r1=p1 undergoes nuclear fission
x1 can be an atom (e.g. U-235), or a mass (e.g. the fissile material in a nuclear reactor). The fission may be spontaneous (e.g. nuclear decay) or have an external cause (e.g. via chain reaction). The fission may also be controlled (e.g. moderated as in a nuclear reactor), or uncontrolled (e.g. a chain reaction in an atomic bomb). The specific type of fission can be specified with tenses and modals. Cf. ratni, porpi, ratmijypopru'e.