FUNDAMENTALS OF NUCLEAR POWER
All the nuclear power stations operating today generate electricity by utilizing energy released when the nuclei of a large atom such as uranium split into smaller components, a process called nuclear fission. The amount of energy released by this fission process is enormous. One kilogram of naturally occurring uranium could, in theory, release around 140 GWh of energy. (140 GWh represents the output of a 1000 MW coal-fired plant operating at full power for nearly six days.)
There is another source of nuclear energy, nuclear fusion, which involves the reverse of a fission reaction. In this case, small atoms are encouraged to fuse at extraordinarily high temperatures to form larger atoms. Like nuclear fission, fusion releases massive amounts of energy. However, it will only take place under extreme conditions. The fusion of hydrogen atoms is the main source of energy within the sun.
The reason why both fission and fusion can release energy lies in the relative stability of different elements. It turns out that atomic species in the middle of the periodic table of elements, elements such as barium and krypton (these are typical products of uranium fission), are generally more stable than either ligh- ter elements such as hydrogen or heavier elements such as uranium. The nuclei of these more stable atoms are bound together more strongly and their nuclear components, the protons and the neutrons, are in fact slightly lighter. It is this difference in mass, equivalent to the stronger binding, that is released during the fission or fusion reaction.
Nuclear Fission
Many large, and even some small, atoms undergo nuclear fission reactions naturally. One of the isotopes of carbon (isotopes are atoms of a single element with different numbers of neutrons) called carbon-14 behaves in this way. Carbon-14 exists at a constant concentration in natural sources of carbon. Thus, living entities that constantly exchange their carbon with the biosphere maintain this constant concentration. However, when they die, the carbon-14 is no longer renewed and it gradually decays. Measuring the residual concentration gives a good estimate of the time since the organism died. It is this property that allows archeologists to use carbon-14 to date ancient artifacts and remains.
Other atoms can be induced to undergo fission by bombarding them with subatomic particles. One of the isotopes of uranium, the element most widely used in nuclear reactors, behaves in this manner. Naturally occurring uranium is composed primarily of two slightly different isotopes called uranium-235 and uranium-238 (the numbers refer to the sum of protons and neutrons each atom contains). Most uranium is uranium-238, but 0.7% is uranium-235.
When an atom of uranium-235 is struck by a neutron it may be induced to undergo a nuclear fission reaction. The most frequent products of this reaction are an atom of krypton, an atom of barium, three more neutrons, and a signif- icant quantity of energy:
In theory, each of the three neutrons produced during this reaction could cause three more atoms of uranium-235 to split. However, this also depends on the quantity of uranium present. If a piece of uranium is too small, then most of the neutrons will escape into the surroundings without ever meeting a uranium nucleus. It is only when the size reaches and exceeds a quantity known as the critical mass that the number of reactions created by each single fission reaction exceeds one. This leads to a rapidly accelerating reaction, called a chain reaction, which will release an enormous amount of energy. A chain reaction of this type forms the basis for the atomic bomb.
In fact, a lump of natural uranium will not explode because the uranium-235 atoms only react when struck by slow-moving neutrons. The neutrons created during the fission process move too fast to cause further fission reactions to take place. They need to be slowed down first. This is crucial to the development of nuclear power.
Controlled Nuclear Reaction
If uranium fission is to be harnessed in a power station, the nuclear chain reaction must first be tamed. The chain reaction is explosive and dangerous. How- ever, it can be managed by carrying away the energy released by the fission reactions, controlling the number of neutron within the reactor core, and then slowing the remaining neutrons so that they can initiate more fission reactions.
An accelerating chain reaction will take place when each fission reaction causes more than one further identical reaction. If the fission of a single uranium-235 atom causes only one identical reaction to take place, the reaction will carry on indefinitely—or at least until the supply of uranium-235 has been used up—without accelerating. But if each fission reaction leads to an average of less than one further reaction, the process will eventually die away naturally.
The operation of a nuclear reactor is based on the idea that a nuclear chain reaction can be controlled so that the process will continue indefinitely but will never run away and become a chain reaction. A reactor in which each nuclear reaction produces one further nuclear reaction is described as critical. Once the product of each nuclear reaction is more than one additional reaction, the reactor is described as supercritical. Operation must be controlled so that the reactor is just—but barely—supercritical.
Naturally occurring uranium can be used as fuel for a nuclear fission reactor.
However, most nuclear reactors contain uranium that has been enriched so that it contains more uranium-235 than it would in nature. Enrichment to about 3% is common. Using enriched uranium makes it easier to start a sustained nuclear fission reaction.
In addition to the uranium, the reactor also contains rods made of boron. Boron is capable of absorbing the neutrons generated during the nuclear reaction of uranium-235. If a sufficiently large amount of boron is included within the reactor core it will absorb and remove the neutrons generated during the fission reaction, stopping the chain reaction from proceeding by keeping the reac- tor subcritical. The boron rods are moveable, and by moving the rods in and out of the reactor core, the number (or flux) of neutrons and therefore the nuclear process can be controlled.
One further crucial component is needed to make the reactor work— something to slow down the fast neutrons. The neutrons from each uranium- 235 fission move too fast to stimulate a further reaction, but they can be slowed by adding a material called a moderator. Water makes a good moderator and is used in most operating reactors. Graphite also functions well as a moderator and has been used in some reactor designs.
When a uranium fission reaction takes place the energy it releases emerges as kinetic energy. In other words, the products of the fission process carry the energy away as energy of motion; they move extremely fast. Much of this energy is carried away by the fast neutrons. These neutrons will dissipate their energy in collision with atoms and molecules within the reactor core. In many reactors this energy is absorbed by the moderator: water. So while the neutrons are slowed, the water within the core becomes hotter. By cycling the water from the reactor core through a heat exchanger, this heat can be extracted and used to generate electricity. Extracting the heat also helps maintain the reactor in a stable condition by preventing overheating.
The operation of a nuclear fission reactor is, therefore, a careful balancing act. As a consequence, a reactor always has the potential to generate a runaway chain reaction. Modern reactor designs try to ensure that there is no possibility of this happening in the event of a component or operational failure.
Nuclear Fusion
The alternative energy-yielding nuclear reaction to fission is fusion. Fusion is the process that generates energy in the sun and stars. In the sun, hydrogen atoms combine to produce deuterium (heavy hydrogen) atoms and then deute- rium and hydrogen atoms fuse to produce helium with the release of energy. The reaction takes place at 10–15 million oC and at enormous pressure.
The conditions in the sun cannot be easily recreated on Earth, although fusion of the type taking place within the sun has been achieved. However, for the purposes of a fusion reactor capable of electricity generation, another reaction offers more potential because it takes place under more benign condi- tions than those in the sun. This is the reaction between two isotopes of hydro- gen, deuterium, and tritium. Deuterium 2 H is found naturally in small quantities in water while tritium 3H is made from lithium. These two will react to produce
helium and energy:
The reaction between deuterium and tritium will only take place at 100 mil- lion oC (but at much lower pressure than in the sun). At this temperature all the atoms separate into a sea of nuclei and electrons, a state called a plasma. Since the constituents of a plasma are all charged, either positively or negatively, they can be controlled and contained using a magnetic field. This is crucial since there is no material that can withstand temperatures this severe. The most promising magnetic field for containing a plasma is torroidal and this has formed the basis for most fusion research. There is an alternative method of containing a fusion plasma called inertial confinement. This relies on generating extreme conditions within a small charge of tritium and deuterium, in essence creating a tiny sun in which the fusion takes place too fast for the particles to escape. Both systems of containment are being developed for power generation.