Explanation

Nuclear power plants primarily use a specific type of uranium 235 for nuclear fission

because its atoms are easily split apart.

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Uranium's most stable isotope, uranium-238,

has a half-life of about 4,468,000,000 years.

Uranium-238 is not fissile, but is a fertile isotope,

because after neutron activation it can produce plutonium-239, another fissile isotope.