DETERMINATION OF THE PRESSURE LOSS COEFFICIENT AT CONTROL-ROD GUIDE TUBE FLOW-HOLE FOR A PWR NUCLEAR FUEL ASSEMBLY BY USING CFD AND BERNOULLI SOLUTIONS

摘要

For accurate prediction of the margins to boiling inside the guide tube of a PWR fuel assembly the bypass mass flow and the guide tube heat source has to be determined. Historically due to a lack of numerical solution methods a conservative approach based on one-dimensional fluid analysis (e.g. Bernoulli principle, sudden contraction and expansion pressure loss) has been used. In reality the fluid moves in a complicated, three-dimensional way in the vicinity of the flow-hole: there is a sharp flow area change from fuel channel through the flow-hole into the guide tube together with two sharp 90 degree bends. In this study, a three-dimensional CFD model with full channel length has been developed to have a numerical determination of the flow-hole pressure loss coefficient. The ISO 5167 standard small sharp-edged cylindrical orifice was selected to validate the CFD model. The calculated values for the pressure loss coefficients (K_(in)) are in good agreement with the ISO standard. The developed model has been applied to calculate the pressure loss under different process parameters, such as: 1) Control rod position - to study the effect of control rod position on the mass flow in the guide tube; 2) Space grid - to study the guide tube mass flow influence on the effect of the pressure drop in the fuel channel; 3) Mass flow -to study the relation between K_(in) and channel flow Reynolds number.