MUSE-4 Pulsed Neutrons Source (PNS) experiments modeling using the Monte Carlo transport code TRIPOLI-4®

In this work, we revisit the modeling of two MUSE-4 Pulsed Neutrons Source (PNS) experiments using the CEA Monte Carlo code TRIPOLI-4® and JEFF-3.1.1 nuclear data library. The MUSE-4 experiments were carried out in the MASURCA zero-power facility at CEA Cadarache between 2000 and 2004, in order to characterize ADS (Accelerator Driven Systems) and ways of monitoring their reactivity. We investigate two subcritical configurations, with respective multiplication factors around 0.97 and 0.957 and detectors placed at several positions across the core, reflector and shield. For each configuration we model the prompt neutron kinetic response after a single pulse of neutrons, the floor plateau resulting from precursor build-up after a long series of pulses using the area-method and the intrinsic source from the plutonium-based (MOx) fuel. The transient, which represents the response of the subcritical core to a burst of neutrons, is measured with a set of fission chambers. Each of these fission chambers is individually modeled in the Monte Carlo code TRIPOLI-4®. Despite the uncertainties in the experimental protocol, such as the exact loading maps of the reactor core and detector efficiencies, the time series of the transient that we obtain match the experimental data in both the timing and shape of the peak, and overall reproduce the key behaviors observed during the MUSE-4 PNS experiments. However, for the lowest reactivity and detectors far from the core, some discrepancies are observed in the shape of the decreasing part of the prompt neutron population. The origin of these discrepancies is likely multifactorial: they may arise from experimental uncertainties or from biases in steel cross-sections at low energies, as neutrons reaching these detectors have traveled long paths. Another possible factor is the hydrogen in the concrete surrounding the core, which could reflect slowed-down neutrons, as observed during the FREYA experiment. The methodology to estimate the plateau value and some tentative explanations on the discrepancies observed are provided in the article. • Reanalysis of MUSE-4’s pulsed neutrons source experiment with the Monte Carlo Code Tripoli-4®. • The area ratio method is used to determine the asymptotic level created by the buildup of precursors. • MUSE-4’s Experiments are reconstructed with the asymptotic plateau modeled. • The ability of Tripoli-4®to model a PNS transient with the reflector effect on very subcritical configuration is demonstrated.