CFD investigations of thermal hydraulic characteristics and consequences of flow blockage in fast reactor fuel subassemblies

Helical wire-wrapped fuel pins of sodium cooled fast reactors (SFR) are subjected to high heat flux of ~ 2MW/m² and hence the fuel clad faces sustained high temperature leading to large creep damage. The gap between the hexcan wall and peripheral pins offers a low resistance flow passage compared to the central region, causing significant flow bypass towards the peripheral subchannels (SC). Apart from supporting the fuel pins against flow induced vibrations, the spacer wires induce prominent transverse flow. As consequences of these, there are periodic flow variations in the subchannels along the stream-wise direction. Comparing the sodium flow and temperature fields in unblocked and blocked bundles, it is found that, the wake induced, temperature non-uniformity persists even up to 3 helical pitch lengths. This suggests that the sodium temperature non-uniformity at the bundle exit can serve as an efficient blockage indicator, provided the cross section temperature is mapped by a proper instrumentation