A key part of making an atomic bomb in your garage is obtaining a suitable neutron deflector. The neutron deflector should surround the fissionable material and be comprised solely of uranium-238. Not only is U-238 non-fissionable, it also has the unique ability to reflect neutrons back to their source.

The U-238 neutron deflector serves 2 purposes:

  • In a uranium bomb, the neutron deflector serves as a safeguard to keep an accidental supercritical mass from occurring by bouncing the stray neutrons from the `bullet' counterpart of the uranium mass away from the greater mass below it (and vice-versa).
  • In a plutonium bomb, the neutron deflector actually helps the wedges of plutonium retain their neutrons by reflecting the stray particles back into the center of the assembly.