Numerical investigation of flow blockage accident in SFR fuel assembly

Abstract

Sodium-cooled fast reactor (SFR) is one of the most promising candidates to meet the Generation IV International Forum (GIF) declare. Due to the good thermo-physical properties of sodium, SFR has drawn a lot of attentions and there are considerable design experiences related to SFR. In the design of SFR, the pitch to diameter ratio (P/D) is kept small. Debris may accumulate in the flow channels and result in the flow blockage accident due to the corrosion of structural material. If single or several sub-channels are partially or totally occupied by debris, flow area will be suddenly reduced and a recirculation region can be expected downstream the blockage. Coolant temperature in this region can be greatly increased which may threaten the integrity of fuel assembly. In this paper, RANS method is employed to evaluate the impaired heat transfer process governed by blockages formed in a sodium-cooled 61-pin wire-wrapped rod bundle. The simulation results were firstly validated against the experimental data obtained from the literature. A good agreement demonstrated the capability of the employed approach to evaluate turbulent properties of sodium flow as well as the impaired heat transfer characteristics caused by blockages. With the validated method, the influence from blockages are systematically evaluated in which blockage size and its location are varied. Simulation results clearly indicate that a sharp increase of cladding and coolant temperature near the blockage can be expected even if the blockage area is not quite large. The local effect from blockage is also identified in the simulation. The variance of cladding and coolant temperature tends to vanish downstream the blockage and it is not feasible to detect the existence of blockage based on cladding temperature near the outlet even if the blockage area is sufficiently large.