Gas-cooled fast neutron reactors have many advantages as far as the fuel cycle is concerned. However, since Helium is not a very efficient coolant, the residual heat removal must be studied in detail. To reach the required performances, even in the case of the residual heat removal after the drop of control rod, it can be necessary to keep Helium under pressure and to keep forced convection. Specific loops are dedicated to the residual heat removal, which by-pass the primary loop and have their own heat exchanger. They are called DHR loops (Decay Heat Removal).
That is why the detailed study of the residual heat removal in an incidental situation is an important issue: in depressurization conditions and sudden stop of the reactor, one must be able to predict the duration and the intensity of the possibly necessary forced convection. The development of natural convection depends primarily on the gas temperature that flows into the DHR loops. Now, this temperature can be highly influenced by the three-dimensional behaviour of Helium in the plenum: if the flow is stratified, the very hot gas is blocked in the plenum and natural convection will take time to start. To study the influence of these 3D effects in the upper plenum, we work on the coupling between the Trio_U and Cathare codes. The Trio_U code is in charge of modelling the mixing and the stratification of the Helium fluxes getting out of the core and of deducing the gas temperature that flows out towards the DHR loops.