For uranium to work in a nuclear reactor it must be enriched to contain 2-3% uranium-235 while weapons-grade uranium must contain 90% or more uranium-235.
The device's primary source of neutron radiation was the radioactive element californium, but the stream of neutrons produced by the californium was multiplied by passing it through a lattice of highly enriched uranium U-235, whose nuclear fission released additional neutrons.
The Russians perfected a far less energy-hogging method that uses centrifuges to spin the uranium hexafluoride at 50, 000rpm so that heavier u-238 separates from lighter u-235.