In an uncontrolled reaction, most of the neutrons released from
the nuclei go on to hit other nuclei. Many atoms are split and
enormous amount of energy is released. This is what happens in
an atomic bomb.
However, to make practical and non-destructive liberation of the energy, we must be able to control the chain reactions so that our nuclear power station would not turn into an atomic bomb.
There are several types of nuclear reactors in which controlled
reactions can take place but they serve similar purposes and will
be discussed in the following section.
Diagram
showing the principal components of power station
They are made from uranium oxide [enriched] with extra uranium-235. The fuel is then converted into small pellets, packed into tubes called pins. The pins are bundled into larger tubes and are ready to be put into the reactor. One kilogram of this fuel gives as much energy as 55 tons of coal.
In the reactors, there are always some neutron moderators which slow down the neutrons so that they have a higher chance of splitting the next nucleus. Materials used for moderators include graphite and heavy water, containing the isotope deuterium. However, a breeder reactor has no moderator. It depends on the liquid metal coolant to control the temperature.
The core also has neutron-absorbing control rods to control the neutrons released. While the moderator promotes stronger interaction of the neutrons with the uranium nuclei, the control rods are there to reduce the concentration of neutrons whenever necessary.
The material used for these control rods is cadmium whose nuclei have a large cross-section for the capture of neutrons. The cadmium rods are inserted into the nuclear reactor to slow down or shut down the chain reaction whereas withdrawing the rods restarts or speeds up the reaction.
The temperature of the reactor is very high although the actual temperature depends on the reactor type. Some reactors operate at temperatures as high as 1500 °C. The coolant flows around the core and carries heat energy to a steam generator. The coolant materials vary between reactors. They may be water or carbon dioxide at high pressures. Some reactors such as fast breeders use liquid sodium as coolant.
The coolant carries the heat energy to a heat exchanger (or steam generator), where it is used to make steam. The steam then drives the turbine, and the turbine drives the generator that produces electricity. Waste steam goes to the cooling towers to be [condensed] back into water.
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