Development and evaluation of a list mode neutron coincidence collar for spatial response measurements of fresh fuel assemblies

Abstract The capabilities of current safeguards neutron instrumentation can be expanded by acquiring and analyzing data in new ways. A new List Mode Collar (LMCL) was created by retrofitting a traditional safeguards neutron coincidence counting system, the Neutron Coincidence Collar, with newly developed preamplifiers on each of its 18  3He tubes in active interrogation mode. By applying list mode data acquisition (LMDA) and analysis, a signal from each  3He tube can now be recorded and analyzed to determine the neutron count rate measured in each individual 3He tube. Further, the count rate distribution across all  3He tube locations in the system can be determined. This is the first time that a full list mode method has been implemented using a traditional neutron coincidence collar and spatial response data has been obtained. The objective of this research was to develop and evaluate a laboratory demonstration list mode neutron coincidence collar that can extract a greater number of useful signatures than are currently generated using the current collar. A new analysis method called the “List Mode Response Matrix” has also been developed for use in combination with these measurements to improve the detection of missing fuel pins (partial defects) during the nondestructive assay of fresh nuclear fuel assemblies, although analysis algorithms will be reported separately from this experimental evaluation. In this paper, 18 channel spatial response data are presented for a 252Cf source moving within the LMCL detector cavity. LMDA, combined with the addition of multiple preamplifiers, facilitates this capability by increasing the number of signals that can be measured simultaneously; allowing for in-depth analysis of neutron coincidence events to determine the location and distribution of nuclear material within the measurement geometry. This approach enables a spatial response measurement to form a “pattern” or item signature unique to the fissioning source’s distribution; in the case of the LMCL, the distribution of fresh low-enriched uranium fuel within the assembly. A major benefit provided by LMDA is that this can all be achieved from a single measurement pulse train in offline analysis. To experimentally demonstrate this concept, a Mirion Technologies (Canberra) model JCC-71 Neutron Coincidence Collar has been retrofitted with new electronics designed at Oak Ridge National Laboratory. This paper presents the results from the experimental evaluation of the laboratory demonstration system. The results indicate equivalent performance of the LMCL to the original collar in addition to improved spatial response capabilities, while maintaining the original system footprint.