Refined control rod worth estimation by kinetics equations with included photoneutrons

In this paper we show that in the BR2 reactor, which is reflected and moderated by a beryllium matrix, the reactivity worth of the control rods is strongly influenced by the delayed photoneutrons, emitted after ( , n) – reactions of photons with beryllium. To analyze this reactivity worth, important to evaluate the correct value of the shut down margin, refined estimations of the control rod worth are performed using the reactor kinetics equations including delayed photoneutron groups. The delayed photoneutron parameters were experimentally determined in the BR02 mock-up reactor. The reactivity worth of the control rods are derived from nuclear measurements, which have been performed during the several BR2 shutdowns in 2009. The experimental program included about 30 rod-drop tests and asymptotic reactor period measurements. The importance of the photoneutrons for the reactivity worth determination depends on the chosen analysis method. The reactivity worth from the rod-drop tests is estimated using an approximate equation for the neutron density decay, the validity of which was verified by comparison with the numerical solutions, obtained by the transient code PARETV7.5/ANL. The contribution of the photoneutrons into the reactivity worth, analyzed by this method for the BR2 reactor conditions is 2%. Neglecting the photoneutrons contribution in the inhour equation leads to underestimation of the rod worth by 18%. Comparing the different experimental methods we conclude that the rod-drop test gives more reliable results for the absolute values of the rod worth vs. the reactor period measurement. Finally, a comparison with reactivity worths, calculated by MCNPX are presented.