Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

Pressurized Thermal Shock (PTS) and Direct Contact Condensation (DCC) were identified by the European project EUROFASTNET as two of the most important industrial needs related to nuclear reactor safety where CFD may bring a real benefit. One typical PTS scenario limiting the Reactor Pressure Vessel (RPV) lifetime is cold water Emergency Core Cooling (ECC) injection into the cold leg during a hypothetical SB-LOCA. The injected water mixes with the hot fluid present in the cold leg and the mixture flows towards the downcomer where further mixing with the ambient fluid takes place. Such a scenario may lead to high thermal gradients in the structural components and consequently to thermal stresses. Therefore, the loads upon the RPV must be reliably assessed. The NURESIM sub-project 2 (Thermohydraulics) Work Package 2.1 focuses on a two-phase flow configuration resulting from a partially or fully uncovered cold leg. In the case of a partially uncovered cold leg, a stratification of cold water on the bottom of the cold leg with counter-current flow of hot water and steam on top of this cold-water layer may occur. There is mixing between hot and cold water. Condensation takes place at the free surfaces between steam and water, e.g. at the cooling water jet. Mixing and condensation are strongly dependent on the turbulence in the fluids. Reliable numerical simulations are required. Two-phase PTS constitutes one of the most challenging exercises for a Computational Fluid Dynamics (CFD) simulation. Presently available CFD tools are not yet able to reproduce all the separate phenomena taking place in the cold leg and the downcomer during the ECC injection, let alone an accurate simulation of the whole process. Improvements of the two-phase modelling capabilities have to be undertaken to qualify the codes for the simulation of such flows. A really accurate simulation of all the phenomena that occur in the scenario will only be possible in the far future and a step-by-step improvement of the quality of the forecasts is necessary. However, a reasonable prediction of the most important phenomena may be reached in a short or medium term and the use of CFD in industrial studies related to PTS is already possible in the frame of some limitations.