Energy resources: Nuclear energy
The transformation of radioactive uranium and, in some instances, thorium isotopes provides vastly more energy per unit mass of fuel than any other energy source, except nuclear fusion, and therein lies its greatest attraction.
The potential of nuclear fuels for energy production became a reality when the first experimental atomic pile, built by Enrico Fermi and Léo Szilárd at the University of Chicago, began functioning in December 1942. That led to the manufacture of fissionable material for the first atomic weapons. The use of nuclear power for electricity production explanded rapidly in the 1960s, a period when the costs of building nuclear power stations and of purchasing the uranium fuel were thought to be less than for fossil fuel plants. The nuclear industry received a boost in the early 1970s, when fossil fuel prices rose abruptly during the oil crisis of 1974: following the Yom Kippur war of late 1973, oil producers in the Middle East quadrupled the price of their crude oil almost overnight.
During the 1980s, however, the costs of building nuclear power stations rose inexorably as stringent safety requirements grew, especially following the accident Three Mile Island in Pennsylvania (1979) and the much larger one at Chernobyl (1986) in the Ukraine. By the early 1990s the global rate of expansion of the nuclear industry had slowed almost to a standstill and fuel got cheaper as the power stations became more expansive.
Today, with growing concern about global warming, the environmental advantage of nuclear power over fossil fuels is becoming increasingly recognised: it produces no greenhouse gases. It also produces no acid rain, unlike coal and to a lesser extent oil.
This course looks at nuclear reactions, reactors and power generation. It looks at the properties of uranium, how and where it is mined, and why nuclear waste is potentially a serious hazard and allows us to consider the advantages and limitations of the situation in which we find ourselves today.
This OpenLearn course provides a sample of level 2 study in Science
After studying this course, you should be able to:
distinguish between energy produced by nuclear fission and radioactive decay
describe the principles behind nuclear 'burner' and nuclear 'breeder' reactors
understand the geoscientific principles underlying the enrichment of uranium in ore deposits
summarise and explain the hazards associated with nuclear wastes and their safe disposal
summarise the fluctuating fortunes of the nuclear power industry.
1 Nuclear energy
The transformation of radioactive uranium and, in some instances, thorium isotopes provides vastly more energy per unit mass of fuel than any other energy source, except nuclear fusion, and therein lies its greatest attraction. The key to that remarkable fact is the conversion of matter (with mass, m) into energy (E), according to Einstein's famous equation E = mc2, where c is the speed of light (3×108 m s−1 ).
The potential of nuclear fuels for energy production became a reality when the first experimental atomic pile, built by Enrico Fermi and Léo Szilárd at the University of Chicago, began functioning in December 1942. That led to the manufacture of fissionable material for the first atomic weapons. The use of nuclear power for electricity production expanded rapidly in the 1960s, a period when the costs of building nuclear power stations and of purchasing the uranium fuel were thought to be less than for fossil fuel plants. The nuclear industry received a boost in the early 1970s, when fossil fuel prices rose abruptly during the oil crisis of 1974: following the Yom Kippur war of late 1973, oil producers in the Middle East quadrupled the price of their crude oil almost overnight.
During the 1980s, however, the costs of building nuclear power stations rose inexorably as stringent safety requirements grew, especially following the accident at Three Mile Island in Pennsylvania (1979) and the much larger one at Chernobyl (1986) in the Ukraine. In addition, the cost of decommissioning nuclear reactors and of developing secure repositories for radioactive waste had not been taken into account in the initial cost-benefit analyses. These considerations have loomed progressively larger as older nuclear power stations approach the end of their useful lives and as the volume of waste grows year by year. The global rate of expansion of the nuclear industry had slowed almost to a standstill by the early 1990s, and as demand for uranium fuel fell, so did its price. In other words, the fuel got cheaper as the power stations became more expensive.
In the early 21st century, however, with growing concern about global warming the environmental advantage of nuclear power over fossil fuels is becoming increasingly recognised: it produces no greenhouse gases. It also produces no acid rain, unlike coal and to a lesser extent oil.
By 2003 over 400 nuclear reactors were generating electricity globally, producing over 350 GW (Table 1). This amounts to about 16% of global electricity generating capacity. An additional 31 nuclear reactors with an additional generating capacity of 25 GW are currently in construction globally. In the UK many reactors are of an early design and only a few of the younger nuclear power stations match the 1 GW capacity of most fossil-fuel stations. Nevertheless, almost a quarter of UK electricity generating capacity is nuclear (Figure 1); we consider the advantages and limitations of this situation shortly.
2 Nuclear reactions, reactors and power generation
The nuclear reactions that produce energy from uranium fuel are complex (Section 2.2), but in principle the generation of electricity is no different from that in fossil fuel power stations (Figure 2). The fuel (fossil or nuclear) is used to heat water to above its boiling temperature in a boiler, and the resultant high-pressure steam drives turbines that generate electricity. The steam is then condensed and returned as water to the boiler. This is a closed circuit, and the steam never comes in contact with the fuel.
2.2 Nuclear fission
Every atom has a nucleus consisting of positively charged protons and electrically neutral neutrons. Protons and neutrons have virtually identical mass and the total number of protons and neutrons defines the mass number of a particular atom. The number of protons in the nucleus is the atomic number and this quantity is always the same for each particular chemical element. However, some elements have several isotopes, each with different numbers of neutrons, but with the same number of protons. In full notation an isotope is represented by its chemical symbol preceded by a subscript number showing the element's atomic number (the number of protons in its nucleus), whereas the superscript is the mass number of the isotope (the sum of protons and neutrons in its nucleus). Uranium has two isotopes of interest:
uranium-235, which has 92 protons and 143 neutrons, written as
uranium-238 , which also has 92 protons but 146 neutrons;
i.e. both isotopes have the same atomic number (92) but different mass numbers (235 and 238). Note: In equations for nuclear reactions we use this formal notation, e.g. , but otherwise the simpler name of an isotope, e.g. uranium-238.
These isotopes have such large nuclei that they are inherently unstable. They spontaneously break down, or decay, by two possible processes:
radioactive decay, a process that emits alpha particles, which are equivalent to the nuclei of helium atoms, with two protons and two neutrons ;
nuclear fission, a much less frequent process, in which the whole nucleus breaks apart, releasing energy.
But how does nuclear fission of uranium release energy?
The main products of nuclear fission are nuclei of other elements whose combined mass is slightly less than the mass of the parent uranium nucleus. The 'missing' mass is converted into energy according to the relationship E = mc2, where m is the 'missing' mass, and c is the speed of light. Because the speed of light is so enormous (3 × 108 m s−1 ), when the number is squared, even a small loss of mass converts into a huge amount of energy.
Of the naturally occurring radioactive elements, only uranium and thorium undergo spontaneous fission and release energy on this potentially vast scale. The other elements decay by emitting either alpha particles (as defined above), or beta particles (equivalent to electrons), or positrons (electrons with positive charge), or gamma rays (very short wavelength electromagnetic radiation).
The isotopes produced by fission may also be radioactive and will themselves decay radioactively until stable nuclei are formed. Radioactive decay can last a long time: the half-lives (Box 1) of most natural radioactive elements are measured in thousands of years (e.g. carbon-14) to billions (109) of years (e.g. uranium-238).
The energy produced by natural radioactive decay is the kinetic energy of the emitted particles, which is converted into thermal energy (heat) when they collide with other nuclei. The heat produced by natural decay of radioactive elements that are dispersed in low concentrations in the Earth's outer layers (crust and mantle) is the source of the Earth's internal heat. This heat is indirectly tapped when we exploit geothermal power. Radioactive decay of uranium is far too 'dilute' a source of energy to exploit directly for electricity generation, as Question 1 illustrates.
Box 1 Radioactive half-lives
The pace of decay of natural radioactive isotopes can be visualised from each isotope's half-life. This is the time it takes for half the total number of atoms to undergo a nuclear transformation (Figure 3). The half-life of uranium-235 is 704 million years (Ma), meaning that half the original atoms will have decayed in this time. Half those remaining will decay in the next 704 Ma, so that three-quarters have decayed after 1408 Ma, seven-eighths in 2112 Ma, and so on. For uranium-238 the half-life is 4468 Ma, so the Earth (which is about 4600 Ma old) has lost a much greater fraction of its uranium-235 than its uranium-238 atoms since it formed. (Natural uranium today contains only 0.7% of uranium-235.)
The rate of heat production by natural radioactive decay of uranium-238 is 3000 J kg−1 yr−1. Roughly how long would it take for 1 kg of uranium-238 to produce the same amount of energy as is contained in 1 kg of coal, which is 2.8 × 107 J?
In one year, 1 kg of uranium-238 undergoing spontaneous radioactive decay will produce 3× 103 J, whereas 1 kg of coal is equivalent to 2.8 × 107 J. So it would take (2.8 × 107 J yr −1)/(3 × 103 J) ≈ 9300 years, or about 104 (10 000) years, for ordinary radioactive decay of uranium to produce the energy equivalent of the same mass of coal.
So, natural radioactive decay cannot be used to fuel power stations. The ability to undergo nuclear fission is what makes uranium a concentrated energy source — but only after some technological intervention.
Natural fission of uranium nuclei occurs much less frequently than radioactive decay. In its natural state, uranium-238 undergoes one nuclear fission for roughly every million alpha emissions involved in its radioactive decay. So, for nuclear fission to become a viable energy source, the fission rate must be greatly increased. Before describing how this is done, you need to understand a little more about uranium-235 and uranium-238.
Natural uranium today consists of 99.3% uranium-238 and 0.7% uranium-235. A greater proportion of uranium-235 was present early in the Earth's history, but its relative amount has decreased because its half-life is about ten times shorter than that of uranium-238 (i.e. it decays faster — see Box 1). Despite its much lower abundance, uranium-235 is the isotope which provides the fissionable fuel in nuclear reactors.
The occasional natural fission of uranium atoms releases neutrons. Nuclei of uranium-235 may capture these low-energy or slow neutrons, each capture increasing the mass number of uranium-235 to produce uranium-236:
The uranium-236 nucleus created in this way is highly unstable and breaks down by fission:
There are three important things to note about this reaction.
Although in numerical terms the mass numbers and atomic numbers total the same on both sides of Equation 2 (236 and 92 respectively), in terms of precise mass, the sum of the products on the right does not quite equal 236. There has been a small loss of mass in the fission reaction, and this has been converted into energy according to E = mc2.
The main fission products (strontium-93 and xenon-140) are radioactive and decay to stable daughter products, releasing thermal energy in the process.
More neutrons are produced — three for every atom of uranium-236 that undergoes fission.
If these neutrons are captured by more nuclei of uranium-235 the reaction will continue. That produces still more neutrons and more energy in an uncontrolled chain reaction, as in atomic weapons, unless it is controlled in some way, as in a power station. Such a chain reaction can only begin, however, if sufficient atoms of fissionable uranium-235 are present in a small volume, as in a reactor core, a nuclear bomb and very rarely in extremely rich uranium deposits: it needs a critical mass for the chain reaction to be self-sustaining. In a nuclear reactor, uranium atoms are bombarded with neutrons . This increases the rate of fission, thereby releasing energy much more rapidly.
Equations 1 and 2 are the basis for calculating the amount of energy produced by uncontrolled fission: the fission energy of uranium. A kilogram of uranium-235 contains 25.5 × 1023 atoms. The fission energy available from one atom of uranium-235 is 3.2 × 10−11 J (Equation 2). So the amount of fission energy available from 1 kg of uranium-235 will be:
The energy released in a year by fission of the 0.7% of uranium-235 in 1 kg of natural uranium is 5.7 × 1011 J, i.e. around 2 × 108 times that released by radioactive decay of both isotopes. The same amount of energy would be released by burning about 20 t of coal over the same time.
So why not exploit fission energy from the much more abundant isotope, uranium-238? The principal reason is that uranium-238 is more stable than uranium-235; slow neutrons just 'bounce off' its nucleus. Only high-energy fast neutrons can penetrate the uranium-238 nucleus to be captured. This creates uranium-239, which in turn is transmuted to neptunium-239 and then to plutonium-239 by loss of electrons:
Plutonium-239 is unstable and undergoes fission in a similar manner to uranium-235, releasing more neutrons and an amount of energy similar to that in Equation 2. This production of plutonium-239 also poses a major hazard in the waste from conventional nuclear power stations, as well as the risks from various radioactive fission products (Section 4).
Clearly, by exploiting the more abundant uranium-238 isotope in this way, the potential of naturally occurring uranium as an energy resource can be increased — in fact by almost 150 times — compared with using just uranium-235. However, establishing an efficient chain reaction based on uranium-238 requires a supply of fast neutrons, and this involves much more advanced and complex technology, as you will see in Section 2.3.
2.3 Nuclear reactors
A critical mass of uranium is necessary for nuclear chain reactions (Equations 1 to 3) to occur. A smaller concentration of uranium, whether in the form of a single nuclear fuel rod, or in most uranium ore deposits, is not in danger of spontaneous fission. In nuclear reactors the chain reaction is controlled by slowing down neutrons and absorbing any in excess of those needed to keep the reaction going at the required rate.
Most operational nuclear reactors make use of the reaction in Equation 2. They are called burner reactors.
2.3.1 Burner reactors
The object of a burner reactor is to use up or 'burn' as much uranium-235 as possible. For some reactor designs the uranium-235 content of natural uranium is increased chemically to produce enriched uranium, which contains up to 3% uranium-235 (compared with the naturally occurring 0.7%). The chain reactions in Equations 1 to 3 produce neutrons with a wide range of energies. Fast neutrons are slowed down using a moderator — graphite, ordinary water or heavy water (deuterium oxide — deuterium is an isotope of hydrogen whose nucleus contains a neutron as well as a proton) — so that they can cause fission of uranium-235. The reaction rate is adjusted using control rods made of boron that absorb neutrons. These rods can be raised or lowered into the reactor core to increase or decrease the heat output.
The core coolant, either carbon dioxide gas or water depending on the reactor type, is heated by nuclear fission in the core.
The heat from the core coolant is transferred to a closed steam-water circuit, which includes the turbine plant.
The steam in the turbine circuit is condensed to water on the low-pressure side of the turbine by a cooling water circuit that is open to the environment.
The requirement for large volumes of cooling water means that nuclear reactors must be sited in coastal locations or near large rivers or lakes.
The main contrasts between the three main burner reactor types are:
Magnox reactors are fuelled by metallic uranium containing the natural proportion of uranium-235 (i.e. 0.7%) held in tubes of a magnesium alloy (Magnox). The moderator is graphite, the coolant is carbon dioxide and the operating temperature is about 400°C.
Advanced gas-cooled reactors (AGRs) are very similar to Magnox reactors (they have the same moderator and coolant), except that uranium oxide, enriched in uranium-235 (2.3% instead of 0.7%) and packed in stainless steel tubes, is used as a fuel. The operating temperature, 800°C, is higher than that in a Magnox reactor, leading to greater conversion efficiency and output by increasing steam pressure. Although the capital cost of building AGRs is very high, they have many additional safety features over Magnox reactors and, because of their greater efficiency, produce cheaper electricity.
Pressurised water reactors (PWRs) are the most widely used globally (Section 2.5). The fuel contained in zirconium alloy tubes is uranium oxide enriched in uranium-235, and pressurised water is both coolant and moderator. They operate at 300-400°C. The UK Sizewell 'B' plant is of this kind (Figure 1).
So, even the most enriched fuel in burner reactors still consists of 97.7% uranium-238. A very small proportion of this is converted into plutonium-239 (Equation 3), which is extracted from spent uranium fuel rods. But the bulk of the uranium in burner reactor fuel rods is never used. Indeed, the wastage is even greater than that: not all of the uranium-235 is 'burned' — as fission products build-up, they interfere with the chain reaction.
However, there is another way of extracting fission energy from the more abundant uranium-238.
2.3.2 Fast breeder reactors
If fast neutrons produced in the chain reactions are not moderated or absorbed, the rate of conversion of uranium-238 into plutonium-239 (Equation 3) can exceed the fission rate of plutonium-239. Reactors that use fast neutrons in this way are called fast breeder reactors.
Their main fuel is uranium-238, together with an initial charge of plutonium-239 which is needed to start the chain reaction that 'breeds' plutonium-239 from uranium-238. (This charge of plutonium-239 is obtained from the spent fuel rods of burner reactors.) The attraction of the fast-breeder concept is the excess plutonium-239 that they produce. About 60% of the fuel used in breeder reactors is converted into useful energy, compared with the 0.5-2% in burner reactors. Not only are breeder reactors more efficient, they generate more plutonium than they consume and can use uranium-238 in the waste from burner reactors.
Fast breeder reactors seem very attractive in theory, but only a handful have ever been constructed. In the UK (the Dounreay reactor), France and the United States all fast breeder programmes have been closed. Japan is considering restarting its own programme, which was closed after a fire in 1995, while India is currently building its first prototype fast breeder reactor (this is being designed to use thorium-232 as a nuclear fuel).
The dearth of fast breeder reactors testifies to formidable technological problems. The greatest of these is that the intense heat generated by the fast neutron chain reaction requires the use of liquid sodium metal as the coolant, rather than water or CO2 (Figure 5). The sodium coolant burns fiercely if any oxygen leaks in at the 600°C operating temperature. Concerns about fast breeder reactors also centre on their fuel, for three reasons:
the concentration of fissile material is greater than in a burner reactor — there is a greater chance of fission becoming uncontrollable;
the necessity for reprocessing nuclear waste (discussed in Box 5);
the central role (and production) of plutonium which is an essential component of nuclear weaponry.
Given this last concern, security has now become a more important issue if such material is not to fall into the hands of terrorists.
2.4 Fuel requirements for nuclear reactors
The fission energy of uranium-235 is 8.2 × 1013 J kg−1. Setting this figure in the context of fuelling a reactor depends on:
The thermal efficiency of the reactor — For modern burner reactors, this is similar to that of modern fossil fuel power stations, about 35% (i.e. 65% of the available energy is lost within the reactor-coolant-turbine system).
The power output — A typical value for a burner reactor is 1 GW. which corresponds to an annual energy output of 3.2 × 1016 J yr−1.
The lifetime of the reactor — This is about 30 years, i.e. about the same as for fossil fuel power stations.
The fuel requirements and products of burner reactors are summarised in Figure 6a. Fuel elements (rods) do not last for 30 years, and the total fuel requirement for a typical 1 GW burner reactor is about 4800 t of natural uranium over a 30-year period. Some 600 t of natural uranium are required as an initial fuelling charge (less if the fuel has been enriched in uranium-235), and an average of about 145 t yr−1 is used to replace spent fuel rods. Spent fuel rods consist of depleted uranium, which has lost most of its uranium-235 component and is mainly composed of uranium-238. About 2-3% comprises fission products from uranium- 235, and a further 1% is plutonium-239. These of course have to be disposed of somehow (Section 4.2). Depending on a 1 GW burner reactor's characteristics, up to 50 t of plutonium-239 will have been produced after 30 years. Yet both uranium-238 and plutonium-239 are potential fuels for a fast breeder reactor.
The fact that less than 5000 t of natural uranium (containing 0.7% uranium-235) is needed to fuel a 1 GW burner reactor over a 30-year lifetime shows the much higher 'energy density' of uranium relative to fossil fuels. A modern coal-fired power station of comparable output needs to burn something like 10 000 t of coal per day (Figure 7). The total volume of waste to be disposed of during the life of a power station is thus also much less for nuclear than for coal, though it is far more hazardous.
The comparison between fast breeder reactors and fossil fuels is yet more favourable, in terms of the required masses of fuel (Figure 6b). The fission energy of uranium-238 is about the same as that of uranium-235, but a breeder reactor uses about 60% of the fuel. Operating at 35% efficiency, a 1 GW fast breeder reactor would need 53 t of uranium-238 during its 30-year lifetime (Figure 6b), about 100 times less than the fuel requirement of an equivalent burner reactor, and potentially available from burner reactors' spent fuel. In 30 years, a 1 GW burner reactor produces enough uranium-238 in spent fuel rods to fuel around 90 breeder reactors with the same capacity. It also produces enough plutonium-239 (Equation 3) to charge 25 of them with the 2 t needed to start their nuclear chain reaction. Each breeder would also produce enough to charge three more with plutonium. So, in theory at least, a global breeder reactor programme could generate much more electricity than do existing burner reactors by using the stockpile of waste from them, without the need to mine uranium ore until some time in the future. Unlike burner reactors, which require continual refuelling during their active life, a successful breeder reactor requires little fuelling after its initial charge. Figure 7 illustrates the annual supply of mined energy resources required to service 1 GW power stations using coal, burner and breeder technologies.
2.5 The growth, decline and future of nuclear power
The Calder Hall Magnox reactor near Sellafield fired the UK's first commercial nuclear power station in 1956, and launched an early UK lead in global nuclear developments. By 1960 six commercial reactors were operating, and Magnox technology had been exported to Italy and Japan. The UK Magnox building programme was complete in 1971 with eleven stations, each producing between 245 MW and 840 MW. Figure 8 shows the distribution of different reactor types in the UK.
Advanced gas-cooled reactors (AGR) were designed to use enriched uranium fuel and higher operating temperatures, thus increasing power output efficiency from 30% to a maximum of 40%. After a long history of technical difficulties and delays, eight 1 GW AGR reactors, originally designed and planned in the early 1970s, were still in service in the UK during the early 21st century.
However, US-designed PWRs became by far the most attractive reactor design for nuclear power plants in other countries. Globally some 240 PWRs are operating successfully, including Sizewell B in the UK, leaving only the UK with AGR stations. But PWRs have not been without their problems (see Box 2). Despite their proliferation, concerns over the operational safety of PWRs dramatically slowed the global growth rate of nuclear power during the 1980s. Utility companies in the US had stopped ordering PWRs by 1978, even before the Three Mile Island incident, because of concern about the relative long-term economics of nuclear and fossil fuel stations. Similar worries in Europe were exacerbated by the catastrophic reactor accident in 1986 at Chernobyl in Ukraine (Section 4.1), and by 1990 global nuclear developments had virtually come to a halt everywhere except in France and Japan.
Box 2 Safety and pressurised water reactors (PWRs)
In the early 1970s, about 1% of manufactured zirconium-clad PWR fuel rods leaked, releasing fission product gases to the cooling water. However, by 1992 fuel fabrication had improved to such an extent that leaking rods were extremely rare.
In PWRs, fuel rods are packed into a much smaller reactor core than in AGRs: the fuel is about 40 times more densely packed. So if the pressurised cooling system were to fail, the reactor core would heat up to danger level much more rapidly than in an AGR. Fears about PWRs reached a peak following the Three Mile Island incident in Pennsylvania during 1979, where safety mechanisms failed to prevent a serious increase in the temperature and pressure of the reactor core, resulting in a leak of radioactive materials. The reactor developed a gas bubble in its core, increasing the danger of a further rise in temperature and ultimately melting of the containment vessel (Figure 4c). The accident report indicated that the damage arose from a combination of technical and operator failures that are unlikely to be repeated, rather than from fundamental design weaknesses, yet there is still considerable opposition to PWRs.
Concerns about the cost of nuclear power were also an important contributory factor in its decline; £15 billion had been spent on constructing nuclear power stations in the UK alone, and the price for decommissioning reactors and managing waste is still unknown. The sheer scale of the decommissioning problem can be illustrated by the nuclear industry's own argument that 'costs will be considerably reduced' if the closure and decommissioning timetable were spread over 135 years instead of 100 years, and if reactor cores and pressure vessels were not dismantled but buried instead. This is what was done at the Berkeley Magnox station in Gloucestershire (Figure 8), which closed in 1989. By 1998 the UK government confirmed these economic doubts with the view that 'at present nuclear power is too expensive to be economic for new capacity and in current circumstances it is unlikely that new proposals for building nuclear plants will come forward from commercial providers'. However, policies change with circumstances. Nuclear reactors provide a means of generating electricity without contributing directly to CO2 emissions, so their increased use is one possible response to combat climate change.
By the mid-1990s the future of nuclear power in the US and most of Western Europe (France excluded) was economically and politically uncertain, even though the industry itself was strong scientifically and its technology still improving. On the other hand, in France, Japan and other countries of the Pacific Rim (Table 1), brighter economic prospects were matched by a favourable political situation. In the former Soviet Republics and Eastern Europe, where scientific and technical developments had lagged somewhat behind those in the West, the economic and political picture was less clear. But nuclear power is potentially a very lucrative market, and by 1993 nuclear industries in several Western countries, faced with declining business at home, had begun competing vigorously to acquire a share of business in more favourable areas of the world.
Because of these uncertainties, forecasts of future requirements for nuclear generated electricity must be viewed with caution. The International Atomic Energy Agency (IAEA) suggested in 2003 that nuclear generating capacity globally will have grown from 362 GW in 2003 to about 400 GW by 2010, an approximate 11% increase in capacity. The IAEA view takes into account decommissioning of old power stations at the end of their lifetime. Notwithstanding such growth, the proportion of nuclear power against other energy sources is likely to remain static. With anticipated growth in global energy demand, IAEA predictions estimate a rise in total nuclear generating capacity to between 427 and 512 GW by 2020.
Whether these estimates are met, or even exceeded, will depend on factors such as:
the extent to which global demand for electricity continues to grow;
public perception of the dangers and costs of nuclear power generation;
whether governments perceive the threat of global warming as real enough to warrant serious attempts at curbing greenhouse gas emissions from fossil fuels;
the extent to which alternative energy sources can meet demand.
In any event, nuclear fuels will continue to be needed. After Question 2, we turn to how they can be supplied.
a.In burner reactors, a moderator is used to slow down the neutrons and the position of the control rods is continually adjusted to absorb excess neutrons. Why it is necessary (i) to slow down neutrons and (ii) to absorb the excess?
b.Figure 5 shows that there is no moderator in a fast breeder reactor. Why is this?
c.All uranium-based reactors breed plutonium but some do so faster than others: why is this?
d.Why does the uranium in ore deposits not all disappear by spontaneous fission?
a.In a burner reactor the chain reaction depends entirely on fission of uranium-235 nuclei, which capture only slow neutrons (Section 2.2). But fission of a uranium-235 nucleus produces both fast and slow neutrons. The fast neutrons must therefore be slowed down so that they can be captured by other uranium-235 nuclei. (ii) Although a large number of neutrons is required to keep the chain reaction going (uranium-235 constitutes only one atom in 40 even in enriched uranium fuel), their action must be controlled. An unlimited number of neutrons would cause the fission of all the uranium-235 at once, creating a nuclear explosion. Excess neutrons above a certain number must therefore be absorbed.
b.Fast breeder reactors depend on the ability of fast neutrons to transform uranium-238 to plutonium-239 (Equation 3), and to sustain the fission of plutonium. Because the moderator in burner reactors is designed to slow down fast neutrons produced in the chain reaction, it is not used in a breeder reactor.
c.In burner reactors, most of the fast neutrons are moderated or absorbed — but it is not possible to remove them all. Those that remain can be captured by uranium-238, which forms the bulk of the fuel elements in all reactors, and plutonium is formed. The breeder reactor is expressly designed to produce plutonium, because it depends on the action of fast neutrons only, which turn uranium-238 into plutonium-239.
d.Even in rich ore deposits, the uranium is 'diluted' by other elements in the minerals of the ore and surrounding rock. Nearly all neutrons are absorbed by surrounding rocks, which effectively act as 'control rods'.
3 The geological occurrence and extraction of uranium
Just how readily available are uranium resources, and do their distribution and cost impose restrictions on nuclear power generation? Compared to a coal-fired power station a nuclear power station requires far less fuel in terms of mass. You have seen that a 1 GW burner reactor requires 5000 t of natural uranium over 30 years, whereas a comparable modern coal-fired power station needs 10 000 t of coal every day. However, uranium does not occur naturally in metallic form, nor in the concentrations required for reactor fuel. Its average abundance is only about 3 parts per million (ppm) by mass in the continental crust — equivalent to just 0.4 cm3 of pure uranium dispersed evenly through a whole cubic metre of granitic rock. Uranium occurs at concentrations up to 100-10 000 ppm in minor minerals in granites, such as zircon (ZrSiO4), apatite (Ca5 (PO4)3(OH)) and titanite (CaTiSiO5). For mining and extraction of uranium to be economic, it must occur at much higher abundances than this.
A typical ore grade for uranium is about 0.3% or 3000 ppm in rock. By how much must uranium be concentrated above its average abundance in continental crust to form an ore of this grade?
The amount of an ore containing 0.3% uranium that must be mined to produce fuel for the 30-year lifetime of a 1 GW burner reactor is 1.7 million tonnes (i.e. 5000 t/0.003 = 1.7 ×106 t). Nevertheless, even that huge quantity is only half the mass of a year's supply of coal to a modern power station.
3.2 Uranium occurrence and ore deposits
In igneous rocks, uranium is more abundant in granites (~3.5 ppm) than in basalts (~1 ppm). The large size of the uranium atom prevents it from easily entering the structures of common rock-forming minerals, so it is an incompatible element that tends to remain in magmas until a late stage of crystallisation, when it enters minor minerals, or even the uranium oxide, uraninite (UO2). In suitable circumstances, following fractional crystallisation of uranium-rich granitic magma, uraninite may be sufficiently abundant in pegmatite sheets and veins to form a disseminated magmatic uranium deposit. Alternatively, when rocks rich in uranium become hot enough to start melting, uranium, being an incompatible element, preferentially enters the melt. If only a small amount of melting occurs, and if such melts crystallise as pegmatite veins before further melting takes place, they may be particularly rich in uranium. One such occurrence is at Rossing in Namibia, where a relatively low grade of ore (~0.03% uranium) has nevertheless been economic to mine because of the huge size of the deposit and the advantages associated with economies of scale. It is said that a year's production of uranium from Rossing could meet the UK's entire power needs for eight years.
Richer occurrences of uranium result from its solubility in water, in particular, hydrothermal fluids — hot, usually saline groundwater that circulates within the Earth's crust. Hydrothermal fluids may circulate through many cubic kilometres of crust for thousands of years, being driven by heat from deep within the crust. The hot water rises through joints, fractures and permeable rocks, drawing in cooler water as part of a convection system. Hydrothermal fluids may dissolve certain components of rocks in one environment only to deposit them in another, as a result of chemical processes within the fluid or its reaction with surrounding rocks. For example, hydrothermal fluids that flow through a granite may dissolve dispersed uranium, migrate until a suitable change in solution chemistry occurs and then deposit and concentrate uranium over a long period. The key to understanding the behaviour of uranium in aqueous solutions is that uranium in its oxidised U(VI) state forms soluble ions (e.g. uranyl, ), but in its reduced U(IV) state it tends to be insoluble, when it precipitates uranium minerals, such as uraninite, in hydrothermal vein deposits.
Some of the world's richest uranium occurrences were formed by hydrothermal processes. Many occur at or below unconformities that are cut by faults (Figure 9), as in the Athabasca Basin of Saskatchewan, Canada, where numerous unconformity related uranium deposits have been discovered. These include the MacArthur River (~24% uranium) and Cigar Lake (~19% uranium) deposits, which are probably the richest in the world. Whether the source of the uranium in such settings is fractured metamorphic basement rocks beneath the unconformity or as a result of 'leaking' from the overlying sediments down faults is uncertain. Whichever, the uranium deposition was clearly associated with a change in oxidation state of the fluid, when large quantities of oxidising fluids containing soluble uranium ions became reduced and precipitated uraninite.
Similar oxidation-reduction reactions are responsible for uranium deposits in quite different geological settings in the Wyoming, South Dakota and Colorado Plateau areas of the western US. These are sandstone-hosted uranium deposits. One form of these is the 'roll-front' deposit, which typically comprises crescent-shaped zones of uranium ore minerals (Figure 10) in sandstones. They formed where oxidising groundwater containing soluble uranium became reduced by carbonaceous fossil plant material in the permeable sandstone. Locally, whole tree trunks have been replaced by the mineralising solutions. The uranium in the groundwater is derived by weathering of uranium-bearing rocks, such as granites, in upland areas. It is then transported as dissolved ions in oxygenated surface water, which infiltrates and migrates through permeable sandstone strata. Where these oxidising waters come into contact with carbonaceous material they are reduced, and deposition of insoluble uranium minerals (along with vanadium and copper) starts.
Continued flow of oxidising groundwater dissolves uranium minerals from the upflow side of the deposit and redeposits uranium under reducing conditions on the downflow side, thus developing the arcuate form of the deposit (Figure 10c).
Some important uranium deposits consist of sedimentary grains of uranium-bearing minerals in quartz-pebble conglomerates, such as those of the Witwatersrand Basin in South Africa and Blind River, Elliott Lake in southern Canada. These conglomerates are unusual. Not only do they contain rounded grains of uranium-bearing minerals, principally uraninite, that were deposited in river gravels in exactly the same form as when they were originally eroded, but also sedimentary grains of gold and pyrite.
What is unusual about the occurrence of sedimentary uraninite and pyrite grains?
Oxygenated surface waters tend to dissolve uranium: weathering today rapidly breaks down uraninite (and pyrite), and ions of soluble uranium (and iron) are carried away in solution.
Under what conditions could surface waters transport uraninite grains?
Only if the surface water and the Earth's atmosphere were non-oxidising.
These conglomerates were deposited more than 2000 million years ago, and there is abundant geological evidence for the Earth's atmosphere containing very low oxygen until about that time. Consequently, uranium minerals were not broken down by weathering as readily as they are today. So, rapidly flowing rivers derived from granitic rocks could have transported and deposited high-density uraninite, to concentrate it in sands and gravels where river energy slackened. Many of these deposits contain very low concentrations of uraninite but are economic to mine because of the gold they contain.
There are many other different types of uranium mineralisation in addition to those mentioned here, so it is no surprise to learn that uranium ore deposits are well distributed globally (Figure 11).
The relative importance of these different kinds of uranium ore deposit in terms of the typical grade and size of deposit is summarised in Table 2.
3.3 Uranium production and economics
Table 3 lists the major uranium-producing countries. Currently, Canada (with 29% of global supply in 2003) is the world's largest producer of uranium, followed by Australia (21%), both having increased production since about 1980, whereas production from the USA, France, and South Africa has declined (Figure 12). The largest single producer is Canada's high grade McArthur River deposit (16.3% of global production in 2003) followed by Australia's Ranger mine (12%).
Uranium is mined at the surface (mostly lower grade, sandstone-hosted and disseminated deposits), in deep underground mines (mostly higher grade deposits, especially unconformity-related) and by in situ leaching (ISL) or solution mining (see Box 3). In 2003 about 50% of global production was mined underground, 30% at the surface and 20% by ISL. These proportions vary depending on the economics of mining and the types of deposit available for exploitation. In recent years some particularly high-grade deposits have been discovered in Canada, e.g. McArthur River and Cigar Lake, where underground mining uses remote methods which help minimise hazards for workers. Elsewhere, it is still economic to mine very low-grade deposits at the surface (e.g. Rossing) or by ISL (e.g. Inkai, Kazakhstan).
Where ore is extracted by mining, it is first crushed and much of the worthless rock separated, then the uranium is leached into solution. This process leaves a massive amount of waste rock which must be disposed of. The solution is then
Box 3 In situ leaching (ISL)
In recent years there has been a growing trend to extract uranium by ISL. It can be used when uranium deposits are below ground in a layer of permeable rock that is confined by impermeable layers (Figure 13). The procedure involves pumping a liquid (such as sulphuric acid) that dissolves (leaches out) uranium down boreholes into the deposit and extracting the resultant uranium-bearing solution (Figure 13), which is then processed to extract the uranium. ISL has several advantages over conventional mining:
it is less hazardous for employees;
it is cheaper;
there is no solid waste for disposal; but it also has disadvantages:
there is a risk of groundwater contamination by leaching solutions;
there is a need to store waste sludge and water after uranium recovery;
it is not possible to restore natural conditions to the leaching zone after operations have finished.
ISL has been used extensively in the USA (93% of US uranium production in 1996) and in eastern Europe, the Russian Federation and Australia. It is a cheap method of mining, appropriate for low-grade ores hosted in permeable rocks, particularly sandstone-hosted deposits.
chemically processed by passing it through ion-exchange columns that selectively extract uranium, then re-dissolving the uraniumin another solution under different chemical conditions. This purified solution is treated with ammonia, which precipitates uranium as an ammonium uranyl salt. This compound is dried and then heated in air to break it down to a mixture of uranium oxides (approximating to U3O8), known as 'yellowcake' (Figure 14).
Solid waste from either underground or surface mines (generally called tailings) always contains traces of the minerals being worked, because mineral processing never achieves complete separation. The amount of waste produced by conventional mining of low-grade uranium ores is prodigious. Processing an economically profitable ore containing 0.5% uranium to yield 1 t of uranium creates about 200 t of waste, which contain traces of uranium and possibly other radioactive and toxic elements. Although tailings may contain only 5-10% of the original uranium in the ore, up to 85% of the radioactivity emitted by this waste is from the radioactive gas, radon (see Box 4). So, mining and processing uranium ore transfers material from a relatively safe underground location to deposit it at the surface, where hazardous constituents are more readily dispersed in the environment.
As with any other form of mineral extraction (Webb, 2006), processing uranium ore requires massive amounts of water. Eventually, this water carries generally fine wastes from ore crushing and separation operations as slurry to settling ponds (called tailing ponds; Figure 15). Contamination from tailings can spread over large areas as wind-blown dust, slurry can be eroded and transported by water, and water can seep into groundwater or surface streams: uranium is highly soluble under surface oxidising conditions. After the collapse of the Soviet Union and former Eastern Bloc countries, it emerged that uranium mining areas in these countries had higher than normal incidence of cancers, particularly lung cancer and childhood leukaemia, as well as various respiratory diseases. Lax safety standards are thought to have led to similar consequences in other uranium mining areas (e.g. Australia, India, Namibia), but this is a nemotive issue and allegations are not easy to prove. Nowadays, compliance with legal requirements has ensured that more mining companies have safety controls in place. Long-term isolation of wastes is a priority, and sites are engineered in a similar fashion to landfill sites (Argles, 2005), with impermeable liners at the base and as a cover. In the USA uranium mining wastes have even been relocated at great expense to make them safe.
Box 4 Radon hazards
Radon is a highly radioactive gas produced from the uranium decay chain. Although radon-222 has a short half-life of only 3.8 days, it disintegrates to form alpha particles and solid radioactive daughter products (e.g. polonium). Once inhaled these radioactive particles can lodge permanently in the lungs, causing damage to lung tissue and long-term disease. Risks to miners are now minimised by the use of powerful ventilation but they are not eliminated.
Radon is not only a hazard to miners. Release of mining waste into the environment creates a long-term source of radon, as its immediate parent, radium-226, has a half-life of 1600 years and its parent in turn, thorium-230, has a half-lif eof 80†000 years. Although ventilation reduces the hazard locally, as in buildings where radon may collect having emerged from uranium-bearing rocks beneath their foundations, the spread of radon is a hazard to the population at large. Although the risk from small doses of radon may be very small, the effect over time on a large population can be significant.
Radon is emitted by uranium decay from any rocks, particularly those that contain higher than normal uranium concentrations, i.e. some granites, limestones and shales, and even coals. Housing with foundations on even moderately uranium-rich rocks can accumulate radon in their foundations or cellars. UK building regulations now require assessment of radon risk from this source, and protective measures for any new constructions in radon-prone areas.
Underground mining is a high-cost operation, as shafts have to be sunk and tunnels drilled in order to access ore and install equipment below ground, and the ore has to be raised to the surface. Surface mining in contrast is a lower cost operation, with the ore directly accessible and often mined by high capacity equipment, which gains enormous advantages from economies of scale. With significant cost differentials you might wonder why underground uranium mines continue to operate.
What single factor can make some underground mines more profitable than surface mines?
Ore grade. A higher concentration of an element in ore requires less ore to be mined and processed to produce the same quantity of the saleable commodity. This offsets the high costs of underground mining.
So underground mining is generally only viable for high-grade ores, but open-pit mining can be used for low-grade ores that are accessible at the surface. Some low-grade ores, in suitable locations and in suitabl erocks, can be extracted underground by ISL (Box 3) at significantly lowe rcost.
A minimum economic grade of 0.3% uranium is usual for open-pit mining, but there are exceptions. The Rossing mine in Namibia continues to operate despit ea grade of only 0.03%. That mining continues there partly reflects economies of scale associated with the huge size of the deposit. Because the infrastructure is already in place from a period of very high prices for uranium, the true cost of continued operation is much reduced. However, long-term contracts for supply from the Rossing mine were agreed when prices were higher than US$25 lb−1. Note that the uranium market quotes uranium prices quaintly in US$ lb−1 for 'yellowcake' (as U3O8 equivalent). At prices lower than US$25 lb−1, few low-grade open-pit mines are viable.
Figure 16 gives uranium prices, nuclear reactor requirements and production statistics for almost six decades, during which attitudes to nuclear power have swung from one extreme to the other. Uranium provides an excellent example of how prices and markets influence resource industries, in ways that have nothing to do with geology.
Prior to the early 1970s the US government was the dominant customer for uranium in the Western World (partly for nuclear weapons), and effectively controlled prices. As demand for nuclear power increased during the 1970s, commercial demand for uranium grew. At the same time US government policy created an artificially high demand. Prices rose to a peak of around US$40 lb−1 in the late 1970s (equivalent, in terms of the decreased buying power of the US dollar, to US$105 lb−1 in 2004). That stimulated exploration and enabled exploitation of relatively low ore grades. Although demand continues to rise, prices and production dropped steadily through the 1980s and into the 1990swhen prices flattened to around US$10-15 lb−1. There were a number of reasons for this seemingly extraordinary situation. The US government first relaxed its contractual policies, then, with the Cold War coming to an end, stockpiles of uranium for military use were no longer needed and were released to the market. In addition, growth of nuclear power installations had slowed as environmental concerns grew. Thus demand for mined uranium fell markedly whilst military and other stockpiles were reduced.
Globally, some 440 nuclear reactors produced about 350 GW of electricity in 2003 (Table 1). This required an annual supply of about 67 000 t of uranium. Mined uranium amounts to only about 36 000 t globally. The remainder comes from recycling spent fuel, from stockpiles created while prices were low and from the use of ex-military weapons-grade enriched uranium (one tonne of which makes at least 30 t of reactor-grade uranium). Reduction of stockpiles and restrictions on the amount of military uranium released will eventually result in a resurgence of mined uranium to satisfy demand. Prices of uranium during 2003 started to rise in response to this (Figure 16a). Renewed interest in nuclear power as a means to check carbon dioxide emissions will undoubtedly spur further rises in the price of uranium. By mid-2005 it had risen to over US$29 lb−1, but still much lower than in the early 1980s, taking into account the decreasing buying power of the dollar. If there is growth in nuclear reactor development, demand for uranium may well increase further, further driving up the world price. Price rises may eventually make more marginal deposits economic again. However, satisfying a rise in demand would not be immediate: lead times for any mine development can be as much as 15 years or more because of protracted planning for mine development and compliance with environmental legislation.
Is there enough uranium to satisfy likely future demands? Quantities believed to exist in an economically recoverable form are listed in Table 4.
What is the lifetime of the global reserves shown in Table 4, i.e. their R/P ratio, assuming a 2003 production of 3.6 × 104 t?
Reserves/annual production = 3.5×106 t/3.6 × 104 t yr−1 = 98 years.
Global reserves would thus last for almost a century at constant 2003 production. However, if the whole of the current reactor demand for uranium (6.7 × 104 t yr−1 in 2003) had to be mined, the reserves would last only half that time. For the foreseeable future there are substantial reserves of uranium to supply global nuclear power needs, even if demand rose considerably. If prices rose, even greater quantities would be available, as this would encourage exploration. The IAEA has estimated that there might be as much as 14.4 × 106 t uranium in conventional resources (Table 2), enough for 200 years. If fast-breeder technology (Section 2.3) became commercially viable, however, requirements for newly mined uranium would actually decrease: fast breeders use mostly spent fuel from burner reactors, and the world is awash with this otherwise waste material.
But all this is highly speculative. The future is uncertain because every country that produces and/or consumes nuclear fuels will make its own political judgements about the competing technical, economic and environmental factors that affect nuclear power generation.
Look at Figure 16 and answer the following:
a.Significant uranium production began in the 1950s, yet requirements for power didn't take off until the 1970s. How do you explain this apparent disparity?
b.How long was the time lag between price rises and increased production during the mid- to late-1970s and how do you explain it? Note: Refer to the price curve in terms of US$ in 2004.
c.What was the reason for the fall in price in the 1980s?
d.Explain the rise in price in the early 2000s.
a.Almost all supplies from the late 1940s to the late 1960s were for military use. Indeed much of production until the mid- to late-1980s went into military stockpiles.
b.Prices rose sharply from $50 kg−1 to almost $300 kg−1 (in terms of the US$ in 2004) in 1974-75 (peaking in 1979), while production took 5-6 years to reach its peak in 1980-81. The lag represents the lead time required to expand production at existing mines and to bring new mines on stream.
c.Uranium prices fell in the 1980s (from over $250 kg−1 to about $30 kg−1), partly because the anticipated rate of growth in nuclear power did not happen and plans for new power stations were cancelled; partly because artificial influences on the market by the US government had ceased and the end to the Cold War cut demand for military uranium and indeed, by the 1990s, stockpiles were starting to be run down.
d.Uranium prices rose from 2003 to 2004, partly because stockpiles and ex-military supplies were running down, and partly because the future for nuclear energy was looking more promising as a means to satisfy increasing energy requirements without increasing CO 2emissions.
Decide, giving reasons, whether each of the following statements about uranium ore deposits and their formation is true or false.
a.The charge and size of uranium ions are such that uranium does not fit readily into the crystal structures of common rock-forming minerals.
b.Mineralised fault zones, particularly those at and beneath erosional unconformities, provide some of the world's richest uranium deposits.
c.Uranium deposits in near-surface sandstones require higher grades for economic mining than underground vein-type ores because the former are more expensive to mine than the latter.
d.Roll-front type uranium ores in sandstones form at an oxidation-reduction boundary where uranium carried in solution in groundwater is precipitated on conversion into insoluble uranium minerals.
e.Yellowcake is an ammonium uranyl salt produced after ion-exchange treatment to purify the leachates of uranium ores.
f.Uranium minerals may be precipitated from surface waters in the anaerobic reducing environments of lagoons and coastal swamps, so that coal and petroleum source rocks have high uranium contents.
a.True. See discussion of incompatible elements.
b.True. See comments on hydrothermal vein and unconformity deposits.
c.False. Underground mining of veins generally will be more expensive in energy and staff costs than surface mining of sandstone-hosted uranium.
d.True. See comments on roll-front type uranium ores.
e.False. Yellowcake is impure uranium trioxide.
f.True. In its reduced form, uranium is insoluble.
4 Side-effects of the nuclear power industry
Nuclear power generation involves concentrated fissionable fuels which, after fission, leave significant quantities of fission-product isotopes, some of which are highly radioactive. Much of the criticism levelled against the industry falls under four main headings to which we have alluded in preceding sections:
the operational safety of nuclear reactors;
the biological effects of abnormal radiation levels arising from fuel transport, processing and reprocessing;
disposal of radioactive waste;
the increased potential for proliferation of nuclear weapons.
The implications of item 4 are beyond the scope of this course, but you should note that so much uranium and plutonium is easily available nowadays that those wishing to purchase materials for nuclear weapons would have little difficulty in doing so. For example, the smuggling of plutonium or enriched uranium from the former Soviet Union became public knowledge in the mid-1990s. Only small amounts are required to produce the high concentrations of uranium-235 or plutonium-239 needed for a nuclear fission bomb, and it is almost impossible to guarantee effective international control over nuclear weapons — despite the existence of the UN's Non-Proliferation Treaty. Such was the concern, that the development of nuclear technologies in Iraq was cited by UK and US Administrations as one reason for going to war in 2003 (although no evidence for such activity was uncovered). With Iran resuming its nuclear development programme and North Korea engaged in similar activity, diplomatic tensions between these countries and the US seemed likely to increase at the time of writing (late 2005).
4.2 Reactor safety: the Chernobyl incident
By far the worst nuclear reactor accident took place on 26 April 1986 when one of four 1 GW reactors at Chernobyl in the Ukraine released a radioactive cloud over Europe (Figure 17). (See S278 video clips document.) The build-up to this accident has been related to a series of complex chemical reactions induced by operator errors during preparation for tests on the reactor — a kind of PWR but with a graphite moderator, normally operating at 700°C. It appears that the tests required operation at 20-25% of full power but that as the power was being reduced, an order was issued to delay the tests because of unexpected electricity demand. This series of events led to the control rods being raised, leaving in the reactor core less than the minimum number specified in the operating instructions. Once the test did proceed, the reactor was extremely difficult to control in this state and it should have shut down automatically. But again the operators intervened, retaining manual control, and as the power output fell, the reactor core started to heat up rapidly. Once it was realised that an accident was imminent, the control rods could not be replaced fast enough. One opinion is that instability in the riverine sediments beneath the reactor complex had led to structural distortion in the plant and it was this that led to the control rods becoming jammed. Both the temperature and power output of the plant rose critically, leading to a massive explosion as molten uranium reacted with the cooling water. The explosion dislodged the 2000-tonne reactor cap, a fire started that took ten days to bring under control, and massive amounts of radioactive fission products were carried in a gas plume high into the atmosphere where they were dispersed by the wind.
The local effects of radiation were severe, with 31 fatalities and 200 incidences of radiation sickness among those working to bring the site under control. Some 130 000 people were evacuated and there were widespread restrictions on the use of fresh foods. After a week, radioactivity had spread across much of Europe (Figure 17). Parts of the UK were subjected to heavy rainfall when the dispersion cloud was overhead, leading to contamination of agricultural land and moorland, particularly by radioactive caesium. Enhanced caesium levels, though below the danger threshold, were still being found in sheep in the mid-1990s in parts of north-west Britain. But the major legacy of this accident will be felt in the Ukraine and in other areas close to Chernobyl where high radiation doses have increased the risk of cancer deaths among millions of people. To be fair, the statistical probability of death by cancer has probably been raised by just a few per cent among the great majority of these people, many of whom will live for several decades. So it is unlikely that the true death toll from Chernobyl will ever be known, though estimates in the tens of thousands are not unreasonable.
Chernobyl highlighted important lessons about reactor safety procedures, about the release and dispersion of radioactivity when a reactor gets out of control, and about long-term clean-up operations which continued into the early 21st century. In particular, risk assessment studies have been improved, and in the UK, for example, where it is claimed that a Chernobyl-like accident could not occur, efforts have been redoubled to assess the risk of seismic disturbance at reactor sites. Such assessments recognise that however well we control the chance of failure due to human error, nuclear reactors can never be totally immune from natural hazards. The Indian Ocean tsunami of 26 December 2004, for example, flooded the construction site of the prototype Kalpakkam fast breeder reactor in India — located only 150 m from the sea. With over 400 reactors operating globally, many of which are located in coastal areas, these are risks that we may have to live with.
4.3 Nuclear fuel transport and processing
Most radioactive materials connected with the UK nuclear power industry are transported by rail in massive transport flasks which have been shown in tests to survive high-speed impacts without fracturing. In fact, it is the highly radioactive spent nuclear fuels being transported to reprocessing sites that constitute the greatest danger.
Power stations unavoidably discharge radioactive gases to the atmosphere, but these escapes are sufficiently dilute not to be regarded as clinically hazardous, although the existence of a link between such emissions and the presence of cancer 'hotspots' is frequently claimed. Spent fuel rods are a far more serious problem.
What are the main chemical changes that occur during the active lifetime of a fuel rod in a burner reactor?
The total uranium content decreases as fission products are generated, mainly from uranium-235 (Equation 2). Thus the uranium becomes depleted (Section 2.4) and, in addition, small amounts of plutonium are formed (Figure 6).
In practice fuel rods are removed before all the fissile uranium has been used up because some of the accumulating fission products impede the fission reaction efficiency (as noted in Section 2.3). The fission products are highly radioactive, and the result is an overall radioactivity typically about eight orders of magnitude higher than in the fresh fuel — hence the potential danger that spent fuel rods present if exposed to the environment. Many countries with a nuclear power capability simply store the spent fuel rods on site with other materials contaminated by radioactive isotopes. However, several countries, notably France and the UK, have developed nuclear waste reprocessing facilities, to which spent nuclear fuel rods are transported. At Sellafield (Cumbria), the state-owned company British Nuclear Fuels Ltd (BNFL) reprocesses depleted spent fuel to recover uranium and plutonium isotopes which can be converted into a new generation of fuel rods (Box 5 overleaf.
As with nuclear power stations themselves, nuclear fuel transport and processing present risks of accidental harm to human populations and the environment as a whole. Table 5 puts into perspective global radioactivity emissions, illustrating that under normal operating conditions the nuclear industry as a whole contributes a very small fraction above natural background radiation (mainly cosmic radiation and that emitted by natural radioactivity in rocks). The problem is that radiation leaks around nuclear facilities may be highly concentrated, making the data in Table 5 deceptive. For example, there is evidence of above-average incidences of childhood leukaemia at Sellafield. Exposure of the children's fathers to radiation at the pre-conception stage is one suggested explanation, but this has been disputed, and other studies have found occurrences of childhood leukaemia at other sites far from nuclear facilities. Another accusation levelled at Sellafield is the discharge and widespread dispersion of liquid wastes containing low levels of radioactivity into the Irish Sea through pipelines 2.5 km long. The early wastes discharged contained some particulate plutonium which, following the intervention of a Royal Commission on Environmental Protection, is now removed before discharge. Radioactive discharges are now at a minute fraction of their original levels. But this opens up the whole question of radioactive waste containment and disposal, which you will examine in the next section.
Box 5 The Sellafield reprocessing facility (THORP)
Formerly known as Windscale, the plant at Sellafield was created in the 1950s to produce plutonium for nuclear weapons. As a secure centre of nuclear engineering expertise, Sellafield became the obvious place to store and reprocess spent uranium fuel rods from UK reactors, initially to retrieve plutonium for the fast breeder programme at Dounreay. In reality, little of this reclaimed plutonium has been used. Although some has been traded with the US, stockpiles of both plutonium and radioactive waste in the UK have grown since the early 1960s.
With expansion of nuclear reactors globally it became clear that large quantities of spent oxide fuels would be produced. After a 100-day public enquiry, plans were approved in 1977 for a new, commercial reprocessing plant at Sellafield, known as THORP (THermal Oxide Reprocessing Plant). The principal object of THORP is to concentrate the most radioactive fission products into a small volume of borosilicate glass for disposal.
THORP was designed to cater for needs beyond those of the UK nuclear programme. Advance payments by contracted customers for storage and reprocessing funded its total cost — £2.8 billion over a 15-year building and development programme. The main customers were Japanese and German nuclear generating companies which, throughout the 1980s, sent their spent fuel to Sellafield. Not surprisingly, this led to accusations that the UK was becoming a nuclear dustbin.
The other objective of THORP was to recycle uranium in the face of an anticipated shortfall in supplies.
How reasonable was this concern at the time when THORP was first mooted in the mid-1970s?
Eminently so, reactor requirements for uranium were rising rapidly during the 1970s (Figure 16) and at the time of the public inquiry.
By the early 1990s the uncertain future of the nuclear power industry, and increases in reserves and fuel stockpiles combined to make the reprocessing of spent fuel an uneconomic proposition. With the shutting down of the UK's fast breeder programme, one of the principal reasons for THORP's existence — to produce plutonium for fast breeders — no longer existed. Moreover, a report from the pressure group Greenpeace, commenting that adopting long-term storage rather than a 'reprocessing and disposal' strategy would reduce nuclear costs, influenced Scottish Nuclear to relinquish its option on reprocessing by BNFL. Similar questions about the long-term economics of reprocessing spent fuel at THORP were raised in Germany and Japan, two of the largest potential customers. Japan in particular already had a significant stockpile of plutonium (about 5 t in storage plus another 5 t at THORP and at reprocessing facilities in France) in anticipation of reprocessing, and was planning to build its own facility. There were also strong representations from members of the US Congress that the project should be halted, on environmental grounds and because of the increased potential danger of nuclear proliferation.
The UK Government finally gave approval for THORP to begin reprocessing in March 1994. Yet there was no guarantee that it would be profitable, given the global surpluses of uranium and plutonium, and continued opposition by environmental groups. Local release of radioactive krypton-85 gas from the plant and other leaks including plutonium dust continued to raise concern. By 2004 the THORP facility, dogged by technical problems, had reprocessed only 5000 of an anticipated 7000 t of spent nuclear fuel. Sellafield provides an example of how long lead times can result in major projects becoming less profitable (and even less socially acceptable) than anticipated when they were first proposed.
4.4 Radioactive waste disposal
Most fission products from nuclear reactors are solid at ordinary temperatures. They cluster around atomic mass numbers 90 and 140 (see, for example, Equation 2). From the point of view of waste disposal, the problem is that most of them are highly radioactive. The common radioactive isotopes produced in nuclear reactors are given in Table 6. The shorter the half-life of these fission products, generally, the more intense their radioactivity.
Besides these fission products is a range of actinide isotopes that do not occur naturally. These are similar in mass and atomic number to uranium, and produced from uranium by neutron absorption and electron emission, plutonium being the most widely known of them. They too are highly radioactive and some are extremely toxic chemically.
From the quantities of each radioactive product and its half-life, a curve that shows overall radioactivity decay can be constructed, which represents the gradual conversion of these materials to non-radioactive isotopes.
Figure 18 shows changes in the heat output — a measure of the level of radioactivity —from radioactive decay of the fission products and actinide isotopes produced in spent fuel from a 1 GW advanced gas-cooled reactor. Note that the axes of the graph are logarithmic. After 100-600 years there is a large fall in the heat production, and therefore radioactivity, from decaying actinide elements (from ~10 to 0.001 kW kg−1). Thereafter, longer-lived fission products (cerium-142, zirconium-93 and caesium-135) dominate heat output, but at levels that decline from about 1 kW kg−1. Yet it is widely agreed that this waste should not be allowed to leak into the biosphere for at least a thousand years. Ideally, it should be isolated for between 104and 105 years. Moreover, it is vital to cool the waste in well-shielded storage for the first few decades until some of the short-lived isotopes have decayed. Spent fuel rods that contain products of nuclear reactions are much more dangerous than new fuel rods, which contain only uranium isotopes with very long half-lives.
Radioactive wastes produced by the nuclear industry, together with those from other sources, are subdivided for disposal purposes into those with low, intermediate and high levels of radioactivity.
Low-level wastes (LLW) include several materials. Gases with half-lives of a few years at most are mainly isotopes of hydrogen, argon, krypton, xenon and radon, which are vented to and diluted by the atmosphere. Liquids, produced during waste treatment, are discharged into the sea or rivers. Solids that include worn-out equipment, crushed glassware, protective clothing, air filters, etc. are burnt in incinerators or buried at purpose-built sites, such as Drigg in Cumbria (Figure 8). Some 40 000 m3 of low-level wastes are produced annually in the UK, but their disposal is not regarded as a major environmental problem. For example, it is often pointed out that some natural substances, such as brazil nuts and coffee beans, are sufficiently radioactive to be classified as low-level waste.
Intermediate-level wastes (ILW) have higher activities and so require more elaborate storage. They include solid and liquid materials from power stations, such as fuel cladding cans, and wastes from the radioisotope industry (e.g. hospital radiography departments) and defence establishments. In total, some 5000 m3 are produced annually in the UK.
High-level wastes (HLW) are the concentrated products of nuclear fuel reprocessing, containing over 95% of the total radioactivity from the nuclear industry's waste products. At present HLW is stored in a low-density liquid solution, of which there is about 1500 m3 at Sellafield, where it is contained by double-skinned, stainless steel tanks surrounded by concrete and cooled by water circulating through sets of stainless-steel coils. It is this waste that may ultimately be incorporated in glass (Box 5), for containment (Figure 19) prior to disposal.
High-level wastes do not occupy large volumes; Figure 6a shows that 4800 t of nuclear fuel for a typical 1 GW burner reactor end up as 4650 t of spent fuel after 30 years, having produced 150 t of HLW. At an average density of 10 t m−3, the total volume of HLW produced after 30 years is 150 t/10 t m−3 = 15 m3, the space occupied by a few filing cabinets. Even though these fission products remain extremely radioactive for centuries (Figure 18), storage and disposal of HLW might seem relatively unproblematic. The central issue is where they are to be kept. Several proposals have been made, one being that they could be lowered into deep-ocean trenches at destructive plate margins, to be covered naturally by sediment and eventually subducted into the mantle. But such areas are seismically active, and at subduction rates of centimetres per year it would take tens of thousand years for them to be subducted to a depth of 1 km; clearly a risky proposition. A more rational means of disposal would be in abandoned deep mines or specially constructed boreholes.
4.5 Geological criteria for safe radioactive waste disposal
Even in the best of circumstances, containers such as the one shown in Figure 19 will survive for only 100-1000 years, although the glass itself may inhibit the migration of radioactive isotopes for a further 1000 years. So, in view of the long decay times (Figure 18), the ideal geological site for waste disposal should also act as an impermeable barrier to any leakage. On land, the prime geological contenders for waste containment in the UK are unfractured clay-rich rocks, salt deposits, and hard crystalline igneous or metamorphic rocks. Salt and some clay deposits have the advantage of being self-sealing, owing to their high plasticity. Clay minerals also have a high capacity for ion adsorption (i.e. ions adhere to their surfaces). Crystalline rocks are generally dry, stable against tectonic movement, extensive and virtually inert to any heating effect from waste canisters.
A very important aspect of nuclear waste disposal is the choice of what form the disposed material should take. THORP is designed to produce borosilicate glass, but all glasses decompose over time and are relatively reactive. It would be better to create artificial minerals, such as apatite or zircon, that would mop up just as much material and are stable unless they are melted at very high temperatures (>700°C).
Taking three groups of impermeable rocks in turn — clay-rich sedimentary rocks, salt deposits (see Argles, 2005) and crystalline rocks — and using Figure 20, suggest which might be the best areas for radioactive waste disposal to be located in the UK.
Clay-rich sedimentary rocks occur in the Mesozoic- Tertiary rocks of Dorset, Hampshire, London and the Yorkshire-Lincolnshire-Home Counties areas, and as mudstones and shales among the Palaeozoic rocks of central Wales, the Lake District, the southern uplands of Scotland, and Northern Ireland.
Appropriate salt deposits of Permo-Triassic age underlie the Midlands, particularly the Cheshire area, Dorset and the North Sea (Argles, 2005).
Palaeozoic and Precambrian crystalline igneous and metamorphic rocks are generally found in the upland regions of Wales, Northern Ireland, northern England and Scotland north of the Midland Valley.
In 1997, the proposal by the UK nuclear waste company Nirex to build an underground repository for HLW at Sellafield failed to obtain government approval after a five-month public enquiry. After rejecting the application the then Secretary of State for the Environment, John Gummer, justified the refusal by saying that he was 'concerned about the scientific uncertainties and technical deficiencies in the proposals presented by Nirex [and] about the process of site selection and the broader issue of the scope and adequacy of the environmental statement'.
Shortly afterwards the newly elected UK Labour Government was confronted with the need for a new technical approach and policy on nuclear waste management. As a result, a consultation was launched in 2001 due for completion in 2006.
Meanwhile the problem of waste disposal is equally acute in many other countries, several of which have technically aware environmental pressure groups. In 2002, the US Senate voted to approve using Yucca Mountain in a remote corner of Nevada as a repository for US waste. It is located in the United States' former nuclear weapons testing area. Although the site has been under investigation since the 1980s, there remain many political and regulatory hurdles before it can be approved for long-term waste storage.
Sweden has a repository for LLW and ILW, having adopted a no-expense-spared approach after widespread public consultation. Possible sites for a similar longterm HLW repository are being investigated but such a repository could not begin to operate until the mid-2010s at the earliest. France has similar plans and Finland's facility should be operational after 2020.
Even if there is no further expansion of nuclear power globally, filling up of temporary stores based at nuclear plants or at reprocessing plants like Sellafield clearly poses a long-term challenge that the world will have to face. The continuing debate about the UK nuclear industry illustrates well how 'Green' groups can successfully stimulate debate and even modify government policy. We leave you to decide whether or not their opposition to nuclear power and reprocessing of nuclear fuels is justified.
a.Why are spent fuel rods from nuclear reactors potentially so much more serious a hazard than new fuel rods if exposed accidentally to the environment?
b.What factors have caused the economics of nuclear fuel reprocessing in the UK to change since 1986?
c.What is the purpose of plans to vitrify high-level radioactive wastes?
a.New fuel rods consist of pure uranium oxide or metal which are radioactive but with a long half-life; spent fuel rods contain a few per cent of highly radioactive fission products, some of which have short half-lives, and so are intensely radioactive (see discussion of Figure 18).
b.Uranium stockpiles that existed in the early 1990s were large because of: release of weapons-grade uranium at the end of the Cold War; cutbacks in nuclear power programmes; uranium overproduction from mines. So there was less need to recycle uranium-235 that remained 'unburned' in spent nuclear fuel. The UK's fast breeder research programme at Dounreay was terminated, so there was no longer a UK market for reclaimed fissionable material from spent fuel rods. Several customers began to reconsider using THORP.
c.Vitrification has two aims: first to concentrate HLW into a small volume for disposal, and second to lock the wastes into a relatively stable glass, which forms part of the multiple containment system resistant to groundwater penetration (Figure 19).
Nuclear power generation results from fission of uranium isotopes when bombarded by neutrons. Conventional burner reactors require relatively scarce uranium-235, whereas fast breeder reactors (which have not yet been developed on any significant scale) would exploit more abundant uranium-238.
In the early 21st century over 400 nuclear — mainly burner — reactors produced 16% of global electricity demand.
The UK played a leading role in nuclear power developments during the 1950s and 60s with its Magnox programme. During the 1970s new, more efficient reactor designs led to the building of AGRs in the UK, and PWRs and other analogous reactors in other countries. Progress was slowed in the late 1970s as concern grew over the operational safety of PWR nuclear reactors.
Including the costs of decommissioning reactors and managing radioactive waste, nuclear power is probably more expensive than power from fossil fuels. Nevertheless, arguments that nuclear power generation produces virtually no carbon dioxide and sulphur dioxide gases have begun to weigh in favour of its further development as fears of global warming grow.
The properties that determine the mobility and concentration of uranium to form ore deposits are: (a) it is an incompatible element by virtue of the high charge and large size of uranium ions, so it becomes concentrated late in the evolution of granitic magmas; (b) uranium-bearing ions are much more highly soluble in water under oxidising conditions than under reducing conditions, thus controlling uranium transport and deposition in groundwater and hydrothermal fluids.
Uranium occurs in many geological settings, including disseminated magmatic and unconformity related hydrothermal deposits, sandstone-hosted and quartz-pebble conglomerate deposits.
Uranium is mined in surface and underground mines, and by in situ leaching. The higher costs of underground mining are offset by higher grade uranium ore. Uranium is extracted chemically from ore to yield uranium trioxide, or yellowcake, subsequently processed and enriched into reactor fuel. Safety precautions from the effects of radioactivity of uranium and its daughter products, especially radon, are essential in mining, processing and disposal of mining wastes.
Uranium demand has been affected by political factors and military requirements as well as the commercial demands of the energy industry. The low uranium prices of the late 1980s and 1990s led to many low-grade deposits becoming uneconomic. A reduction of stockpiles and the possibility of a brighter future for nuclear energy led to a rise in prices in the early 21st century.
Current estimates of reserves of uranium amount to about 3.5 million tonnes, sufficient to maintain global nuclear power supplies for many years even if demand (∼6.6 × 104 t annually in the early 21st century) increases significantly.
Public concerns about nuclear reactor safety were exacerbated in 1986 by a major accident at the Chernobyl reactor in the Ukraine, after which widespread atmospheric dispersion spread radioactive contamination over most of Europe.
The intensity of nuclear-waste radiation is greater from short-lived radioactive fission products than from uranium and plutonium with long half lives. The reprocessing of spent nuclear fuel rods therefore constitutes a potential hazard. However, by 2005, this has been tentatively linked only to increased incidences of childhood leukaemia. The small, but finite risk of accidents at facilities such as the controversial THORP plant on the Sellafield site could be much more serious.
Plans to dispose of radioactive waste by burial in the UK have had a chequered history, and plans to bury high-level wastes (HLW) in the UK and in other countries have either been postponed indefinitely or are awaiting approval.
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Figure 17 Gittus, J.H., et al. (1987) The Chernobyl Accident and its Consequences, United Kingdom Atomic Energy Authority
Figure 18 Johnson, K.D. B. (1980) Energy in the Balance, papers from the British Association meeting 1975, Westbury House
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