ИСТИНА

Development of nuclear power plants required refinement of thermal hydraulic codes. Usage of best estimate three-dimensional code BAGIRA for modeling design-basis accidents, beyond-design-basis accidents and operating conditions of nuclear power plants revealed the weakest points where we faced the most of difficulties. The code BAGIRA has two versions: first – two momentum conservations equations (one for liquid phase and one for gas phase); second – one mixture momentum conservation equation along with drift-flux based closure accounting for slip velocity. In the first version (two momentum conservations equations)the bottleneck is to add different stabilizing summands depending on gradients of flow parameters to the interphase force. General bottlenecks (for both first and second versions) were as follows: low pressure when inaccuracies of themrophysical properties of water and steam vapor leaded to non-physical results; high partial pressure of non-condensable gas together with low partial pressure of steam; problems with analysis of mixture parameters in water boiling or water complete evaporation; problems with implicit numerical scheme when describing interphase heat-mass exchange in cases when the system of differential equations is non-linear. Finite-difference schemewas emendated for qualitative improvement of the code in mentioned above cases. Calculations of three-dimensional flows in VVER-1200 vessel in accident and transient regime were performed.