Scratches head ... U-235 is only extracted from Uranium ore after a long and difficult process. That's weapons grade uranium, and it's relatively scarce.
U-238, having an extra neutron thrown at it, instead of exploding, generates Plutonium-239, which is weapons grade Plutonium.
This is why only the Plutonium bomb was tested in WWII ... Plutonium was easier to make, they had enough for the test shot, and tested the less-straightforward assembly method of the Fat
Man, crossing their fingers and hoping the Little Boy shotgun design would work the way the numbers said it would.
(I don't know a damn thing about physics, but I do know the history of the construction of the Bomb)
nice plutonium fact sheet for anyone who cares