Determination of neutron multiplication of subcritical HEU systems using delayed neutrons

This report presents a new method for the nondestructive analysis of highly enriched uranium (HEU). The method uses a source of 14-MeV neutrons to induce fission in the HEU sample. The 14-MeV neutron source is pulsed at a rate of 50 Hz. Between pulses, neutrons are detected with a medium-efficiency 3He-based neutron detector. These neutrons are delayed neutrons from fission products that result from HEU fission events induced during the pulse and from fission events induced by the delayed neutrons from the fission products. The time history of neutron detection is recorded and subsequently analyzed according to the Feynman reduced variance method to yield rates of detection of one (N1/s), two (N2/s), and three (N3/s) neutrons from fission events. Ratios of (N2/s)/(N1/s) and (N3/s)/(N1/s) are compared with model calculations to extract the neutron multiplication. The rate N1/s is used to deduce the HEU system mass. Measurements were performed on a set of spherical HEU samples enriched to 93% metal. The samples were assembled from a set of nesting hollow hemispheres. The mass of the HEU samples ranged from 592 to 13722 g. Experimentally determined total neutron multiplications for the HEU samples were compared with calculations performed with the ONEDANT code. Overall agreement was better than 5%. Experimentally determined mass values agreed with the stated mass values to better than 5%.