During protected loss of flow (PLOF) accidents in fast breeder reactors the temperature at
the core outlet experiences variation depending on the pump coastdown. In case of long flow
coastdown the temperature at the core outlet will decrease and sodium stratification will
occur in the upper plenum due to the effect of gravity. The geometrical characteristics of
nuclear reactor such as Monju (presence of shrouds with circumferential holes), can
moreover create currents with different temperature which move towards the hot-leg piping
system. As a result uneven temperature distribution is likely to appear at the inlet of the pipe
which can set the conditions for occurrence of thermal stratification in horizontal piping.
In the present work CFD tools are employed as a validation of an experimental benchmark
for the understanding of essential phenomena occurring in a thermally stratified flows in
pipes with bends. Moreover the discussion of such events is connected to their influence in
association with natural convection establishment inside the reactor.
Finally the numerical results point out the impact of the chosen turbulence modeling on the
prediction of the experimental data and discussion is provided as an attempt to motivate the
encountered issues as a starting point for a future development.