The work in the presented paper is focused on the investigation of plant behavior in case of Large Break Loss of Coolant Accident (LB LOCA) along with Station Blackout (SBO) sequences for Pressurized Water Reactor (PWR) reactors. A spectrum of large break loss of coolant accident simulations has been performed with sizes for the internal diameter (ID) = 300 mm (12.5%), 500 mm (34.6%) and guillotine break 2x850 mm (2 x 100.0%). The main objective of the presented analysis is to assess the influence of the break size for LB LOCA. The places of selected breaks are located on the cold leg between Main Coolant Pump (MCP) and inlet nozzle of reactor vessel for maximum facilitation of the loss of coolant. The presented analyses are performed with integral computer code ASTEC V2.1.1.3, developed by IRSN and GRS for investigation of severe accident behaviour of different Nuclear Power Plants (NPPs). The selected referent NPP is Kozloduy NPP, unit 6, equipped with Water Water Energy Reactor (VVER 1000) / V320 reactor type. The work is oriented on a core degradation investigation during accident progression for in-vessel phase. The investigation is focused on a comparison of main in-vessel phenomena arising during the progression of the accidents for the selected three large breaks, such as reaching a total reactor core uncover, reactor core heat up, hydrogen generation, failure of fuel cladding and fission product release, core material degradation, melting and relocation to the reactor vessel bottom head.