The results of the Chi-Nu physics experiment at Los Alamos National Laboratory have contributed essential, never-before-observed data for enhancing nuclear security applications, understanding criticality safety and designing fast-neutron energy reactors. The Chi-Nu project, a years-long experiment measuring the energy spectrum of neutrons emitted from neutron-induced fission, recently concluded the most detailed and extensive uncertainty analysis of the three major actinide elements — uranium-238, uranium-235 and plutonium-239.