In a heavy-water nuclear reactor, when neutrons bombard U-238, some uranium atoms absorb an additional neutron and are transformed into Pu-239. On the eve of World War II, scientists both in ...

Nuclear energy is a controversial subject ... is harder to use for weapons purposes than uranium-235 or plutonium-239. This means there is less of a risk its wider use will pose a proliferation ...

It had undergone nuclear fission ... As he bombarded a metal called uranium, he expected to create a slightly heavier nucleus, perhaps with 239 or 240 particles instead of its usual 238.