Coolant stratification in NPP pipelines

Coolant temperature stratification in thermoisolated WWER reactor cooling system pipelines, connecting elements of the circuit with different temperature is experimentally investigated. A facility, allowing one to simulate operational conditions of the NPP main circulation pipeline with temperature disbalance by separate loops, as well as a pipeline for emergency cold water supply from hydrotanks to a ring gap during the primary circuit depressurization is applied. Temperature field by vertical pipeline diameter was measured using 10 mobile microthermocouples. The flow picture was visually observed by introducing aluminium powder or colour. The cold water flow rate was changed within Reynolds number internal Ra=102-104, temperature difference between hot and cold water made up 10-50 deg C. The data obtained show that under Froude member values Er < 3 liquid temperature stratification occurs. A hot water wedge which propogates to a certain lenth and is stabilized is formed under effect of thermogravitational forces in upper part of the pipeline. With cold water flowrate increase, waves with increasing amplitudes appear at the boundary. With turbulence intensity growth in the cold zone the boundary becomes unstable and at Fr>3 destruction and displacement of the upper hot area occurs