Analysis of prompt decay experiments for ADS reactivity monitoring at VENUS-F facility

The strategy chosen to monitor a subcritical core reactivity combines two approaches: 1) the on-line monitoring of the relative fluctuations of the reactivity ρ(t), due to the measurement of the ratio of the beam intensity to the reactor power; 2) some calibration measurements, performed regularly, providing an absolute level of ρ. These absolute or calibration measurements are performed during beam interruptions and are based on the analysis of the time decay of the reactor neutron population, N(t). Different analysis techniques can be applied, but most of them require analysing both prompt and delayed neutron contributions (as the area method). Consequently, to give precise results, these techniques require the use of long beam interruptions of several ms durations. This paper focuses on the prompt decay part of N(t) and presents a method to determine the prompt multiplication factor k_p, from which the k_eff can be inferred. This method requires shorter beam interruptions of a few tens of microseconds. The method is applied to Pulsed Neutron Source (PNS) measurements at the VENUS-F facility. The VENUS-F Facility is a zero power ADS mock-up and consists in the vertical coupling of the VENUS reactor (at SCK-CEN, Mol, Belgium) with a deuteron accelerator GENEPI-3C