The paper introduces the fracture accident happened in the bottom mounted instrumentation and secondary core support assembly in RVI of Unit 2 of Yangjiang NPP. Besides, the paper also introduces the damage caused by the bolt fracture to the primary circuit equipment. The paper analyses the cause of fracture in terms of design, manufacture, mounting and operation, and puts forwards some improvement measures. The key measures put forward by this paper can be used as reference for the welding on site for the ...