²It is noteworthy that ²³⁸U can be converted into a different, fissile element called plutonium through the process of neutron bombardment. Likewise, naturally-occurring thorium 232 (²³²Th) may be converted into ²³³U which is fissile. However, converting non-fissile uranium into fissile plutonium, or converting non-fissile thorium into fissile uranium, requires intense neutron bombardment at a scale only seen within the core of a nuclear reactor running on some other fuel such as ²³⁵U, which makes ²³⁵U the critical ingredient for any independent nuclear program.