Numerical Analysis on Thermal-Hydraulics of Supercritical Water Flowing in a Tight-Lattice Fuel Bundle

To evaluate thermal hydraulic characteristics of a tight-lattice fuel bundle of a supercritical water reactor (Super Fast Reactor), a simplified 19-rod fuel assembly was analyzed with a three-dimensional two-fluid model analysis code ACE-3D which has been enhanced by Japan Atomic Energy Agency. In ACE-3D, a two-phase flow turbulent model based on the k- model was adopted. In this calculation, a one-twelfth model is adopted as the computational domain and takes advantage of symmetry. As boundary conditions, mass velocity, inlet enthalpy and power per rod are to be the same as the steady state condition of the Super Fast Reactor. Cross-sectional local power distribution in the fuel assembly is set to be flat. The effect of grid spacers is taken into account in the analysis. Calculated rod surface temperatures take peak values near the top of the rods. The maximum cladding surface temperature (MCST) is observed at the position facing the narrowest gap on the center rod near the outlet and the value is 901 K (628 °C). It was confirmed that the predicted MCST satisfies the thermal design criteria to ensure fuel and cladding integrity: the MCST should be less than 650 °C.