By Sunil Bhardwaj

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The material or elements which can undergo fission phenomenon directly are known as fissile material or elements. For example: $$_{ 92 }^{ 233 }{ U }, _{ 92 }^{ 235 }{ U }$$ and $$_{ 94 }^{ 239 }{ Pu }.$$

These elements undergo fission phenomenon even without any bombardment of projectile.

Some of elements such as $$_{ 90 }^{ 232 }{ Th }$$ does not undergo fission by their own so these elements require bombardment of neutrons which converts them to fissile element. Such elements which can be converted into fissile by means of neutron bombardment is known as fertile elements. E.g. $$_{ 90 }^{ 232 }{ Th }$$ $$_{ 90 }^{ 232 }{ Th } + _{ 0 }^{ 1 }{ n } \underset { slow }{ \longrightarrow } \left[ _{ 90 }^{ 233 }{ Th } \right] \underset { -\beta }{ \longrightarrow } _{ 91 }^{ 233 }{ Pa } \underset { -\beta }{ \longrightarrow } _{ 92 }^{ 233 }{ U }$$ where Pa = Protoactinium.

From this reactions it is clear that $$_{ 92 }^{ 233 }{ U }$$ can be produced from $$_{ 90 }^{ 232 }{ Th }$$, therefore $$_{ 90 }^{ 232 }{ Th }$$ is Fertile element and $$_{ 92 }^{ 233 }{ U }$$ is fissile element.

Similarly, $$_{ 92 }^{ 238 }{ U } + _{ 0 }^{ 1 }{ n } \longrightarrow \left[ _{ 92 }^{ 239 }{ U } \right] \underset { -\beta }{ \longrightarrow } _{ 93 }^{ 239 }{ Np } \underset { -\beta }{ \longrightarrow } _{ 94 }^{ 239 }{ Pu }$$ in this case $$_{ 92 }^{ 238 }{ U }$$ is fertile and $$_{ 94 }^{ 239 }{ Pu }$$ is fissile.