The VVER-1200/AES-2006 is one of the most promising nuclear reactors for power generation. This work provides an in-depth analysis of the void fraction effect on the operation of VVER-1200 using MCNPX code. The void fraction in the VVER-1200 may be result from the loss of coolant accident (LOCA) or boiling the moderator/coolant materials. The fission multiplicities (υ), the estimated of the recoverable energy per fission (Q) and the effective delayed neutrons fraction (βeff) have been investigated at different fuel enrichment and a different void fraction. It is beneficial to calculate the βeff at the different void fraction and different enrichment, due to its impact on the reactor power change rate. The excess reactivity in the reaction can be controlled using the gadolinium dioxide or boron carbide. Increasing the void fraction decrease the burnup rate of gadolinium dioxide and boron carbide in the VVER-1200 assembly. The Doppler coefficient has a large effect on the reactivity of the reaction. Therefore, it is important to calculate the Doppler coefficient at different fuel temperature. The existence of the void fraction increases the concentration of the most hazardous radioactive nuclear waste products.