The SGTR studies correspond to the double-ended guillotine rupture of a steam generator tube, which allows unimped blowdown from both ends of the severed tube. Occurrence of the SGTR accident leads to an increase in contamination of the secondary system as well as activity release to atmosphere due to leakage of radioactive coolant from the RCS. Therefore the automatic or operator actions should be taken in time to terminate the leakage. For ACP1000, the time window of the operator action is prolonged by adding new protection signals and equipment. In present work, the maximal time interval of the operator action after the SGTR is studied with these new measures. And after the operator takes actions, in accordance with the procedure, no SG overflow takes place till the pressure of the primary side is equal to that of the secondary side.