Thermohydraulic CFD study on the residual power's extraction in a IV Generation Sodium Cooled Fast Reactor

Thermohydraulic CFD study on the residual power's extraction in a IV Generation Sodium Cooled Fast Reactor : In 4th generation Sodium-cooled Fast Reactor a situation of unprotected loss of flow without scram can lead to a severe accident scenario with a partial melt down of the core. The mixture of steel and fuel (debris) is relocated in the core catcher. A 3-D CFD simulation, with NEPTUNE_CFD code coupled with SYRTHES (for the conduction in solids), has been performed so to analyse the behaviour of the reactor in this condition focusing on the role of two decay heat removal systems (one in-vessel and the other ex-vessel). An assessment of the ex-vessel system (named RRC) needs to be done before the CFD. For this purpose, a steady state 0-D model has been implemented in which a sensitivity analysis on the emissivity of the safety vessel has been carried out. This analysis allowed the derivation of a function for the heat flux extracted through the main vessel by the RRC. It has been shown that the flux extracted, and so the power too, can be approximated to a 2nd order polynomial function on the primary sodium temperature. The temperature of the primary sodium is so the main parameter affecting the performance of the system. The power extracted with a temperature of 700°C has been computed being more than three times higher than the one extracted at 400°C. A 0-D transient study has been performed to show the evolution of the cold and hot pool average temperatures. It is observed that the in-vessel system (named RRB) affects more the transient of the average quantities than the RRC. The functions extracted from the 0-D model have been used as boundary condition applied to the main vessel in the 3-D CFD simulation. The purposes of this simulation consist in having a good prediction of the natural convection in all the parts of the reactor, estimating the thermal stratification and analysing the debris coolant interaction (monitoring local temperatures).