General description of a nuclear power system

The principal parts of a nuclear power plant are shown in Fig. 1.1. 

In operation, heat is generated in the fuel, which is usually an oxide of uranium or metallic uranium. The 

During the operation of the reactor, U235 is destroyed, forming highly radioactive fission products. At the same time the U238 reacts with neutrons and, as an eventual result of this reaction, forms a new fissionable material (plutonium), mostly as the isotope Pu239. As this isotope builds up, some of this will fission too. 

In most reactors the amount of plutonium formed will not fully compensate for the loss of U235, and the ability of the fuel to sustain the nuclear reactions (its reactivity ) falls until it is necessary to replace the fuel to keep the plant producing adequate levels of power. 

The fuel retains much of the radioactive fission products but is wrapped in a sealed can which, under normal operating conditions, forms a further barrier to their release. 

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