During Double End Break Loss of Coolant Accident (DEBLOCA) in VVER1000 reactor, in the first second, pressure waves had been noticed in all points hitting the main components of the primary circuit. Studying the stresses due to DEBLOCA is important to avoid equipment failure. The main attention is focused on the pressure drops on the equipment, because this parameter determines the dynamic loads on the equipment, which can lead to its failure. ATHLET code is used to obtain pressure and temperature variation with time during the accident for the VVER1000 reactor Kalinin-3. The results are used in ANSYS Finite element package to calculate stresses in steam generator collector. The stress just before the emergency (at time 0.014 sec) was 4.3e7 pa and becomes 4.38e6 pa just after the emergency (at time =0.022 second). Fluctuations appear with two peaks of highest amplitude of 1.87e7 and 2.38e7 pa at time of 0.06 and 0.094 sec respectively which is well below the allowable stress. The results indicate safe operation of the equipment.