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In a nuclear power plant, the emitted neutrons trigger fission in other uranium nuclei. The process repeats itself and a chain reaction is set up to generate energy continuously in the reactor as illustrated in Fig. 8.

Fig. 8   The chain reaction of uranium fission

As the fission of one uranium nucleus takes one neutron while producing 2 or 3 neutrons, fission of one nucleus can trigger 2 or 3 further fissions and so on. If this is allowed to go on unchecked, the energy released will increase very rapidly to a dangerous level. In an atomic bomb, the design is to use highly concentrated uranium-235 and allow the nuclear chain reaction to continue by itself, resulting in an atomic explosion. In a nuclear power plant, however, the uranium-235 concentration in the fuel is never high enough to produce such an explosion, and the chain reaction is very carefully controlled to ensure steady and stable energy generation.

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Fig. 9   In the nuclear reactor, control rods are used to absorb excess neutrons and control the rate of the chain reaction.

To control the chain reaction and thus the energy release, control rods made of neutron absorbing material may be inserted into the reactor to absorb excess neutrons. On the one hand, enough neutrons to sustain a stable chain reaction and maintain a constant power output are needed while on the other hand, too many neutrons can cause an excessive reaction rate.

When the control rods are lowered deeper into the reactor, fewer neutrons are available for the chain reaction and the power output decreases. To increase the power output the control rods may be raised.

The following animation shows how different parts of the Guangdong Daya Bay Nuclear Power Station work together to produce electricity:

Flash animation: Nuclear Power Plant


Refuelling

It is necessary to replenish the reactor core with a fresh supply of fuel typically once a year. Since the reactor core is fully enclosed during operation, it is necessary to shutdown the plant for the refuelling procedure while replacing about one third of the fuel assemblies. Fuel at the centre of the reactor is consumed faster. During refuelling, one possible choice is to move the fuel assemblies at the edge of the reactor towards the centre and place fresh fuel assemblies at the edge.

The replaced fuel assemblies are removed from the core and transported within the plant under water, which acts as a shield against the radiation emitted. The fuel assemblies are stored under water in the fuel storage building next to the reactor building until their radiation levels are reduced sufficiently before being transported away from the plant. These fuel assemblies are subsequently treated to extract their useful constituents and the remaining highly radioactive material is solidified in glass and stored deep underground, permanently encased and embedded in concrete.