Ablation of a solid by an immersed jet

Abstract

Issues related to the occurrence of a severe accident with melting of the core are considered as early as in the design phase of nuclear reactors. For sodium cooled fast reactors (SFR-Na), two mitigation devices are set up in the reactor vessel. The first one is ”transfer tubes” that are used to relocate the corium in the lower part of the vessel; these tubes are arranged in the fuel assembly. The second one is called ”core-catcher” and is supposed to receive the corium melt in the lower part of the reactor vessel. This device must absolutely be correctly designed to resist to the thermal ablation which undoubtedly will occur since the corium melt temperature is very high. In this context, experimental studies are carried out to fully understand the ablation process with simulating fluids, the final goal being to accurately simulate it in realistic conditions. Thus, an experimental study of a simplified situation is exposed using simulating materials: the core-catcher material is replaced by transparent ice and corium by a hot water jet. In this paper, the ablation of an ice block by an immersed water jet is investigated. Experiments were performed with an initial nozzle-to-solid distance H/Dj=10 and the Reynolds number ranged between ∼104 and ∼1.5×105. First, heat transfer is characterized at the stagnation point by analyzing pictures taken during cavity formation. In a second step, the cavity shape evolution is deeply analyzed to get more information about heat transfer in the whole cavity. These results help understanding the complex mechanisms at play during ablation and will be used to validate numerical simulations.