1/3 fraction of the original uranium-235 should be left after 3.5 billion years
The only naturally occurring isotope of this radioactive metal, uranium-235, is capable of supporting a nuclear fission reaction, making it special. A variation of an element called an isotope has a different number of neutrons in its nucleus. Approximately 93.5 percent of the uranium used in modern nuclear bombs is enriched.
On average, plutonium-239 makes up 93% or more of nuclear bombs, whereas plutonium-240 makes up less than 7% and other plutonium isotopes are present in extremely minute amounts. A nuclear bomb requires around 33 pounds (15 kilograms) of enriched uranium to function, according to the Union of Concerned Scientists.
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