Numerical simulations of sodium mixing in a T-junction

Thermal fatigue is a major problem for liquid metal fast reactors due to the high temperature differences of the coolant in the circuits and because liquid metals efficiently transmit thermal fluctuations to walls. Detailed thermal hydraulic investigations have been carried out to quantify the amplitudes and frequencies of the temperature fluctuations in the secondary sodium circuit of the Phenix pool type fast reactor. Computational fluid dynamics calculations have been performed using either Reynolds Averaged Navier Stokes equations or Large Eddy Simulations. The characteristics of the mixing jet are well reproduced by the simulations and the high thermal fluctuation zones fit the thermal crack locations observed in the T-junction. Simulations with conjugate heat transfers, which allow inter- changing heat at the wall between fluid and solid, lead to noticeable heat transfers in the pipe wall and to high surface heat flux at fluid/solid interface.