 In short, antiprotons would react inside the nucleus of uranium, causing a release of energy that breaks the nucleus apart as in conventional nuclear reactions.Even a small number of such reactions can start the chain reaction that would otherwise require a much larger volume of fuel to sustain.Whereas the "normal" critical mass for plutonium is about 11.8 kilograms (for a sphere at standard density), with antimatter catalyzed reactions this could be well under one gram.