Methodology for modal analysis at pulsed neutron source experiments in accelerator-driven systems

• Modal analysis of the PNS spatial correction factor (SCF) is performed.
• Many modes are needed to obtain a precise PNS SCF for ADS.
• For VENUS-F, the PNS SCF disappears at the zero crossing of the 9th mode.

The evaluation of experimental techniques for the determination of the subcriticality level of an ADS core relies on reactor point kinetics. The measured parameters depend on the detector position, and the reactivity values are subject to spatial correction factors (SCFs). In this paper the contribution of different eigenmodes to the SCF for the pulsed neutron source (PNS) technique is assessed. Moreover by pulse simulations, precise values of the correction factor are obtained.As case study, the VENUS-F SC1 subcritical core is investigated, with a pulsed neutron source in the center of the core, generated by the GENEPI-3C deuteron accelerator. Much more than 100 modes need to be taken into account to precisely obtain the spatial correction factor for the area method evaluation of this core. Especially the modes with a maximum in the center of the core contribute significantly to the SCF. No spatial correction needs to be applied close to the zeros of the first mode with a maximum in the center of the core (different from the fundamental one). In the reflector zone, the absolute reactivity level is overestimated.