Subcriticality Measurements with HEU (93.2) Metal Annular Storage Castings

These carefully performed and documented measurements with unreflected and unmoderated highly enriched uranium (HEU) castings can be used to benchmark calculational methods for the time decay of the fission chain multiplication process as measured with small (1 x 1 x 6 in. thick plastic scintillators with 1/4-in.-thick lead on all detector surfaces) detectors adjacent to the tightly fitting stainless steel cans that contained the HEU (∼93 wt%) metal. Prompt time decay measurements were performed stimulating the fission chain multiplication process with a timed, tagged Cf spontaneous fission source that emitted fission-spectrum neutrons and a time and directionally tagged 14.1-MeV neutrons from the DT reaction in a steady state generator with an embedded alpha detector. Time decay measurements were performed with HEU masses varying from 18 to 90 kg for a wide variety of source-detector-casting configurations. The use of a DT generator provided no addition information about the fission chain behavior beyond that provided by a time-tagged Cf spontaneous fission source. The main quantities obtained in the measurements were (1) the time distribution of the counts in a detector after a neutron fission in the Cf source or after the alpha detection coincident with the emission of a neutron from the DT generator (the equivalent of a pulsed neutron measurement with a randomly pulsed source) and (2) the time distribution of counts in one detector after a count in another detector (the equivalent of a two-detector Rossi-alpha measurement). Monte Carlo calculations using the MCNP-PoliMi coupled gamma-neutron transport code generally agreed with the measurement results except for some differences early in the fission chain decay process. The measurements that were performed with the HEU about 1 m above the floor were considerably affected by room return neutrons at times as early as 100 ns, and at times after 300 ns, a major portion of the time response was associated with the interaction of the HEU assemblies with the floor. This room-return effect increased with the size of the assembly because the larger assemblies subtend a larger solid angle to a neutron returning from the floor.