In the context of GEN-IV Heavy Liquid Metal-cooled nuclear reactors safety studies, the flow blockage in a fuel sub-assembly is one of the most important and realistic accident in Lead Fast Reactor. Because of the obvious and inherent problems with experimental tests, modelling the accident via a 3D CFD code is the only available option. This paper shows that a detailed analysis of the flow blockage using a 3D CFD model provides a detailed thermo-fluid dynamic picture in the analyzed cases of blockage. The closed hexagonal, grid-spaced fuel assembly of the LFR ALFRED (Advanced Lead Fast Reactor European Demonstrator) is modeled and the analisysis performed with ANSYS FLUENT 15.1. The results of the 3D CFD model are in agreement with a system code while providing better spatial and temporal resolution and indicate that beyond-design conditions are reached only with blockage larger than 30% in terms of area fraction, bringing to clad temperatures of ~900 °C.
The 3D CFD model shows two effects: a local effect mainly in the wake/recirculation region immediately downstream of the blockage and a global effect caused by the lower mass flow rate in the blocked subchannels; the former effect gives rise to a temperature peak right behind the blockage and is dominant for large blockages (>20%), while the latter effect causes a temperature peak near the end of the active region of the fuel sub-assembly and is dominant for small blockages (<10%).
A number of transient analyses at different values of blockage were carried out with fully resolved SST-omega turbulence mode. These analyses show that a blockage of ~15% (in terms of area fraction) leads to a maximum clad temperature of ~800 °C in ~3-4 s. Localized clad temperature of ~1000 °C is reached with blockages of 30% or more. Blockages of >15% could be detected via thermocouples at selected locations in the plenum region of the FA.