&gt; even if he had a startup neutron source<p>In the words of my [nuclear engineering] professor: &quot;criticality is a property of the system and is independent of any external source.&quot; 1kg of uranium isn&#x27;t enough to form a critical mass, period. Nothing else really matters and adding a source doesn&#x27;t change that. It&#x27;s possible this person built something that experienced induced fissions, but a self-sustaining chain reaction (i.e., a reactor) is not.