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 helium atoms and release energy. The reaction takes place at 10-15 million°C and at enormous pressure.

The conditions in the sun cannot be recreated on earth, so here a different fusion reaction has been studied, involving two isotopes of hydrogen called deuterium and tritium. These differ in the number of neutrons their atoms contain: deuterium contains two and tritium three. Deuterium (1H) 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 million°C. At this temperature the atoms involved separate into nuclei and electrons, a state called a plasma. Since the constituents of a plasma are all charged, either positively or negatively, both 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 shape for containing a plasma is torroidal and this has formed the basis for most fusion research. However, while fusion has been demonstrated, its commercial realisation remains a long way off.

Nuclear reactors

Nuclear reactor is the name given to the device or structure in which a controlled nuclear reaction takes place. There are a number of different designs but these have many features in common.

The core of the reactor is its heart, the place where the nuclear fuel is placed and where the nuclear reaction takes place. The fuel is most frequently formed into pellets roughly 2 cm in diameter and 1 cm long. These pellets are loaded into a fuel rod, a hollow tube of a special corrosion-resistant metal; this is frequently a zirconium alloy. Each fuel rod is 3-4 m long and a single reactor core may contain close to 50,000 such rods. Fuel rods must be replaced once the fissile uranium-235 they contain has been used up. This is a lengthy process which can take as much as 3 weeks to complete.

In between the fuel rods there are control rods, made of boron, which are used to control the nuclear reaction. These rods can be moved in and out of the core. The core will also contain a moderator to slow the neutrons released by the fission of uranium atoms. In some cases the moderator is also the coolant used to carry heat away from the core.

The outside of the core may be surrounded by a material which acts as a reflector to return some of the neutrons escaping from the core. This helps maintain a uniform power density within the core. There may also be a similar reflecting material in the centre of the core.

The coolant collects heat within the core and transfers to an external heat exchanger where it can be exploited to raise steam to drive a steam turbine. The coolant may be water (light water), deuterium (heavy water), a gas such as helium or a metal such as sodium. The core and its ancillary equipment is normally called the 'nuclear island' of a nuclear power plant while the boiler, steam turbine and generator are called the 'conventional island'. The coolant system will link the nuclear and conventional islands.

A nuclear power plant will contain a host of systems to ensure that the plant remains safe and can never release radioactive material into the environment. The most important of these is the containment. This is a heavy concrete and steel jacket which completely surrounds the nuclear reactor. In the event of a core failure it should be able to completely isolate the core from the surroundings and remained sealed, whatever happens within the core.

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