Study of the control-rod shadowing effect on the thermal and fast neutron fluxes in the Bushehr VVER-1000 reactor using the MCNPX Code

In this study, the input data required to model and perform neutronic calculations for the Bushehr Nuclear Power Plant (BNPP) with the MCNPX code were prepared. A comprehensive workflow was developed and implemented to solve the statistical transport equations, from which the neutron flux was obtained. Subsequently, key parameters, including the thermal and fast fluxes, were evaluated at Hot Zero Power (HZP) under three core conditions: a clean core, insertion of control-rod group~10, and insertion of all control-rod groups. The results indicate that, owing to the relatively large spacing and arrangement of control-rod groups~9 and~10, they do not shadow one another; instead, an anti-shadowing effect (ASE) is observed. Consequently, inserting these rods increases the neutron flux in the core, thereby enhancing the overall worth of the control rods.