Optimization of the Photoneutron Flux Emitted by an Electron Accelerator for Neutron Interrogation Applications Using MCNPX and TRIPOLI-4 Monte Carlo Codes and Performance Assessment on 220 Liter Nuclear Waste Mock-Up Drums

Neutron interrogation is a widely used technique to detect special nuclear material. First, neutrons induce fission reaction on the actinides, and then, prompt and delayed particles emitted enable to indicate presence of such material. In a previous study, we demonstrated the feasibility of this technique using the photoneutron flux emitted by a 16 MeV linear electron accelerator. This approach enables to reach average emission intensity on the order of one decade beyond the one produced by deuterium-tritium neutron generators traditionally used for such applications. Higher average emission intensities of the photoneutron flux would enable to expand boundaries of neutron interrogation and enhance mass detection limits. This new study aims at optimizing the photoneutron flux emitted by an electron accelerator with the objective of improving performance of neutron interrogation for nuclear waste drums characterization. In order to ensure accuracy and reliability of our simulation results, two Monte Carlo particle transport codes were used in parallel in this study: MCNPX developed by Los Alamos National Laboratory, and TRIPOLI-4 developed by CEA. Potential discrepancies between results obtained with the two codes were investigated and careful attention was given to minimize the high-energy photon beam contained in the photoneutron flux in order to reduce spurious photofission reactions during measurements. In order to assess experimentally performance enhancement of neutron interrogation further to our photoneutron flux optimization study, experiments were carried out with a neutron interrogation cell based on an electron accelerator at the SAPHIR facility (CEA LIST, France). We considered the case of a 220 liter mock-up drum containing an iron matrix. Different samples of uranium were positioned at the center of the drum and interrogated. Prompt neutrons and delayed neutrons were detected during the irradiation using 3He detectors embedded in polyethylene and surrounded by cadmium. Measurements were conducted with different conversion targets and mass detection limits determined for each target. Experimental results obtained enable to show the potential of electron accelerators to challenge the limits of neutron interrogation.