Abstract: For CPR1000-type reactor units of Hongyanhe Nuclear Power Plant to be built in cold regions, building structure modifications have to be made in the main steam line and main feedwater line (VVP/ARE) compartments. Simulations of pressure response in the compartments were performed to assess the overpressure risks using thermal hydraulic calculation code, and several suggested plugging schemes were investigated to demonstrate their feasibilities for main steam line break (MSLB). The sensitivity analysis was also made to study the impacts of different blasting methods and the best plugging scheme was given.