The Member States 25 March 1957, Consolidated version of the Treaty establishing the European Atomic Energy Community , TITLE FIVE - GENERAL PROVISIONS, ARTICLE 198

Article 197 For the purposes of this Treaty: 1. ‘Special fissile materials’ means plutonium 239; uranium 233; uranium enriched in uranium 235 or uranium 233; and any substance containing one or more of the foregoing isotopes and such other fissile materials as may be specified by the Council, acting by a qualified majority on a proposal from the Commission; the expression ‘special fissile