In order to enhance the efficiency and obtain more safety margin in the new type nuclear power plant design, it is of great necessity and significance to study the potential probability of vessel structure design improvement which is based on CPR1000 NP. In this paper, LB LOCA is chosen as the typical accident with CATHARE GB code and the DRM applied. Sensitivity study of in-vessel structure, length of fuel rods and the water inventory are performed. The result shows that by reducing the length between the vessel bottom and the core inlet, extending the fuel rod length and increasing primary water inventory, the PCT declines respectively. More than 200 K temperature benefit can be obtained when the three modifications mentioned above combined together. Due to the sufficient margin increased, a proper power uprating is feasible.

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