Natural convection flow is established in KMRR(Korea Multi-purpose Research Reactor) reflector tank at the loss of reflector circulation. At some spots of the inner shell and bottom plate of the reflector tank, high temperature is maintained due to the nuclear heating inside the tank wall and it is a design concern to prevent any local boiling inside the reflector tank not only during steady state but also transient like loss of reflector circulation. To simulate the natural convection flow inside the reflector tank and the associated heat transfer a laminar natural convective heat transfer test in an open cavity with 'L' type heated wall made by the combination of a vertical and bottom horizontal plate was performed using the Mach-Zehnder interferometer. As a test result, the fluid temperature distribution in the test section and the local Nusselt number distributions at the heated wall were obtained for various Rayleigh numbers. Also a numerical model which predicted the experimental results well was developed using SIMPLER algorithm. It was confirmed through the experimental results and the numerical analysis that the heat transfer rates were highest at the lower region of vertical plate and the inlet region of horizontal plate and comparatively high at the middle portion of the inner shell and bottom plate. It is thought that this heat transfer rate distribution shows a desirable trend for the effective natural convection cooling of the KMRR reflector tank considering that the highest wall temperature occurs at the middle portion of the inner shell and bottom plate. It was also confirmed that the average Nusselt numbers at the 'L' type heated wall were lower than those obtained from the existing natural convection heat transfer correlations for the vertical and horizontal plate.
