Occurrence of thermal stratification in sodium cooled fast reactor piping

•Specific governing parameters of thermal stratification in piping are proposed.•Occurrence and amplitude of stratification based on water tests are reported.•Thermal stratification test performed in Phenix reactor is described.•Water and sodium results are compared on the basis of proposed parameters.

Sodium cooled fast reactors (SFRs) are operating in a wide range of temperature. For some off-normal conditions, the flow rate is drastically reduced and large temperature variation occurs. In such transient situations, buoyancy influence can be very important and thermal stratification can appear in the horizontal portions of the piping, particularly in secondary circuits. From the viewpoint of thermal stress analysis in the piping, the occurrence of thermal stratification must be predicted to prevent any crack or damage leading to a sodium leak.The paper first introduces a qualitative description of the influence of the buoyancy forces on the flow behaviour in piping, depending on the piping geometry. Then, dimensionless parameters which play a role in the occurrence, the amplitude and the duration of thermal stratification are recalled. Past experimental results obtained in a long horizontal pipe using water as a simulant fluid are presented and new dimensionless parameters are proposed. Measurement of thermal stratification in one secondary circuit of Phenix reactor during the ultimate tests performed in 2009 is described.The water and sodium results are compared on the basis of the proposed dimensionless parameters. A prediction of the risk of thermal stratification in piping based on these parameters seems possible. Nevertheless, it is shown that the influence of piping geometry and specific boundary conditions can play a significant role in the stratification amplitude.