LASL Builds the World's Smallest Nuclear Reactor

You can now build a critical mass with 242 grams of $U235$ if you do it right. Carroll B. Mills and George A. Jarvis of Los Alamos Scientific Laboratory have done it right. By interleaving 0.003‐cm‐thick foils of $U235$ with 0.316‐cm‐thick polyethylene foils, they achieved criticality with a rectangular solid 15.4 cm square by 12 cm high. Each face of the core was covered with a layer of $U235$ foil, and the entire core was surrounded with a cubical beryllium reflector 30.4 cm thick.