Reactivity worth calculation for control rods groups of the VVER-1000 nuclear reactor core considering shadowing effect using Monte Carlo method

Global core calculations use the diffusion equation to predict theoretically the nuclear reactor behaviour. However, this equation is not valid in strong absorbing media where the neutron spectrum is a rapidly varying function of the position, such as control rods or burnable poisons. In this paper, to overcome this misleading, the Monte Carlo simulation has been performed and the VVER-1000 reactor core in the MCL (Reactor Minimally Controlled Power Level) condition is modelled using the MCNPX code to calculate the reactivity worth of the control rod groups. The calculations in this model are divided into four steps. At first, the integral and differential worth are calculated for control groups 8, 9 and 10 with 50% overlapping and shadowing effect is considered. And in three other steps the integral and differential reactivity worths of control groups 8, 9 and 10 are calculated separately (without overlapping). In each step, the core is maintained critical by variation of the boron concentration. In these processes, the boric acid coefficient is achieved while the core is critical. The results are compared with experimental values and are in good agreement with them.