Numerical Analysis of Thermal and Hydro Dynamical Processes in Upper Fuel Channel Part of Boiling Water Reactor

This article is the second part of the 3D numerical analysis of the thermal and hydro dynamical processes which take place inside the fuel channel of the RBMK-1500 reactor during the interaction between the coolant and the fuel bundles. This article covers the results devoted to the upper part of the fuel channel, meanwhile, the preceding investigation [3] focuses on the processes, which run inside the lower part of the fuel channels of the RBMK-1500 reactor. The evaluation of the pressure drop, coolant temperature rise, flow velocity and steam mass fraction changes at the distance grids (spacers) and at the heat transfer intensification grids-intensifiers of the upper part of the fuel channel was carried out using the ANSYS CFX software. It was stated that the heat transfer intensification grids (intensifiers) increased the turbulence of the coolant (water-steam mixture) and influenced on the rise of the heat transfer rate and on the intensity of the steam generation. The average value of the heat transfer coefficient in the upper part of the channel was equal to 40000 W/(m(2).K) and was about 14% higher than in the lower part of the channel. Differently from the lower part of the channel, the heat transfer coefficient increased at the heat transfer intensification grids and did not return back to the previous values after passing through them. The main reason of that was the turbulence of the water-steam mixture and vortex formation. All those processes have influenced on the more intensive wetting of the fuel elements by the water droplets and on the more intensive droplets evaporation.