Abstract: According to the equation presented in paper, the intervals from the time the operator is made aware of an unplanned boron dilution accident to the time a loss of shutdown margin occurs on the vari- ous conditions are calculated by using the primary coolant volume, the volume flow of boron dilution water and the initial and critical boron concentrations on each condition. The assumptions are conservative and so the results computed are safe. The comparison is made between results of this paper and the safety analysis report of Qin Shan power plant.