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Topic 7 Nuclear energy
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Don't try this at home but if, instead of eating, you could supply your body's energy by nuclear fission, 1 kilogram of U-235 – the size of a chocolate bar – would provide you with enough energy for about 500,000 years! You'd be getting hungry again about now, if you had 'eaten' your bar of uranium before our prehuman ancestors had learnt to use fire!
Of course, we use a lot more energy than the amount it takes just to keep us fed. A single 100 watt light bulb uses about the same amount of energy as your body does. And then there's your family car, computer, television, hot water, air conditioning, dishwasher, plus the movie theatre, shopping centre, and all of society's infrastructure! The total energy used each year by an average Australian (or other citizen of the developed world) is 20 X 107 kilojoules (kJ). This is a lot – nearly 40,000 times more energy than that used by your body alone.
How do you think we should generate this energy? Do we need to use so much? Certainly we could and should improve our energy efficiency, for example installing insulation instead of air conditioning. However, modern society seems likely to use large amounts of energy for some time to come.
Nearly 30 countries around the world use nuclear power plants to generate electricity. The USA has more than 100 nuclear power plants, nearly a quarter of the world total. France generates 75% of its electricity by nuclear reactors, although for most countries the proportion is much less.
In the 1950s and '60s, there was great excitement about the potential of nuclear power, and many plants were ordered and built. Many countries in Asia and elsewhere continue to build new reactors. In Western countries, enthusiasm for nuclear power has dampened somewhat since the 1970s, though even some environmentalists concede it offers benefits.
No new reactors have been approved in the US since its nuclear accident at Three Mile Island in 1979. However, some that had already been approved or begun were completed as late as the 1990s. In Europe, orders for new reactors dropped right off after the nuclear accident at Chernobyl in 1986. However, in Asia the nuclear power industry continues to grow rapidly. This reflects population growth and a lack of availability of other energy resources.
However, we continue to use ever-increasing amounts of energy, even as the damaging effects of our heavy fossil fuel use become apparent. For some people, nuclear energy appears to be a good solution to the problems of pollution of the atmosphere associated with coal or oil fired power stations.
Nuclear reactors
There are several different types of nuclear power reactors, but about 90% are pressurised light water reactors (LWRs). In any case, there are many basic similarities between all nuclear power reactors.
- The reactor itself has four main elements: the fuel rods, coolant, control rods and moderator. These are contained within a sealed steel reactor vessel.
- The fission of uranium in the fuel rods generates enormous heat, which is transferred via the coolant to the water, which is then converted to steam to turn the turbine to generate the electricity, just like a conventional coal or oil power plant.
- The transfer of the heat to the water also acts as a cooling system, playing a part in preventing the whole reactor vessel from overheating. This could lead to a potentially disastrous meltdown, so it is very important that the coolant systems do not fail.
Figure 7.1 Nuclear light water power reactor.
Unlike the fission in a nuclear bomb, fission in a reactor's fuel rods proceeds in a controlled chain reaction, which is just barely self sustaining. This is achieved by controlling the number and the speed of the neutrons causing nuclear fission.
The number of neutrons is controlled in a several ways. First, it is controlled by keeping the purity of the fuel at about 3% fissile U-235 in a LWR. The rest is mostly non-fissile U-238, which absorbs neutrons without undergoing fission. This greatly slows down the chain reaction which means that a reactor cannot explode like a nuclear weapon, which uses almost pure fissile fuel for a lightning-fast chain reaction. Exploding a nuclear weapon uses about eight kilograms of fissile fuel in under half a microsecond! This amount of fuel in a lower grade form lasts the Australian Nuclear Science and Technology Organisation (ANSTO)'s small research reactor a whole year. Large power reactors require about 200,000 kg of low-grade uranium per year.
 Figure 7.2 In comparison to this model of the High Flux Australian Reactor's core, the core of a full-sized nuclear power plant would be 7 metres high with a diameter of 11 metres.
The number of neutrons causing fission in a reactor is also controlled more precisely by using control rods made of a highly neutron-absorbing material such as boron or cadmium. The control rods can be lowered or raised to decrease or increase the rate of the chain reaction. If something goes wrong, and the reactor starts going 'supercritical' (ie the fission rate goes above the constant rate needed for a self sustaining reaction), the control rods drop down automatically to quench the reaction.
The nuclear reactions inside the reactor vessel are also influenced by the speed of the neutrons bouncing around. 'Fast' neutrons move too quickly to cause fissile U-235 atoms to split. The speed of the neutrons is controlled by the moderator – which also acts as the primary coolant. It slows the neutrons down to about 1/10,000th of their initial speed. These 'thermal' neutrons are slow enough to cause fissile nuclei to split when hit. An important safety feature of the LWR is that the light water that is used as the coolant also acts as the moderator. A loss of coolant (such as occurred in the Three Mile Island reactor) therefore reduces reactivity in the core because the neutrons are moving too fast. In contrast, the Chernobyl reactor used graphite rods as the moderator. In the Chernobyl reactor, water was used only to cool the core, and not as the primary controller of the speed of the neutrons and, therefore, the rate of the reaction. In fact, by absorbing some neutrons, the water did help to slow the reaction. Consequently, loss of coolant during the Chernobyl disaster also increased the rate of reaction, and accelerated the reactor towards its supercritical meltdown.
Research reactors
Research reactors, such as ANSTO's High Flux Australian Reactor (HIFAR), operate on exactly the same principles as a power reactor but with much smaller cores. The only difference in their design is the lower temperatures and pressures involved, because they are not required to make steam to drive a turbine to make electricity. This makes them safer. Research reactors are used to generate beams of neutrons for scientific research, and to irradiate materials to make radioactive isotopes for use in medicine, the environment and industry. ANSTO's HIFAR reactor will be replaced by the more powerful and sophisticated replacement research reactor in 2006.
Figure 7.3 The containment building of ANSTO's research reactor.
Nuclear weapons
The first nuclear weapons were developed by the US's top secret Manhattan Project during World War II, with support from the British and Canadian governments. Most scientists had concerns about developing such terrifying weaponry. Many of the key physicists involved were Europeans who had fled Europe and the Nazis. These scientists feared that the Nazis would develop their own bomb.
How does a nuclear bomb work? A fission bomb (or 'A-bomb') contains two carefully separated masses of nearly pure fissile material: either uranium-235 or plutonium-239. Production of these materials requires sophisticated technology and expertise, so building a nuclear weapon is not easy. To detonate the weapon, the two masses are combined into a critical mass, beginning a chain reaction of nuclear fission. This reaction spreads almost instantaneously, explosively unleashing vast amounts of energy and causing destruction.
 Figure 7.4 A fission bomb is detonated by conventional explosives, which combine two (or compresses a single) non-critical mass of pure fissile material to create a single critical mass. This begins an explosively fast nuclear chain reaction.
The hydrogen bomb
The first 'H-bomb' was detonated in 1952 during a test in the US. Hydrogen bombs are about a thousand times more powerful than fission bombs, but thankfully they have never been used in conflict. Rather than using fission, a hydrogen bomb uses nuclear fusion - the reaction that occurs within stars like our sun. In a nuclear fusion reaction hydrogen and other light elements (with small nuclei) are heated to enormous temperatures, to the point at which their nuclei are moving so fast that they begin to fuse (join together) to form heavier elements. As with fission, the process generates enormous amounts of energy and neutrons, and causes a chain reaction in surrounding materials. Fusion weapons use a small fission explosion to achieve the tremendous temperatures required to start the nuclear fusion reaction.
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