In this paper, the influence of the reactor power behavior on the LBLOCA hydrodynamics at the initial moment of the process (0-1 second) is considered. The calculations are carried out for the first loop in case of the beginning of loading, end of loading and constant power. The parameters of a typical reactor plant V-320 (VVER-1000) were used for the calculation, particularly, the 3rd unit of the Kalinin NPP. All initial data for the calculation were obtained from the materials of the international standard problem Kalinin-3 [1]. The fuel composition and kinetics of the reactor core was considered for the case of the end of the cycle. For the calculations, the computational best estimate code ATHLET [2.3] was used, which is included in the software package AC2 and officially acquired by the national research Nuclear University MEPhI according to the license agreement with Gesellschaft fur Anlagen-und Reaktorsicherheit (GRS) gg GmbH, Germany [3]. Code ATHLET is certified in Russia for stationary and transient conditions at reactors with water coolant [4].