The World Nuclear Association(WNA) states:
Weapons-grade uranium is highly enriched, to over 90% U-235 (the fissile isotope).Weapons-grade plutonium has over 93% Pu-239
However, what WNA or any other nuclear weapon watchdog do not state is why Uranium or Plutonium is enriched in the first case before they can be used in nuclear warheads?
Is it because the amount of space available in the warhead is limited and therefore only a small amount of Uranium or Plutonium can be used & so they have to be enriched first?