Numerical comparison of the effect of different lower plenum flow diffusers on the core inlet flow distribution of a PWR with Code_Saturne

Reactor pressure vessel internal structures, especially those located in the lower plenum, may have a significant influence on the core-inlet flow-rate distribution; strongly depending on their geometry. EDF aims to design a new reactor with a more homogeneous core-inlet flow-rate by optimizing the flow diffuser. For that EDF have realized the comparative study of six existing designs with adapted structures on internal components in the lower plenum, which serve as flow distributor. The typical structures are ringed matrix, drill drum, flow skirt, support columns, perforated plates and so on. Each of them is intended to both inhibit the formation of vortices and swirls induced by the U-turn flow from the down comer and make the flow distribution more homogenous as the coolant enters the core inlet. Based on a Physical Analysis and Ranking Table (PIRT) methodology, the important thermal-hydraulic phenomena and key parameters of different flow diffusers, which will influence the flow distribution on core entrance, were identified and listed for further parametric studies. The flow diffuser structure of the six types reactors were adjusted into 1/5 scale EDF generic “BORA¹” 4 loops mock-up to have a comparable basis. The strategy is to only change the flow diffuser structure in a fixed bottom head vessel in order to better analysis the diffuser geometry impact on the flow distribution. In this study, in order to examine the effect of the internal structure in the lower plenum, on the accuracy of prediction for the BORA mock-up flow distribution, simulations with simplification based on real geometry were conducted with the EDF in-house open source software, Salomé² and Code_Saturne³. Finally, it has been shown that CFD: - can be used to help compare the efficiency of the different existing world-wide diffuser concepts, - has the capacity to help the reactor designers conceive new models.