CFD simulation of fibre material transport in a PWR core under loss of coolant conditions

The aim of the numerical simulations carried out in this study was to determine how and where mineral wool fibres transported to the core by ECC water during a LOCA are deposited across the grid spacers of the fuel elements of a German PWR. The spacer grid is modelled as a strainer which completely retains the insulation material carried by the coolant and reaching the plane of the spacers. The accumulation of the insulation material gives rise to the formation of a compressible fibrous cake whose permeability to the coolant flow is calculated in terms of the local amount of deposited material and the local value of the superficial liquid velocity. The calculations showed that the fiber material at the uppermost spacer grid plane is not evenly distributed. First, it is accumulated at the positions of the break-through channels. Later when the inner circulation in the core has stopped, the insulation material can also be distributed into other regions of the spacer plane. Further investigations are necessary to determine the accumulation of insulation material for a longer period of time. Also steam production in the core or re-suspension of the insulation material during back flow should be considered. Moreover, the geometry modeling should be improved taking into account the real structures in the upper plenum and the geometry of the ECC injection nozzle ('Hutze'). (orig.)