Subcritical measurements of uranium shells

The authors have begun a program of measuring the degree of subcriticality of highly enriched uranium (HEU) systems. They have used the Feynman variance technique and point kinetics to extract a value for keff. The measurements were done with the Rocky Flats shells. For the bare uranium systems, the HEU was driven with an internal source of 252Cf, AmF, and AmBe. The two (α,n) sources were chosen so that one (AmF) would be close to the neutron energy spectrum for fission and the other would have a much higher average energy. To reach higher multiplications, they reflected the HEU with metallic shells. For the reflected systems, the HEU was driven either by an internal (AmF) source or an external deuterium-tritium (D-T) neutron generator