Evaluation of the postulated fresh fuel drop scenario in the RBMK Ingalina nuclear power plant - 131

This paper is devoted to investigation of single fresh fuel drop scenario that is postulated to occur in the Ignalina Nuclear Power Plant with the modified ARROTTA code in combination with the CASMO-4 and WIMS-D4 lattice codes for cross section generation. The modified ARROTTA code can model the large size of a typical RBMK core, and the unique control rod configuration. It was also extensively validated. In order to properly analyse the dynamic consequences on the fresh fuel drop event, a simple hydraulic model was developed to estimate the drop velocity of the fresh fuel as a function of time. The force balance accounted for gravity, friction between fuel and coolant, local resistance, and buoyancy. The ARROTTA results show that the increase in reactivity is only about 0.3$ when a fresh fuel is dropped into a central empty channel without initiation of any control rod action. The overall core power is increased only by 3.5% although the local power variations are much larger. Therefore, for the Ignalina plant, it has been demonstrated that no core damage occurs for the postulated single fresh fuel drop event without any control or protection system activation. (author)