Thermal hydraulic investigations on porous blockage in a prototype sodium cooled fast reactor fuel pin bundle

• We simulate flow and temperature fields in FBR fuel bundle with porous blockage.
• We perform RANS-based CFD simulation for 217 pin bundle of 7 axial pitch lengths.
• Flow reduction in fuel bundle due to porous internal blockage is estimated.
• Monitoring bulk sodium outlet temperature does not guarantee blockage detection.
• Admissible blockage length to avoid sodium boiling is determined.

Thermal hydraulic characteristics of sodium flow in a prototype fuel subassembly with porous internal blockage have been investigated by computational fluid dynamics (CFD) simulations. CFD solutions for a subassembly having 217 pin bundle with seven helical pitch length were obtained by parallel processing. The CFD model has been validated against benchmark blockage experiment reported in literature. Wide parametric ranges for blockage radius, porosity, mean particle diameter and location of blockage have been considered. Critical length of blockage that would result in local sodium boiling as a function of aforementioned blockage parameters has been estimated and the parametric zone posing risk of sodium boiling has been identified. Attention has been paid to coolant mixing and flow and temperature fields downstream of the blockage zone. It is seen that for a prototype subassembly with various sections contributing to pressure loss, the total flow reduction is <2.5% for all blockages that can lead to local sodium boiling. This suggests, that global bulk sodium temperature monitoring at subassembly outlet is unlikely to detect slowly growing blockages. Comparing the sodium flow and temperature fields in unblocked and blocked bundles, it is found that the wake-induced temperature non-uniformity persist even upto 3 helical pitch length, highlighting that the sodium temperature non-uniformity at the bundle exit can serve as an efficient blockage indicator, provided that the cross-section temperature is mapped by a proper instrumentation. The peak clad temperature is found to be a strong function of porosity, with enhanced clad temperature for smaller porosity. Fuel-clad that are partly exposed to blockage are subjected to large circumferential temperature variation and the resulting huge thermal stress. Further, for a six subchannel blockage with 40% porosity and 0.5 mm mean particle diameter the critical length is 80 mm, whereas for the same blockage the critical length reduces to <7 mm when its porosity reduces to 5%. Six subchannel blockage with 60% porosity and 0.5 mm mean particle diameter, does not induce boiling even up to a blockage height of 400 mm. For a single subchannel blockage with one helical pitch length, there is no risk of sodium boiling even for porosity as low as 5%. The results of the present study would act as safety and monitoring criteria during the operation of the reactor.