Heat transfer in vertical concentric cylinders in a high level nuclear waste repository
Modeling free convection heat transfer in a cylindrical annular enclosure is still an active area of research and an important problem to be addressed in the high level nuclear waste repository. The waste containers are vertically emplaced in the borehole 300 meters below ground. and in a horizontal grid of 30 x 8 meters apart. The borehole will be capped after the container emplacement and the container is surrounded by an enclosed vertically concentric air-gap inside the borehole. The expected initial heat generated is 4.74 kW per container and it degenerates exponentially with time. A computer simulation model is used to find the thermal performance for the air-gap and the problem is solved in two steps. The first step is to solve for the heat conduction in the rock. The second step proceeds by using the results of the first step as boundary condition to solve the more complicated convection and radiation in the air-gap. Finally a parametric study was done for three different air-gap thicknesses (5 cm, 10 cm, and 15 cm) to obtain the effect of the gap thickness on that heat transfer. Eventually the goal of this study is to obtain the temperature at the central axis of the vertical container as a function of time so that the maximum temperature of the zirconium cladding on the fuel pellets can be determined.