A CFD study was carried out on fluid flow and heat transfer in the Lead-cooled Fuel Pin Bundle of the ALFRED LFR DEMO. In the context of GEN-IV heavy liquid metal-cooled reactors 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 paper is a first step toward a detailed analysis of such phenomena, and a CFD model and approach are presented to have a detailed thermo-fluid dynamic picture in the case of blockage. In particular the closed hexagonal, grid-spaced fuel assembly of the LFR ALFRED was modeled and computed. At this stage, the details of the spacer grids were not included, but a conservative analysis has been carried out based on the current main geometrical and physical features. Reactivity feedback, as well as axial power profile, were not included in this analysis. Results indicate that critical conditions, with clad temperatures around ∼900 °C, are reached with blockage larger than 30% in terms of area fraction. Two main effects can be distinguished: a local effect in the wake/recirculation region downstream the blockage and a global effect due to the lower mass flow rate in the blocked subchannels; the former effect gives rise to a temperature peak behind the blockage and it is dominant for large blockages (>20%), while the latter effect determines a temperature peak at the end of the active region and it is dominant for small blockages (<10%). The blockage area was placed at the beginning of the active region, so that both overmentioned phenomena can fully take place. The mass flow rate at the different degree of blockage was imposed from preliminary system code simulations. Transient analyses with fully resolved SST-omega turbulence model were carried out and results indicate that a blockage of ∼15% (in terms of blocked area) leads to a maximum clad temperature around 800 °C, and this condition is reached in a characteristic time of 3-4 s without overshoot. Local clad temperatures around 1000 °C can be reached for blockages of 30% or more. CFD simulations indicate that blockages >15% could be detected by putting some thermocouples in the plenum region of the FA. © 2014 Elsevier B.V.
|Titolo:||A CFD analysis of flow blockage phenomena in ALFRED LFR demo fuel assembly|
|Data di pubblicazione:||2014|
|Appare nelle tipologie:||1.1 Articolo in rivista|