Simulation of Active Assay of Fresh Fuel Assemblies with Burnable Poisons using Organic Scintillators

The uranium neutron collar (UNCL) is one of the current instruments for assaying fresh low-enriched uranium fuel assemblies; this instrument uses He-3 gas proportional counters embedded in polyethylene to detect the neutron doubles rate. The system uses at least one neutron interrogative source (typically americium-lithium (AmLi)) to induce fissions within the assembly of interest. A current challenge in fresh-fuel assay is the presence of burnable poisons (BPs) in the assemblies. To achieve higher burnup levels, the fuel in modern reactors is more enriched in U235 than the fuel of traditional reactors, and thus modern reactors require more safety measures. BPs, such as gadolinium, have a large thermal neutron capture cross section and they are used to maintain a safe, constant reactivity. While BP capture of thermal neutrons is beneficial in reactor safety, it is detrimental to assay efforts since BPs can capture the thermal neutrons before the He3 proportional counters can. If two assemblies with equal amounts of U-235 and different amounts of gadolinium are measured with a UNCL, the assembly with more gadolinium will yield a smaller neutron doubles rate. The estimated mass of the assembly containing more gadolinium will therefore be smaller than the estimated mass of the assembly with less gadolinium. Organic scintillators are fast-neutron detectors that do not require any moderation of the neutron prior to detection and could be used in an UNCL-type system. A system that observes only fast fission neutrons may be less affected by the change in fission dynamics due to the BPs. Operating at a higher detection threshold could allow for a mass-calibration curve that is less dependent on the gadolinium content. Currently, an a priori declaration of the gadolinium content is required to implement suitable correction factors. It is of interest to investigate techniques that do not rely on this declaration. Furthermore, organic scintillators allow new signatures to be measured (e.g. gamma rays and the angular/spatial distribution of emitted neutrons) while also improving measurement precision. Simulation studies were performed in MCNPX-PoliMi to emulate an active assay system comprising organic scintillators. Several configurations and analysis techniques beyond counting neutron doubles are studied. This paper will detail the simulated comparison of the organic scintillator based UNCL to the current He-3 based UNCL.