Gun-type fission weapons are the simplest approach and they work with Uranium-235 bombs as you can ignite it with just one explosion.
But Gun-type fission weapons don't work with plutonium, and weapon grade Plutonium is cheaper than weapon grade Uranium, so it wasn't much used.
This isotope shows up as an inevitable contaminant in Plutonium-239 for nuclear weapons, because it emits neutrons too fast and makes it harder to assemble the critical mass without fizzle.
It is the presence of this contaminant that made implosion-type fission weapon a necessity: Section "Gun-type fission weapons don't work with plutonium".