The description of the complex flow of coolant water with particles is necessary to evaluate safety relevant effects of the sedimentation of isolation material on sump-sieves in nuclear power station. Classifying and modeling of the different phenomena maybe important in the case of a coolant accident, because the isolation material can be transported into the reactor containment, the building sump of the containment and into the associated systems [1]. In order to ensure the heat dissipation from the reactor core and the containment the cooling systems transport the water from the sump into the condensation chamber and then into the reactor pressure vessel. The functionality of the pumps can be affected by a high allocation of the sieves with fractionated isolation material. In this case the heat dissipation from fuel elements is not guaranteed. The transport of the material will be simulated with the CFD-code. The modeling of the flow with particles is very complex, because of the structure of the particles and their interaction with the fluid. There are different classes of particles with different attributes, e.g. sinking velocity. So one needs more than one disperse phase to describe the whole process, which is associated with a lot of computing power and not realizable for large geometries. The paper deals with experimental and methodical activities for the description of the agglomeration and the break-up of isolation material in fluid flow. The aim of this work is to describe the evolution of the volume parts of the different particle classes turbulent flows depending on the time in. The modeling phase starts with a very simple model to describe 3 particle classes (x, y, z) and results in a differential equation system with 3 equations. To describe all classes the model has to be expanded. Therefore the Lindenmayer-System approach has been adopted. These systems can be taken in cases where self-similarity takes place. The result is a differential equation system with iterations for the three classes (x(i), y(i), z(i)), with i as the parameter for the number of subclasses. The values for the agglomeration and break-up rates will be taken from experiments. As a result a model has been created which describes the evaluation of the different particles classes in turbulent flow. It helps to choose the correct particle class in the CFD simulation depending on the situation to simulate.

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