Validation of mox core reactivity versus flux variation feedback on delayed neutron data

International audienceThe reactivity of commercial Light Water Reactors (LWR) is given by the reactimeter, which computes the core reactivity from the flux variation measurement. The correspondence is obtained from the kinetics equations with a large 6-10 percent uncertainty due to the delayed neutron (DN) data. This paper presents the first direct validation of the relationship between the measured flux variation and the MOX core reactivity, thanks to the boron reactivity worth measurements in EOLE-MISTRAL2 experiment. The kinetics relationship based on JEFF3 DN data is satisfactory. On the contrary, using B-VII DN data leads to an under-prediction by -8.3 percent $\pm$ 2.7 percent of MOX core reactivity. Considering the satisfactory prediction within 2 percent of the measured $\beta_{eff}$ value, we can conclude that this large boron worth bias is mainly due the DN time dependence in BVII dat