Effects of Irradiation on Nuclear Fuels

As the fuel in a nuclear reactor is irradiated, it undergoes nuclear transmutations that cause its composition to change in the following ways  

Heavy nuclides, mainly isotopes of uranium and plutonium, are formed. 

These changes in composition bring about changes in reactivity of the fuel, which eventually decreases to such an extent that the reactor will no longer be critical unless the spent fuel is replaced with fresh fuel. 

One of the principal objectives of fuel-cycle analysis is to follow quantitatively the changes in concentration of fissile and fertile nuclides and fission products during neutron irradiation. 

Another important objective is to follow the changes in reactivity that take place as fissile nuclides are depleted or formed from fertile nuclides, and as neutron poisons are formed through buildup of fission products or burned out through reaction with neutrons. 

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