Monte Carlo assessment of spatial and energy effects in the VENUS-F subcritical configurations and application for reactivity determination

Experiments with a subcritical core and an external pulsed neutron source (PNS) have been performed at the VENUS-F reactor of the SCK-CEN during the year 2012 within the frame of the FREYA project of the 7th European Framework Programme. Different reactivity levels, obtained by varying the position of the control rods and the safety rods, have been investigated, with values of k_eff ranging between 0.9 and 0.96. The area-ratio and the prompt decay constant techniques are well-known techniques to determine the reactivity of the system from the results of PNS experiments. However, both techniques are based on the point kinetics model and hence they are affected by the spatial and energy effects present in real systems, which will cause them to produce biased values of the reactivity. In this work, we apply a methodology based on Monte Carlo simulations to assess the spatial and energy effects present in the VENUS-F core and to determine relationships between the reactivity and the measured parameters (area-ratio and prompt decay constant) that take into account these effects and allow the application of the reactivity determination techniques to obtain accurate reactivity values beyond the limits of the point kinetics model. The results of the validation of this methodology against the experimental results obtained in the VENUS-F reactor will also be discussed.