Abstract
Five highly enriched (93.2 wt% 235U) uranium (HEU) and three depleted (0.2 wt% 235U) uranium (DU) castings were available for passive time coincidence measurements with two arrays of eight total liquid scintillation detectors and two arrays of 32 total moderated 3He detectors. In addition to measurements with each type of casting, measurements were also performed with both types of castings intermingled. The spontaneous fission rate for the casting is primarily due to the presence of 238U. Thus, the DU castings have ~15 times more spontaneous fission that the HEU castings with very little addition in multiplication with each addition of a DU casting. The characteristic time behavior of the fission chain multiplication process is determined by the 235U. The presence of the DU castings in these assemblies has three physical effects: provision of an additional source of neutrons to initiate fission chains in the HEU, reflection of neutrons back into the HEU, and shielding of the detectors from fission chain neutrons and gammas. The effects are investigated within the context of the time coincidence distributions using the signal-triggered multiplets and Feynman variances.
