Numerical Analysis of Flow Blockage Phenomena in the ALFRED FA: Preliminary Study on Neutronic Feedback

In the context of GEN-IV Heavy Liquid Metal safety studies, the flow blockage in a Fuel sub-assembly is considered one of the main issues to be addressed and the most important and realistic accident for LFR Fuel Assembly. The present document is a first step towards a detailed analysis of such phenomena, and a CFD model and approach is presented to have a detailed thermo-fluid dynamic picture in the case of blockage. In particular, the internal flow blockage in the ALFRED hexagonal wrapped Fuel Assembly is presented here. At this stage, a conservative analysis has been carried out basing on the main geometrical and physical features. In the first part of the work, reactivity feedback, as well as axial power profile, was not considered. Nevertheless, relevant indications derives from the analysis of the results. Results indicate that critical conditions, with clad temperatures around ~900°C, are reached with blockage larger than 30% in terms of area fraction. Transient analyses with fully resolved SST- turbulence model were also carried out. In the second part of the work, a first evaluation of the neutronic feedback is given. The quantification is performed by a one-way coupling of the neutronic deterministic code ERANOS and the CFD code CFX. Results indicates that the influence of the neutronic feedback on the power is around 10% and therefore a preliminary safety evaluation provides results which support current fuel assembly design.