Abstract
Tagged neutron measurement systems use an external neutron source to simultaneously perform transmission and fission site imaging of fissionable objects. This work builds off of the one energy group point kinetics model traditionally used in neutron coincidence and multiplicity counting to develop an expanded point kinetics model applicable to tagged neutron measurements. The expanded model takes advantage of additional knowledge available from a tagged neutron measurement, specifically the timing, energy, and direction of the interrogation source relative to the fissionable object. A method for estimating total multiplication of bare uranium metal objects is developed using this expanded model. Both MCNPX-PoliMi simulations and data from active interrogation measurements of highly-enriched and depleted uranium geometries using a D-T neutron generator as an interrogation source are used to evaluate the potential of this method. Threshold reactions such as (n,2n) and (n,3n) are shown to introduce a non-trivial source of systematic error into the expanded point kinetics model if the interrogation source energy is above the threshold energy for these reactions in uranium isotopes. Despite this source of systematic error, total multiplication can be estimated to within 12% of its simulated value. The magnitude of this systematic error varies with the degree to which the point kinetics model assumptions are violated. Systematic error can be reduced if the enrichment of the uranium object is known. In this case, simulations show that total multiplication can be estimated to within 3% of its simulated value.
