An original method to determine the onset of conditions for reliabilitycritical hydraulic impacts for reliability analysis of active safety systems of nuclear power installations is proposed. The suggested method is based on determining the effect of head-flow characteristic delay onto hydraulic impact preconditions during the changes of pipeline system hydrodynamic parameters under transitional modes (e.g. in pump start-up). The delay time of responses to change in the hydrodynamic system parameters embodies the determining factor of head-flow characteristic’s inertance, depending on both design and technical parameters of system components (including pumps), and the hydrodynamic parameters change rate under transitional modes. Using the proposed method, the analysis of conditions for critical hydraulic impacts is performed for the primary high-pressure safety injection system of serial WWER-1000 nuclear power plants. The analysis results allow a conclusion that for this system the conditions leading to hydraulic impact due to the pump start are not reached. The developed method can be applied to any thermal and nuclear power facilities’ pipeline systems equipped with pumps.

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The analysis of well-known studies in modelling conditions for water hammers in equipment and components of pipeline systems has revealed that definition of conditions and parameters of water hammers in the transonic modes of single-and two-phase flows (at a speed of propagation of acoustic disturbances) is the least studied problem.The original method is proposed for determining the conditions and parameters of water hammers in transonic flow modes subject to the transition of the kinetic energy of the flow stagnation into the energy of the water hammer pulse.It was found that the simulated hydrodynamic loads in transonic modes can significantly exceed the corresponding known recommendations of N. Zhukovsky.The proposed method of equations computer modelling served to determine the criteria range for water hammers due to aperiodic thermohydrodynamic instability in transonic flow modes. K e y w o r d s: water hammer, transonic modes, head-flow characteristic.

The research presents an analysis of the known results in modeling the maximum design accident (MDA) using the code RELAP5/V3.2 whith Westinghouse fuel assemblies' (WFA) diversification in WWER-1000 reactors. According to the known results of MDA calculated model simulation with RELAP5/V3.2 code at the maximum allowable water temperature in the heat WWER emergency cooling system exchanger (90 ºС), the fuel elements' claddings temperature reaches 1320 ºС and exceeds the admissible nuclear safety limit (1200 ºС). Thus, according to known results, these MDA whith WFA engaged pass from the "design" accident status to the "severe" acciden status and means a decrease in safety in relation to the FA-A fuel assemblies. The alternative MDA analysis for WFA-equipped plants showed that, unlike the known calculations, the nuclear safety limit on the maximum permissible fuel cladding temperature is not violated and never reduces the overall safety level in WWER diversification with WFA fuel assemblies.

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