自じ発はつ核かく分ぶん裂れつMeaningsNoun1. spontaneous fissionKanji used自self発let out核nucleus分part裂splitComposed of自じ発はつspontaneity; spontaneous passiveNoun核かく分ぶん裂れつnuclear fission; karyokinesis (division of a cell nucleus during mitosis or meiosis)Noun