Theoretical description of two-phase flow in the primary circuit of AST-500 heat surrly reactor

The paper deals with the problems of theoretical modelling of two-phase flow in the core and riser section of AST-500 reactor. A two-phase model realized in FLOCAL code is presented that has been developed for this purpouse but is actually employed in a wider area. This model has been used for a methodical study of thermal-physical effects. Calculations performed for non-stationary regimes revealed coolant flow rate oscilations that were interprited as a dynamic instability with density waves. The oscilations arrearance and amplitude depend in the first instance on the steam weight content at the core outlet. An oscilation amplitude decrease was observed with the power reduction and the rise of hudraulic resistance at the channel inlet at fixed steam content levels. The established stability conditions are in a good agreement with the published data