Three dimensional numerical simulation is conducted on the CANDU Moderator Test Facility (MTF). Heat generation inside the tank is modeled through surface heating. Transient variations of the temperature and velocity distributions inside the tank are determined.

The results show that the flow and temperature distributions inside the moderator tank are three dimensional and no symmetry plane can be identified. A high temperature zone, located on the top left corner of the tank, is identified. The inlet jets are found to flow along the walls of the tank and impinge on each other at the top of the tank. This impingement point is located more towards the right side of the tank. The impingement of these two flow result in a secondary downward moving jet, which penetrates into the tube bundle. This secondary jet divides the tank into two sides. One side contains high temperature liquid and the other side contains low temperature liquid.

The temperature contours along the length of the tank have a saddle shape, with high temperatures towards the edges of the saddle. This is due to strong wall jet flows in the middle planes pushing the hotter fluid towards the end walls of the tank. Competition between the upward moving buoyancy driven flows and the downward moving momentum driven flows, results in the formation of circulation zones inside the tank. The numerical results for MTF indicate that the moderator tank operates in the buoyancy driven mode. Any small disturbances in the flow or temperature can make the system unstable and asymmetric. Once the system comes out of symmetry, it cannot go back to symmetry. This results in circulating buoyancy driven flow at one side and a momentum driven flow at the other side of tank.

Different types temperature fluctuations are noted inside the tank: (i) large amplitude temperature fluctuations are mainly at the boundaries between the hot and cold; (ii) low amplitude temperature fluctuations are mainly in the core of the tank with more uniform temperature distributions; (iii) high frequency fluctuations are in the regions with high velocities; and (iv) low frequency fluctuations are in the regions with lower fluid velocities.

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