Year
2017
Abstract
We demonstrated the application of a fast-neutron multiplicity counting (FNMC) system to detect small variations in an assembly of uranium-oxide pins. The current version of the FNMC includes eight liquid organic scintillators (EJ-309 7.62 cm diameter x 7.62 cm) and eight crystal organic scintillators (stilbene 5.08 cm diameter x 5.08 cm) in a checkerboard assembly. We measured time-correlated fast neutrons, emitted by an assembly of 32 uranium-oxide pins, using two Am-Li sources to induce fissions in the sample. Each pin is a cylinder of approximately 1 cm diameter and 10 cm length, with a mass of 75.52 g and 16.37 wt% enrichment. We removed up to 16 pins from a sample consisting of 32 pins, and determined the minimum number of pins that resulted in a statistically significant decrease of neutron coincidence counts. All the measurements were performed at the Zero Power Physics Reactor facility of Idaho National Laboratory. We found that a diversion of the four innermost pins could be detected by the system in 5 minutes with 95% confidence. Fast-neutron measurements have several potential advantages compared to thermal and epithermal neutron counters the most important being a shorter die-away time and therefore reduced spurious coincidences due to accidental counts. This advantage may result in a higher sensitivity in timely detecting diversion of nuclear material from peaceful uses.