The original intent of the duoplasmatron breeder was to eliminate the dangers of going supercritical, actually to eliminate HE fissile material entirely from th

The original intent of the duoplasmatron breeder was to eliminate the dangers of going supercritical, actually to eliminate HE fissile material entirely from the bulk, as the plutonium would have to be neutron-bred by the fusion reactions triggered by the tritium beam. Also short pulse control. This was mainly to make it safe and cheap. You could also just use a subcritical mass of HE-fissile material surrounded by a neutron reflector with aperture in the direction of thrust. Recommended only if you are certain to be far enough away from humans to avoid recovery and conversion to a weapon. Aliens too; they might send one back. There is a design for a safer and simpler cylindrical slow-burn wave reactor that may also work for low levels of neutron propulsion.