Natural convection in high heat flux tanks at the Hanford Waste Site / [by] Mark van der Helm and Mujid S. Kazimi

Abstract

A study was carried out on the potential for natural convection and the effect of natural convection in a High Heat Flux Tank, Tank 241-C-106, at the Hanford Reservation. To determine the existence of natural convection, multiple computations based on analytical models were made knowing the tank geometry and contents' thermal characteristics. Each computation of the existence of natural convection was based on the determination of the onset of natural convection generalizing the tank as a 1-D porous medium. Computations were done for a range of permeabilities considering the porous medium alone, with a superposed fluid layer, and with a salt gradient. Considering only the porous medium, the higher permeability value, 3.2 *10-10 ft2, allowed convection, though the lower permeability, 2.6*10-14 ft2, did not. The presence of the superposed layer induced convection throughout the porous medium for the full range of permeabilities.

Considering the effect of the salt gradient and superposed layer together, the effect of the superposed layer is expected to induce convection despite the stabilizing salt gradient. Therefore, natural convection is expected to exist in Tank 241-C-106. Secondly, because temperature measurements indicated lower temperatures at a location near the center of the tank, a thermal model was used to compute the local effects of a convective annulus around a thermocouple tree at that location. A conduction model of the tank and surroundings was used to bound the local model. The local model allowing convection in the annulus set the size of the annulus based on the known temperature measurements of the thermocouple tree and the boundary conditions set by the conduction model. Previous published calculations on Tank 241-C-106, allowing for only conduction within the tank, reported a steam region at the bottom of the tank with an approximately 24 foot radius.

In the present analysis, using the computer code, TEMPEST, it is found that the cooling effect of the annulus creates a region with a 12 foot radius surrounding the thermocouple tree in which the temperature is suppressed below the saturation temperature due to the effects of the convective annulus. The annulus gap width for matching temperatures and the boundary conditions is on the order of 1 inch.