastes from conventional reactors.
The other role of a subcritical nuclear reactor or ADS is the destruction of heavy isotopes. In the case of atoms of odd-numbered isotopes heavier than thorium-232, they have a high probability of absorbing a neutron and subsequently undergoing nuclear fission, thereby producing some energy and contributing to the multiplication process. Even-numbered isotopes can capture a neutron, perhaps undergo beta decay, and then fission. This process of converting fertile isotopes to fissile ones is called breeding.
Therefore in principle, the subcritical nuclear reactor may be able to convert all long-lived transuranic elements into (generally) short-lived fission products and yield some energy in the process. But the main benefit would be in making the management and eventual disposal of high-level wastes from nuclear reactors easier and less expensive. However, much of the current interest is in the potential of ADS to burn weapons-grade plutonium, as an alternative to using it as mixed oxide fuel in conventional reactors.
See also paper on Accelerator-driven nuclear energy
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