For the success of treaties on nuclear non-proliferation and disarmament, an understanding of the materials needed to manufacture a nuclear weapon is required. The fission process, in which atomic nuclei are split by neutrons, is central to the detonation of a nuclear weapon. In the fission process, neutrons are released, potentially resulting in a chain reaction in the material. However, not all atomic nuclei can undergo fission, and not all nuclei that can undergo fission can sustain a nuclear chain reaction. This means that some types of nuclides can be used to initiate or sustain the chain reaction in a nuclear weapon, whereas others are less useful for this purpose. To facilitate successful non-proliferation and disarmament agreements, a clear definition of which materials to control is needed. One fundamental input to this definition is knowledge of whether a material can at all undergo fission and if it can sustain a nuclear chain reaction.
The need to classify the properties of different atomic nuclei in connection with fission and neutron chain reactions has led to definitions of several “nuclear f-words”: one can speak of fertile, fissile, fissible and fissionable nuclei. However, not all of these terms are universally used and may have unclear, or even conflicting, definitions. Well-defined and agreed upon terminology may be beneficial to the negotiation and verification of future treaties. In this poster, the physical motivation for the definitions of these important concepts will be described.