[en] It is necessary to determine switch over time to simultaneous cold an hot leg safety injection during LOCA long-term core cooling. It has a significant effect on avoiding boron crystallization in the core and any return to criticality. The author introduces the analyses of LOCA long-term core cooling for Guangdong Daya Bay Nuclear Power station 18 Months Fuel Cycle Project. The boron concentrations in the reactor core and in the containment sump versus time during LOCA are calculated using REFLET computer code. The minimum and maximum allowable boron concentration in the refueling water storage tank as a function of switch over time are provided. The operator must change the SI configuration from cold injection model to simultaneous cold and hot leg safety injection before 6h when boron concentration in RWST is 2204 mg/L